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Chapter 5Contaminant Fate and Transport

5.1 Introduction

The fate and transport of chemicals in the study area are discussed in this chapter. The fateand transport are described to support the human-health and ecological risk assessments andto aid in defining remedial alternatives.

Fundamental to describing fate and transport at the site is the conceptual site model, and themodel is described in this chapter. The conceptual site model qualitatively defines thevarious contaminant sources, release mechanisms, relative rates of migration and persistenceof contaminants, and migration pathways for contaminants at the site. The conceptualmodel was used to evaluate sources, release mechanisms, and pathways during the RI.

The chapter is divided into the following sections:

• Introduction• Contaminant Mobility and Persistence• Contaminant Migration• Summary• References

5.2 Contaminant Mobility and Persistence

The probable behavior of the COPCs at the site is determined by their physical, chemical,and biological interaction with the environment. The mobility and persistence of thechemicals in the environment are two key characteristics in determining probable behavior.Mobility is the potential for a chemical to migrate from a site, and persistence is a measureof how long a chemical will remain in the environment. Environmental factors that affectthe mobility and persistence of the contaminants include pH; concentration of otherchemicals in the media; soil moisture; oxidation-reduction potential (ORP), measured as Eh;water chemistry; organic-matter content; and the presence of microorganisms.

5.2.1 Contaminant Groups

A wide range of chemicals has been detected in environmental media in the study area.Tables 4-1 and 4-2 list the COPCs. The lists reduce the number of chemicals that requirefurther assessment as risks to human health and the environment. Chapter 4 discusses thenature and extent of selected chemicals from the lists of COPCs and several otherchemicals. Not all of the chemicals evaluated in Chapter 4 are discussed in Chapter 5.Instead, specific chemicals were selected to represent the range of COPCs associated with

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the site. The representative chemicals from the COPC list were selected on the basis ofhigh concentrations, common occurrence, variable migration potential, and likelycontribution to overall risk to human health and the environment. The chemicals discussedin this chapter are listed in Table 5-1 and are grouped according to general type.

Table 5-1REPRESENTATIVE CHEMICALS

Halby Chemical Site

Inorganics

AluminumArsenicBerylliumChromiumLeadManganeseAmmoniaThiocyanateCyanide

SVOCs

Benzo(a)pyreneFluorene

Nonchlorinated VOCs

Carbon disulfideToluene

Chlorinated VOCs

Vinyl chloride

Pesticides

Endrin

Note: The chemicals in this list were selected on the basis of high concentration,common occurrence, and variable migration potential. The chemicals are from thelists of COPCs presented in tables 4-1 and 4-2.

5.2.2 Physical and Chemical Properties

Various basic physical and chemical properties affect the transport of chemicals in theenvironment. The following are the most important properties:

• Sorption• Volatilization• Degradation• Transformation• Density• Bioaccumulation

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Sorption is the tendency for chemicals to adsorb to and desorb from materials in the mediain which they occur. The materials susceptible to sorption typically are clays and organicmaterial, both of which exist in abundance in the environment at the site. In addition,inorganic chemicals adsorb onto iron, manganese, and aluminum oxyhydroxide or oxidecoatings and organic coatings on sediment grains. Adsorption of inorganic chemicals(particularly metals) can be irreversible because of the process of fixation. Theconventional measure of sorption is the distribution coefficient (Kj) of soil and geologicmaterial for the chemical. The Kd for organic chemicals is the product of a partitioncoefficient (K̂ ) and the fraction of organic carbon. In general, chemicals with a K,,,. greaterthan 10,000 ml/g (such as many SVOCs) have high degrees of adsorption andconsequentially low mobility, whereas chemicals with a K̂ , lower than 1,000 ml/g (such asmany VOCs) have lower degrees of adsorption and consequentially higher mobility. Ahigher K̂ also contributes to greater bioaccumulation. The Kd for inorganic chemicals is acomplex function of pH, organic content, oxide coatings, and other factors; therefore, Kd isnot easily estimated by methods other than site-specific testing.

Volatilization is the tendency for some chemicals, particularly VOCs, to change from aliquid or adsorbed state to a gas. A conventional measure of volatility is Henry's LawConstant (K,,). Compounds with Kh values higher than 10"3 atmosphere-cubic meter permole (atm-m3/M) are expected to volatilize readily from water to air, whereas COPCs withK̂ values lower than 10"5 atm-m3/M are relatively nonvolatile. Most inorganic COPCs arenot volatile under normal temperature and pressure conditions.

Degradation is the transformation of one chemical to another by such processes ashydrolysis, photolysis, and biodegradation. Degradation is commonly expressed as a half-life that composites the degradation by whatever processes may be operating. Hydrolysis isthe reaction of a chemical with water and photolysis is the result of exposing the chemicalto light. Biodegradation results from the activity of microorganisms, which transformchemicals under either aerobic (oxidizing) or anaerobic (reducing) conditions.

Transformation occurs when metals are increased or reduced in valence state by oxidationor reduction, respectively. Transformation may have a significant effect on the mobility ofa metal, either increasing or decreasing it. Transformation can be caused by Eh and pHchanges and microbial or nonmicrobial (abiotic) processes.

Density effects can be significant if the concentration of a chemical in water exceeds itswater solubility and forms a nonaqueous-phase liquid (NAPL). The density of a chemicalgenerally is characterized by its specific gravity, the ratio of the density of the chemical inits pure form to that of water. If more dense than water, the dense NAPL (DNAPL) willtend to sink through the water-bearing unit until other factors, such as variability in thepermeability of geologic materials, change its direction of movement.

Bioaccumulation is the extent to which a chemical will partition from water into thelipophilic parts (e.g., fat) of an organism. Bioaccumulation commonly is estimated by theoctanol-water partition coefficient (K̂). Chemicals with high values of Kow tend to avoid

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the aqueous phase and remain in soil longer or bioaccumulate in the lipid tissue of exposedorganisms. Accumulation of a chemical in the tissue of the organism can be quantified bya bioconcentration factor (BCF), which is the ratio of the concentration of the chemical inthe tissue to the concentration in the water. BCFs are both contaminant- and species-specific. Inorganic chemicals and SVOCs tend to have higher K̂ values, so theybioaccumulate more extensively than VOCs.

Table 5-2 contains data for the representative chemicals on the physical and chemicalproperties that are relevant to fate and transport. Tables 5-3, 5-4, and 5-5 contain data ontotal organic carbon (TOC), pH, conductivity, temperature, Eh, and dissolved oxygen ingroundwater obtained during groundwater sampling.

5.2.3 Representative Chemicals

The following chemical-specific profiles briefly describe how the chemical and physicalproperties of the representative chemicals affect their mobility and persistence in theenvironment.

5.2.3.1 Aluminum

Aluminum is highly reactive and, in nature, is found in combination with other substances,such as oxygen, fluoride, and silica. There is only one oxidation state for aluminum, +3.Major transport processes include leaching from geologic formations and soil and sedimentparticulates to water, complexation, and adsorption onto soil or sediment particulates. Ingeneral, the mobility of manomeric forms of aluminum increases as the pH decreases.Adsorption onto clay and suspended particulate is a significant and rapid process.

5.2.3.2 Arsenic

The predominant form of arsenic in oxidizing environments is arsenate—As(+5). Underslightly reducing and acidic conditions, such as temporary flooding, the more toxic andmobile arsenite—As(+3)—form dominates. Arsenite and methylated arsine predominate inmoderately reducing soil, such as tidal marshes and consistently flooded soil. All of theseconditions are likely to exist at the site. However, at the site, no arsine was detected usingDraeger tubes, which monitor vapors arising from soil, sediment, or surface-water samples.

Transport and partitioning of arsenic (and all other metals) in water depend on the oxidationstate of the arsenic and on interactions with other materials present. Organic matter,divalent metals, and dissolved sulfide enhance the reduction of the arsenic valence state toa more mobile form. Soluble forms move with water, but arsenic may be adsorbed fromwater onto sediment or soil, especially clays, iron oxyhydroxides and oxides, aluminumhydroxides, manganese compounds, and organic material. Adsorption to oxyhydroxides isthe most important natural adsorption process. Microbes are capable of methylating arsenicto trimethylarsine gas, which is a more volatile and mobile form than inorganic arsenic.

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Table 5-3GROUNDWATER PARAMETERS IN BACKFILL, COLUMBIA WATER-BEARING

UNIT, AND SHALLOW RECENT SEDIMENTSHalbv Chemical Site

Well

SMW-1

SMW-2

W-2A

W-3A

W4-C

W5-B

SMW-6

SMW-8

SMW-9

BMW-10

SMW-1 0

SMW-1 1

SMW-1 2

TOC (mg/l)

14412641.718.729

32.9B12.64.912672.935

20.5B36.25.51 B3882993883371261544091.314.29.64B68.223.6

PH

7.206.416.335.236.746.296.466.487.007.346.416.468.306.707.726.348.288.995.906.905.956.326.025.866.78-

Conductivity(umhos/cm)

2300147037004930185026701752804040

L 38404180616068059022006710364043701700206015751750328033002680-

T(C)

16.0013.0017.4014.0016.0014.0016.00

-17.5017.0019.0016.0019.0016.0019.5014.0017.2013.0019.0014.0017.0014.0017.0014.0017.00

-

Eh (mv)

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-136-

-219--60--62-+25---

DO (ppm)

-1.69-

5.10-

0.560.800.42-

4.43-

0.99-

3.72-

1.07-

1.144.100.740.702.66-

1.58--

TOG = Total organic carbonT = TemperatureEh = Reduction-oxidation potentialDO = Dissolved oxygen- " = Not measuredB = TOC detected in associated blank sample

Table 5-4GROUNDWATER PARAMETERS IN UPPER POTOMAC WATER-BEARING

UNIT AND INTERMEDIATE RECENT SEDIMENTSHalby Chemical Site

Well

IMW-1

IMW-2

W-2B

IMW-3

W-3B

W4-B

W5-A

IMW-6

IMW-8

IMW-9

IMW-1 0

IMW-1 1

IMW-1 2

TOC(mg/l)97.91423.93.28.99.211412.2

L_ 23.71.96B2.6

500006540014101200871007070860092003703904200237014021043504690260015502900-

T(C)

16.7014.0016.5013.0016.8014.0015.0020.0015.5014.0019.0014.0016.5014.0016.0015.0016.7014.0013.7013.0016.0014.0020.0025.0017.00

-

Eh (mv)

+99-+46-

+196--55--18--40--6-+4--37---+39--6---

DO (ppm)

1.602.04-

1.95-

2.05-

1.002.500.80-

3.28-

3.93-

1.11-

1.15-

1.092.201.091.600.60--

TOC = Total organic carbonT = TemperatureEh = Reduction-oxidation potentialDO = Dissolved oxygen- " = Not measuredB = TOC detected in associated blank sampleR = Rejected data value

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Table 5-5GROUNDWATER PARAMETERS IN LOWER POTOMAC WATER-BEARING

UNITHalby Chemical Site

Well

DMW-1

DMW-2

DMW-3A

DMW-3B

DMW-4

DMW-5

DMW-7

W4-A

TOC(mg/l)12.41.47B2.11.01B4.2

Bioaccumulation of arsenic occurs in aquatic organisms, particularly algae and lowerinvertebrates. Although some fish and invertebrates may contain high levels of arseniccompounds, the predominant arsenic form, arsenobetaine, is relatively inert.Biomagnification in aquatic food chains does not appear to be significant.

5.2.3.3 Beryllium

Soluble beryllium salts are hydrolyzed in water to form insoluble beryllium hydroxide.Adsorption to clay and metal oxides is most effective at low pH. Beryllium can formcomplexes, oxycarboxylates, and chelates with a variety of materials, which increase thesolubility and mobility of beryllium species. In natural waters, most beryllium is found onparticulates, either adsorbed or precipitated. Beryllium is not known to bioconcentrate inaquatic species.

Certain fossil fuels contain beryllium compounds and may therefore serve as sources to theenvironment. Beryllium (as well as arsenic, nickel, vanadium, and other metals) artificiallyenters the environment from coal combustion and can emerge from slag and ash dumps(HSDB, 1994).

5.2.3.4 Chromium

Chromium (Cr) forms two compounds that depend on oxidation state, chromic (+3) andchromates (+6). Trivalent (+3) chromium is the dominant species under the pH and redoxconditions generally present in subsurface environments. Chromium is soluble in acids andstrong alkalies. Trivalent chromium is converted to hexavalent chromium (+6) underoxidizing conditions. Hexavalent chromium is highly mobile and more toxic than the +3oxidation state.

Hexavalent chromium is soluble, existing in solution as an anionic complex, and is notreadily adsorbed by clays or hydrous metal oxides. Trivalent chromium is readily adsorbedin the subsurface. Hexavalent chromium is a moderately strong oxidizing agent and reactswith organic or other oxidizable material to form trivalent chromium. Trivalent chromiumcombines with aqueous hydroxide ions (OH") to form insoluble chromium hydroxide(Cr(OH)3). Precipitation of this material is thought to be the dominant fate of chromium innatural waters. Chromium is bioaccumulated by aquatic organisms, and the passage ofchromium through the food chain has been documented. Chromium in soil can occur as theinsoluble oxide dichromate (CR2O3) and may enter the atmosphere as an aerosol or betransported to surface waters and groundwaters in runoff and leachate.

5.2.3.5 Lead

The dominant species of lead in aqueous solutions is dissolved divalent (+2) lead underacidic conditions and divalent lead carbonate complexes under alkaline conditions. Lead isremoved effectively from water to soil and sediment by adsorption to organic matter, clay

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minerals, and hydrous iron and manganese oxide. Adsorption is the predominant mode oflead removal from water and increases with increasing pH. Only a small fraction of lead incontaminated soil appears to be in water-soluble, potentially mobile, forms.

Precipitation of lead increases with pH, most lead precipitating out at pH higher than 6.Lead also precipitates out as oxides, hydroxides, hydroxycarbonates, sulfates, and sulfides.Under oxidizing conditions, the least soluble forms of lead are carbonates, hydroxides, andhydroxycarbonates. In reducing conditions where sulfur is present, lead sulfide is the stablesolid.

Lead is an extremely stable metal. Because of its very-low vapor pressure, volatilization oflead from soil to water is negligible. If released into water, metallic lead will sink intosediment.

Under appropriate conditions, dissolution by anaerobic microbial action may be significantin subsurface environments. Lead does not appear to bioconcentrate significantly in fish.

5.2.3.6 Manganese

As with all metals, elemental manganese (Mn) and inorganic manganese compounds havenegligible vapor pressures but may exist in air as part of or adsorbed to suspendedpaniculate matter. Dry deposition is the main force that removes manganese (and othersuspended particulates) from the atmosphere.

Divalent (+2) manganese predominates as the dissolved form in most natural water. Theprincipal anion associated with Mn(+2) in water usually is carbonate. The concentration ofmanganese is limited by the relatively low solubility of MnCO3 [Mn(+2)] and the extremelylow solubility of MnO2 [Mn(+4)]. In extremely reducing water, the fate of manganese iscontrolled by formation of the poorly soluble sulfide.

Soil adsorption constants for Mn(+2) may span five orders of magnitude, ranging from 0.2to 10,000 mg/1, increasing as a function of the organic content and the adsorption of thesoil. In some cases, adsorption of manganese to soil may not be a readily reversibleprocess. At low concentrations, manganese may be fixed by clays and iron oxyhydroxidesand will not be released into solution readily. At higher concentrations, manganese may bedesorbed by ion-exchange mechanisms with other ions in solution. As with most metals,adsorption of manganese is reduced as the pH decreases.

Manganese in water systems may be bioconcentrated significantly by aquatic organisms.

5.2.3.7 Ammonia

Ammonia readily dissolves in water, primarily as ammonium [NH4(+1)], when the pH isbelow 8.5. With a K^, of only 3 ml/g, ammonia is only slightly adsorbed to soil and

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sediment and is therefore very mobile. Under anaerobic conditions, ammonia is even lessstrongly adsorbed. Ammonia may be generated from nitrate and other sources of nitrogen(e.g., cyanide, thiocyanate, and organic enzymes) by microbiota. Oxidation of thiocyanatein the presence of water generates bicarbonate, ammonium, and sulfate. Thiocyanate is asite-related chemical but also could be attributable to the petroleum coke piles (seesubsection 5.2.3.8).

In clay soil, ionized ammonia competes with major ions for ion-exchange sites on clay andcan displace the other ions from the exchange sites.

Ammonia occurs primarily as a gas in the atmosphere. The predominant fate process ofatmospheric ammonia is reaction with acidic gas air pollutants, such as nitric and sulfuricacids. In the atmosphere, ammonia combines with sulfate and washes out rapidly inprecipitation (HSDB, 1994). Some ammonium ions may be oxidized to oxides of nitrogenand to nitrate ions and then can be removed readily by precipitation. The atmospheric half-life of ammonia is estimated to be a few days.

Other important removal processes include wet deposition with rain and snow, dissolutioninto surface water, uptake by plants, and reaction with photochemically produced hydroxylradicals. In soil, surface water, groundwater, and sediment, ammonia can undergo trans-formations that render it a nutrient source. In surface water, ammonia is converted rapidlyby aerobic microbes into nitrate by nitrification.

5.2.3.8 Thiocyanate and Cyanide

Thiocyanate is the predominant compound of cyanide detected in the study area. Singlethiocyanate salts (such as the sodium and potassium thiocyanate used at the former processplant) typically are soluble and mobile in the environment. When thiocyanate formscomplexes with metals, the complex may be relatively insoluble and stable.

Thiocyanate converts to cyanide with the release of elemental sulfur in the presence of astrong oxidizer, such as chlorine. In the absence of such an oxidizer, thiocyanate will tendnot to convert. Strong oxidizers, such as chlorine, typically are available at sufficientconcentrations only in such processes as water treatment and not in the natural environment.Hexavalent chromium is a strong oxidizer and may have contributed to the conversion ofthiocyanate to cyanide in the past. The analytical data indicate that hexavalent chromiummakes up only a small percentage of total chromium, so this oxidizing ability probably nolonger exists at the site at significant levels.

Under moist reducing conditions (such as those in landfills and wetlands), thiocyanateconverts to HS(-l) and ammonium (Lagas et al., 1982).

The predominant form of cyanide in the environment is hydrogen cyanide (HCN), a weakacid that is soluble in all proportions in water and which will volatilize rapidly from water

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and soil surfaces. Hydrogen cyanide is the weak-acid dissociable cyanide discussed in thisreport. Ultimately, HCN is biodegradable.

Cyanide also may exist as simple cyanides (e.g., NaCN), as metallocyanide complexes (e.g.,with iron, calcium, cadmium, sodium, cobalt, copper, nickel, and zinc), and as organiccompounds. Total cyanide values measured in the study area are the sum of theconcentrations of all cyanide species. Typically, if no cyanide was detected, then no weak-acid dissociable cyanide was detected, either. In addition, if cyanide was detected, then theconcentration of weak-acid dissociable cyanide rarely exceeded 10 percent of theconcentration of total cyanide and often was not detected at all. These data lead to theconclusion that most of the cyanide in the environment at the site is in a metallocyanidecomplex. Cyanide tends most to complex with cobalt and iron and least with zinc andcadmium. The predominance of iron in groundwater at the site suggests that an iron-cyanide combination is the likely metallocyanide complex.

Cyanide mobility is pH-dependent. In general, cyanide is relatively mobile when the pH isless than 9.2 but is less mobile when the pH is very low, iron-oxide concentrations are high,or clay is present. Although the pH of groundwater in the study area is moderate (pH = 6to 8), there is much disseminated clay in the geologic materials and iron oxide likely ispresent extensively because of the high concentration of iron in the groundwater. Thisaccounts for cyanide not being detected in Columbia wells along the Christina River andLobdell Canal or in upper Potomac wells east of the site.

Cyanide is not known to bioaccumulate.

In the study area, the petroleum coke piles represent potential sources of cyanide andthiocyanate, both of which are created during the coking process (Luthy, 1979). During thecarbonization of coal to coke, coal is combined with ammonia and oxygen at hightemperature. This process generates HCN as a by-product. Because the coke probably wasnot washed thoroughly after processing, HCN likely remained on the coke and has beenwashed off during storage in the study area. Mixing HCN with sulfides would generatethiocyanate.

5.2.3.9 Benzo(a)pyrene

Very low vapor pressure, very low solubility in water, and high K^ and K^ values indicatethat benzo(a)pyrene (BaP) is strongly adsorbed to organic material in soil. Volatilization isan insignificant transport mechanism. Because of the low solubility of BaP in water and itspreference for adsorbing to organic material in soil, groundwater transport of BaP isunlikely.

PAHs such as benzo(a)pyrene do not contain functional groups that are susceptible tohydrolytic reactions. Therefore, hydrolysis is not a significant degradation mechanism forthis contaminant. Biodegradation of PAHs is well-documented and is probably the ultimate

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fate of BaP. Limited availability of nutrients and oxygen and the presence of such potentialmicrobe inhibitors as arsenic could limit the effectiveness of biodegradation. The half-lifeof BaP in soil is 2 months to 1.5 years.

5.2.3.10 Fluorene

Biodegradation is the dominant process for removing fluorene from soil. On the basis ofthe log KO,, value, fluorene is considered immobile in soil. When released to water, fluorenewill adsorb strongly to sediment and suspended matter, indicating that adsorption is animportant fate process in surface water. In experimental studies, fluorene was observed tohave a half-life of 6 to 27 days. On the basis of vapor pressure, fluorene is expected toexist in the vapor phase in the ambient atmosphere. Fluorene in the vapor phase willdegrade readily in the ambient environment by reacting with photochemically producedhydroxyl radicals. Removal of fluorene in the air will occur through wet and drydeposition.

Aquatic organisms that lack a metabolic detoxification system, such as phytoplankton,mussels, scallops, and snails, tend to accumulate PAHs such as fluorene.

5.2.3.11 Carbon Bisulfide

Carbon disulfide's high vapor pressure and K̂ causes it to volatilize fairly rapidly from soiland water. The moderate K^ value indicates that carbon disulfide has medium mobilitythrough soil and will not partition significantly from water to soil. Carbon disulfide isknown to migrate more rapidly under reducing conditions, which predominate in thegroundwater at the site, than under oxidizing conditions. The relatively low Kow valueindicates that carbon disulfide tends to remain in the aqueous phase and is not readilybioaccumulated. The greater adsorption observed in moist soil than in dry soil suggests thatsome adsorption is caused by microbial action (HSDB, 1994).

The high concentrations of carbon disulfide detected in groundwater (on the order of330 mg/1 during the first round of RI sampling) are almost 15 percent of the solubility limitof 2,300 mg/1 (Montgomery and Welkom, 1990), suggesting the possible presence of carbondisulfide as a DNAPL near well cluster MW-1. Witco (1996) reported carbon disulfide at1,500 mg/1 (about 65 percent of solubility) in a groundwater sample from a test pit near theprocess plant drainage ditch. Small masses of carbon disulfide were reported in excavationsby USEPA. The specific gravity of carbon disulfide is 1.26, so it would form a DNAPL.

Carbon disulfide in the atmosphere reacts with oxygen and photochemically produceshydroxyl radicals with a half-life of a few days. The compound is oxidized by someheterotrophs; the degradation half-life probably is longer than 1 year (HSDB, 1994).

Carbon disulfide reportedly occurs naturally in marshlands where biological activity is high.Carbon disulfide can be derived from natural anaerobic biodegradation. Carbon disulfide

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can be generated by the oxidation of iron sulfide (pyrite), which is naturally ubiquitous inestuarine and tidal marsh environments, particularly associated with organic debris (Tayloret al., 1982). Under natural mineralized conditions, the gas is continuously generated byiron sulfide that is oxidizing. However, at the site, the highest concentrations of carbondisulfide are not associated with these sorts of environments.

As with most organic compounds, biodegradation rates of carbon disulfide in subsurface soiland groundwater vary considerably with type of soil, water chemistry, hydrologicconditions, types of microbes, humus content, temperature, pH, Eh, amount of oxygen, andthe presence of other nutrients. The expected half-life of carbon disulfide under aerobicconditions in soil ranges from 0.3 to 1 year.

5.2.3.12 Toluene

The primary transport process for toluene is volatilization from soil and water. If toluene isreleased to the atmosphere, it will degrade by reacting with photochemically producedhydroxyl radicals with a half-life of 3 hours to slightly over 1 day or will be washed out inrain. Toluene is not subject to direct photolysis.

If toluene is released to soil, it will be lost by evaporation from near-surface soil, byleaching to groundwater, and by microbial degradation. Biodegradation occurs both in soiland groundwater but it is apt to be slow, especially at high concentrations that may be toxicto microorganisms. Soil biodegradation is not impeded by adsorption. The expected half-life of toluene in soil is 4 to 22 days and in groundwater is 1 to 4 weeks.

Toluene will not hydrolyze significantly in soil or water under normal environmentalconditions. If toluene is released into the water, its concentration will decrease because ofevaporation and biodegradation. The removal can be rapid or take several weeks,depending on temperature, mixing conditions, and acclimation of microorganisms. Toluenewill not significantly adsorb to sediment or bioconcentrate in aquatic organisms.

Toluene also may form an light nonaqueous-phase liquid (LNAPL). Toluene that isdissolved in groundwater up to about 10 percent of its solubility suggests the presence ofLNAPL toluene.

5.2.3.13 Vinyl chloride

The low KQC value indicates that vinyl chloride exhibits high mobility in soil and should notpartition from water to soil. The low Kow and the high solubility of 1,100 mg/1 indicatesthat vinyl chloride tends to remain in the aqueous phase and is not readily bioaccumulated.

Vinyl chloride has a very high vapor pressure and a high Kh, which allow it to volatilizerapidly from soil and surface water. Once vinyl chloride is in the atmosphere,photooxidation reduces it to hydrogen chloride and carbon monoxide.

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Within both saturated and unsaturated soil, vinyl chloride can form by the biodegradation ofTCE, PCE, and 1,2-dichloroethene (1,2-DCE). Further biodegradation of vinyl chloride tocarbon dioxide has been documented under laboratory conditions, but it may not be asimportant a factor in natural environments. The half-life for vinyl chloride in groundwaterranges from a few weeks to almost 8 years. Half-life in soil and surface water is muchshorter.

5.2.3.14 Endrin

Endrin is known to persist in soil for 14 years or longer. Endrin has a low water solubilityand strong adsorption to soil, thus making leaching to groundwater unlikely. Endrin that isreleased to surface-water systems will be subject to photoisomerization to ketoendrin.Endrin will adsorb significantly to sediment. Endrin is known to bioconcentrate in fish andshellfish.

5.3 Contaminant Migration

This section discusses the site-specific source areas and potential mechanisms forcontaminant release and migration from the source areas.

5.3.1 Source Areas

The environmental media at the site appear to have the following sources of contaminants:

• Surface and subsurface soil in the former process plant area and the processplant drainage ditch

• Subsurface soil, including sediment from the former lagoon and backfilledmaterials in the former onsite lagoon

DNAPLs

Offsite source areas of contamination may exist west of the site (e.g., the asphalt plant),north of the site (e.g., the salt piles), and east of the site at the petroleum coke piles andperhaps other facilities and operations.

The former process plant area is a probable source because of the elevated concentrations ofcontaminants, such as arsenic and carbon disulfide, in wells and soil in and near the areaand past use of these chemicals in the area.

Carbon disulfide and other VOCs were detected, in particular, near the railroad tracks in theformer process plant area and along the process plant drainage ditch. VOCs essentiallywere nonexistent in surface soil. The concentrations observed in subsurface soil probably

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represent the residuals of even higher past concentrations. Large quantities of carbondisulfide probably were released to the surface of the ground or to the onsite drainagesystem, or both, causing extensive contamination of soil in the former process plant areaand in the ditch. Although carbon disulfide may be derived from natural sources, the highconcentrations at the site are not associated with environments expected to have naturallyhigh levels of carbon disulfide. The carbon disulfide then would have migrated downwardto the groundwater and along the drainage ditch to the onsite lagoon. Some plantdischarges may have gone directly to the tidal marsh.

The water main between the process plant drainage ditch and the Conrail tracks is incontact with soil with high concentrations of arsenic and carbon disulfide. No structural orcorrosion problems were observed at the main, so the soil contamination apparently has notaffected the main to date. Were the water main to be breached in the area of thecontaminated soil, the large volume of water that would flow from the main would preventcontaminated soil from entering the main. Once the main was shut off, some contaminationmight enter the pipe if water is allowed to flow back into the pipe.

The fill materials are identified as a probable source of groundwater contamination becausethey contain concentrations of chemicals well above background that also are identified asCOPCs in shallow groundwater. The chemicals include arsenic, beryllium, manganese, andvanadium. For example, well BMW-10 in the backfill and well SMW-10 in the Columbiahad very high concentrations of manganese and thiocyanate, among other chemicals.Nearby soil borings SB-8 and SB-20 contained high concentrations of manganese, and thesoil sample from the borehole for well IMW-10 had a high concentration of thiocyanate.Other direct examples of the close association between soil and groundwater contaminationare difficult to identify because of the many processes affecting the concentrations ofchemicals as they migrate from soil to groundwater.

The sediment from the former lagoon, under the backfill, may be a source. The sedimentoccurs over much of the northern and central parts of the site at depths ranging from 8 to12 feet. Arsenic, beryllium, cadmium, and manganese were detected frequently atconcentrations well above background.

Under the aerobic conditions expected above the water table in the backfill, metals wouldbe in higher valence states. The higher valence state may enhance mobility (as withchromium) or reduce mobility (as with arsenic and vanadium). Hexavalent chromium isonly a small fraction of total chromium measured in onsite soil, indicating depletion of thisvalence state in comparison to the less mobile trivalent state.

The mass-balance approach described by Summers et al. (1980) was used to estimate theeffects of soil contamination on groundwater. The exchange of several metals fromunsaturated soil to groundwater was analyzed. The concentration that could be left in thesoil that would not cause the groundwater MCL (or RBC for thiocyanate) to be exceededwas estimated. The details of how the approach was applied are provided in TechnicalMemorandum 7 (Appendix A).

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The results of the analysis are summarized in Table 5-6. The maximum allowableconcentration to be left in the soil is shown. The maximum allowable concentration waswell below the maximum observed in the soil at the site for all chemicals except thallium.The results indicate that contamination that remains in the soil could contaminategroundwater in the future to concentrations above acceptable levels.

Table 5-6ESTIMATE OF EFFECTS OF SOIL CONTAMINATION ON GROUNDWATER

Halby Chemical Site

Chemical

Arsenic

Beryllium

CadmiumManganese

Nickel

Thallium

Thiocyanate

MCL(mg/1)

0.050

0.004

0.005

0.050

0.10

0.002

NA

RBC(mg/1)

0.000045

0.000016

0.00078

0.038

0.031

0.00013

0.16

Concentration in Soil (mg/kg)MaximumAllowable

4.6

9

0.1

24.5

52

10.2

0.55

HighestObserved11,900

116

9926,100

252

4.4

940

MCL = Maximum contaminant levelRBC = Risk-based concentrationNA = Not available

As more-mobile metals migrate downward and encounter the organic silt of the sedimentfrom the former lagoon, the valence state is likely to drop where reducing, more-acidicconditions predominate. TOC in groundwater from the backfill well (BMW-10) was veryhigh (well above 100 mg/1), and the Eh was measured at -60 millivolts (mV), bothsuggesting reducing conditions. For metals such as chromium, the reduction in the valencestate should make the metals less mobile, and they could concentrate in this layer, whereasthe arsenic and vanadium become more mobile and migrate through. This may explain whyarsenic and vanadium are present in groundwater at concentrations well above RBCs butchromium is not. The presence of arsenic and vanadium in the sediment from the formerlagoon attest to the sediment as a continuing source of groundwater contamination.

A nearby source that may be providing several of the same contaminants as the site(particularly some metals and SVOCs) is the area to the east that contains petroleum cokepiles. The piles have been located in the area for many years and probably havecontributed contaminants to groundwater, surface water from surface runoff, and airborne

5-18

dust, which may have distributed contaminants widely. The fact that ammonia, cyanide,thiocyanate, and several site-related metals, such as arsenic, beryllium, and vanadium, couldhave been derived from the petroleum coke piles is discussed in subsection 5.2.3. Highsulfate concentrations in wells east of the tidal marsh attest to oxidation of sulfides in coal.

Carbon disulfide probably exists as a DNAPL in the Columbia near well SMW-1. Evidenceof this is the very high concentration (up to 330 mg/1) of carbon disulfide in the shallowwell (Columbia water-bearing unit), 1,500 mg/1 in a test pit, and the elevated concentrationof carbon disulfide in the intermediate-depth well (upper Potomac water-bearing unit) at thelocation, IMW-1. No direct evidence of DNAPL (e.g., a sample from the bottom of a well)has been detected in groundwater but it has been observed in the shallow excavations on thesite.

The primary mechanisms for contaminant release and transport from the sources at the siteare:

• Volatilization of organic compounds from soil and surface water

• Entrainment by wind of dust and soil containing contaminants

• Leaching of contaminants to groundwater and subsequent migration off thesite in groundwater to surface water (onsite lagoon, tidal marsh, or ChristinaRiver) or well-discharge points

• Release of contaminated surface runoff or soil to surface-water bodies and tothe sediment of drainage ditches, the onsite lagoon, adjacent wetlands, andthe Christina River

5.3.2 Releases to the Atmosphere

VOCs, including carbon disulfide, cyanide gas, and chlorinated hydrocarbons, could bereleased by volatilization through the pore spaces in the soil or by volatilization directly tothe atmosphere if site soil is excavated. The potential for SVOCs, such as PAHs, to bereleased to the atmosphere is much more limited because of their physical and chemicalproperties (see subsection 5.2.2).

Such VOCs as carbon disulfide and chlorinated solvents are characterized by relatively highvapor pressures, Henry's Law Constant, solubility in water, and generally low organic-carbon partition coefficients. These chemicals can be released by volatilization through thepore spaces in the soil or directly to the atmosphere if soil is excavated. The potential forSVOCs to be released to the atmosphere in this way is limited because of their lower vaporpressures and Henry's Law Constant.

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Sampling air by using a photoionization detector during fieldwork at the site indicated noemissions of VOCs from the site. VOCs were largely absent from soil and surface water,indicating that they have left these media by some process, probably volatilization.

Wind erosion and truck traffic have the potential for transporting contaminated soil.Contaminants that tend to bind to the soil (e.g., PAHs and metals) can be released to the airas dust. Several SVOCs, including BaP, and metals, including arsenic, beryllium,manganese, and vanadium, were detected in soil at the site and could be transported off thesite. Future activities at the site, such as excavation, could create pathways for releasingSVOCs to the atmosphere by generating contaminated dust. The presence of surface-soilcontamination and the fact that dust has been observed blowing off the site attests to thefeasibility of paniculate transport at the site.

5.3.3 Releases to Groundwater

Percolation of precipitation, both rainfall and snowmelt, through the unsaturated soil and fillcan dissolve contaminants and transport them to the groundwater through leaching.Contaminants then can be transported off the site in the direction of groundwater flow. Thefollowing mechanisms influence the migration of contaminants dissolved in groundwater:

• Advection, the transport of dissolved contaminants by the bulk motion offlowing groundwater, is the primary transport mechanism for dissolvedcontamination along the hydraulic gradient.

• Dispersion, the spreading of dissolved contaminants from the path theywould be expected to follow during advection, results from the spatialvariation in aquifer permeability, fluid mixing, and molecular diffusion.

• Sorption, degradation, and transformation, which are described in subsection5.2.2.

Not all contamination moves through the aquifer matrix in the dissolved phase in ground-water. NAPLs may migrate in somewhat different directions than groundwater does.Because of their physical characteristics, such as viscosity, the preferential path for DNAPLmovement may be through coarser sands and gravels.

Contaminated groundwater can discharge to surface-water bodies, such as wetlands andlagoons. Pumping of residential or industrial wells is another form of groundwaterdischarge. Regardless of how the contaminated groundwater is discharged, the contam-inants can be subjected upon discharge to biodegradation, oxidation, volatilization, and theother processes discussed in subsection 5.2.2.

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5.3.3.1 General Conceptual Model of Groundwater Flow

Maps of the water table and the potentiometric surfaces of the Columbia and the Potomacwater-bearing units (figures 3-13 through 3-18 and figures in Appendix C) were used todetermine groundwater-flow paths. Nine sets of water-level measurements in each water-bearing unit were taken between August 1993 and May 1994; two were taken onconsecutive days at high and low tides. Water-level fluctuations in all wells in the threeunits typically varied by 1 foot or less during the period of measurement. In general, thewater-level maps generated by using the data from each round of measurement areconsistent in their general directions of groundwater flow. Measurements taken at high andlow tides also are generally consistent with the periodic long-term water-levelmeasurements.

Individual Units. The following generalizations can be made about groundwater-flowdirections in each water-bearing unit.

Backfill. Only one well (BMW-10) is available for measuring water level in the backfilland the well is partially screened in the underlying Columbia. But evidence discussed inChapter 3 (subsection 3.5) shows that the water table occurs over most of the site in thebackfill, when present. The assumption is that flow in the backfill is to the east-northeasttoward the nearest surface-water discharge area, the onsite lagoon. This assumption isbased on the tendency for the water table generally to slope (and shallow groundwatergenerally to flow) in the direction of the topographic slope, toward surface discharges.

Columbia Water-Bearing Unit. According to the water-level data, flow in the Columbia istoward the northeast, in the direction of the lagoon, the tidal marsh, and the Christina River.Groundwater presumably flows laterally underneath the petroleum coke piles and dischargesinto the Recent sediment near the Christina River and ultimately into the river.Groundwater in the Columbia likely discharges at times upward into the tidal marsh (and,less likely, into the onsite lagoon on occasion), particularly at low tide.

Wells SMW-6 and W-3A, upgradient of the site, typically have very low or nondetectableconcentrations of the chemicals that were evaluated in Chapter 4 as being site-related.Therefore, they appear to be background wells for the site.

Arsenic, manganese, thiocyanate, cyanide, sulfide, and ammonia increased in concentrationfrom background in several monitoring wells on the site and are presumed to be derivedfrom the site. Carbon disulfide was detected at very high concentrations in well SMW-1 inboth RI rounds of sampling but not at particularly high concentrations elsewhere.Detections of PCE and TCE were limited to a few wells near the former process plant area.Downgradient, well SMW-9 contained high levels of arsenic, manganese, thiocyanate,cyanide, sulfide, and ammonia; well SMW-2 contained carbon disulfide; and wells SMW-2and W4-C contained elevated levels of thiocyanate and cyanide, all of which are site-relatedand support the concept of northeasterly groundwater flow in the Columbia.

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AR30214QL*

Contaminated groundwater is the likely source of contamination at the surface near theBio-7 sampling station, downgradient of the site. For example, the highest concentration ofammonia detected in Columbia wells was detected in well SMW-9, just a few yards fromthe Bio-7 station. Vinyl chloride was detected on the site during the second round ofgroundwater sampling and in well W4-C during both rounds. Vinyl chloride and 1,1-DCEare likely to be degradation products of PCE and TCE. The detections lend further supportto the concept of groundwater flow to the northeast in the Columbia.

Commonly, well SMW-12, near the Christina River, had water levels higher than those inwells just east of the tidal marsh. Less frequently, well W5-B periodically had water levelshigher than those east of the lagoon. This suggests occasional westward or southwestwardgroundwater flow from the vicinity of these wells back toward the site. Because waterlevels in wells just west of the tidal marsh typically are higher than water levels in wellsjust east of the marsh, the tendency is for groundwater to flow from west to east under themarsh, perhaps with discharge into the marsh at times of low tide. Any westwardgroundwater flow near wells SMW-12 and W5-B is a localized phenomenon and ultimatelygroundwater would be diverted to the east or southeast toward Lobdell Canal rather thancontinuing back under the site.

The presence of thiocyanate in well W-2B, north of the site, during the first round of RIsampling can be attributed to one or more of several causes: (1) periodic variations ingroundwater-flow direction; (2) other sources, such as the sediment from the former lagoon;and (3) former flow toward pumping wells, particularly the one at Forbes Steel.Thiocyanate also was detected in W-2B by EBASCO (1990) but was not detected in thesecond round of sampling during the current RI.

Upgradient well SMW-6 contained thiocyanate and cyanide; both chemicals have beendetermined to be site-related. Water-level data obtained during this RI and in previousinvestigations indicate that groundwater flow typically has been from the location of wellSMW-6 toward the site. It is unlikely that tidal fluctuations have pushed groundwatercontaminated with these chemicals from the site back to the location of SMW-6, becausethe tidal range is not sufficient (except perhaps when the tide is unusually high). Thethiocyanate and cyanide in SMW-6 may have been drawn into the well by pumping of theresidential wells at Hamilton Park (Figure 3-12), which is generally upgradient of the siteand well SMW-6. Although the wells are not used extensively now, pumping likely wasgreater in the past. The possibility also exists that there are sources of thiocyanate andcyanide upgradient of the site that have not been identified.

The generalization of groundwater flow to the northeast in the Columbia is complicated bythe possible interactions of the water-bearing unit with the surface water in the lagoon andthe tidal marsh. Fluctuations in the tidal levels obviously affect water levels in theColumbia (figures 3-13 and 3-14) and may change the flow direction in the Columbia attimes. The expectation, however, is that changes in flow direction in the Columbia wouldoccur only during periods of unusually high tide and would be very transient and localized.Therefore, the assumption is that fluctuations in the tide cause only minor, localized, and

5-22

very short-term deviations from the general trend of groundwater flow to the northeast inthis unit.

Upper Potomac Water-Bearing Unit. According to the water-level data, groundwater in theupper Potomac flows toward the northeast, in the direction of the Christina River.Groundwater presumably flows laterally underneath the petroleum coke piles and dischargesinto the marine sediment near the Christina River and ultimately into the river.

Wells IMW-6 and W-3B, upgradient of the site, typically have very low or nondetectableconcentrations of the chemicals that are evaluated in Chapter 4 as being site-related.Therefore, the wells appear to be background wells for the site.

Thiocyanate, cyanide, and ammonia (among other chemicals) increased in concentrationover background values in several monitoring wells on the site and are presumed to bederived from the site, and carbon disulfide was detected at elevated concentrations in wellIMW-1 in both RI rounds of sampling. Downgradient, wells IMW-9, IMW-2, and W4-Bcontained elevated levels of thiocyanate, cyanide, and ammonia, and well IMW-2 containedcarbon disulfide, all of which are site-related and support the concept of northeasterlygroundwater flow in the upper Potomac.

Commonly, the water level in well IMW-12 near the Christina River was higher than thewater levels in wells just east of the tidal marsh. This suggests westward or southwestwardgroundwater flow from the vicinity of this well back toward the site. However, thegroundwater levels west of the tidal marsh typically are higher than the well groundwaterlevels east of the marsh, indicating that westward groundwater flow would be divertedultimately to the east or the southeast toward Lobdell Canal or near well W5-B rather thancontinuing back under the site.

Lower Potomac Water-Bearing Unit. According to the water-level data, groundwater flowin the lower Potomac is toward the south (and occasionally toward the south-southeast andsoutheast) in the direction of pumping centers off the site. Recharge for the lower Potomacunder the study area may be coming from the lower Potomac east of the Christina River ifit extends upgradient under the river. In places, the lower Potomac has been truncatedcompletely with Recent sediment that is deposited directly on bedrock. At these locations,water from the Christina River likely recharges the lower Potomac through the Recentsediment. Water levels in the lower Potomac are consistent with recharge from the river.

Groundwater-Flow Velocities. Velocities for each water-bearing unit were estimated byusing the product of the hydraulic conductivity and hydraulic gradient divided by theeffective porosity. The effective porosity was estimated to be 0.20; this value, roughly two-thirds of the total porosity of a typical sand, acknowledges that not all of the porosity is

5-23

available for groundwater (or contaminants) to flow through. According to subsections3.5.3.3 and 3.5.3.4, the following values for hydraulic conductivity are representative:

• Columbia

Range of 0.92 to 12 ft/dayGeometric mean of 4.2 ft/dayAverage hydraulic gradient of 0.0015

• Upper Potomac

Range of 1.1 to 32 ft/dayGeometric mean of 8.7 ft/dayAverage hydraulic gradient of 0.00125

• Lower Potomac

Range of 0.18 to 4.3 ft/dayGeometric mean of 1.4 ft/dayAverage hydraulic gradient of 0.0014

The following estimated groundwater-flow rates were calculated by using these values:

• Columbia

Range of 0.0069 to 0.090 ft/day (2.5 to 33 feet per year [ft/yr])Geometric mean of 0.032 ft/day (12 ft/yr)

• Upper Potomac

Range of 0.0069 to 0.20 ft/day (2.5 to 73 ft/yr)Geometric mean of 0.054 ft/day (20 ft/yr)

• Lower Potomac

Range of 0.0013 to 0.030 ft/day (0.47 to 11 ft/yr)Geometric mean of 0.0098 ft/day (3.6 ft/yr)

Vertical Interactions. The assumption is that vertical hydraulic interaction exists at thesite, and, therefore, the potential for vertical migration of contaminants exists. There do notappear to be confining layers between water-bearing units that are of sufficiently lowpermeability to prevent migration completely if a hydraulic-head gradient across theconfining layer exists.

5-24

The backfill and organic silt of the former-lagoon sediment lie directly on top of theColumbia water-bearing unit and so are hydraulically connected to the Columbia. Only theorganic silt would be expected to serve as a confining unit between the fill and theColumbia because the fill typically is silt with sand or gravel and blocks of rubble andcinder and the Columbia is fine to coarse sand. The organic silt is rarely more than 2 feetthick and in some places is nonexistent, so it does not represent much of a confining unit.The water levels in wells BMW-10 (screened primarily in the backfill) and SMW-10(screened in the Columbia) generally differ by less than 0.4 feet and more commonly differby less than 0.2 feet. The water levels also vary over time in a similar pattern. Therefore,the assumption is that there is hydraulic connection between the backfill and the Columbiawater-bearing unit.

Data are insufficient for quantitatively defining the hydraulic interconnection between thetidal marsh and lagoon and the Columbia. However, because the tidal-elevation rangetypically is between 1.8 and 4.5 feet msl and water levels in the Columbia generally arebetween 3.0 and 3.5 feet msl, the assumption is that the Columbia likely discharges into thetidal marsh (and, less likely, into the lagoon). The direction of flow may be reversed whenthe tide is at unusually high levels.

The relationship of water levels in the Columbia and the underlying upper Potomacgenerally indicates the potential for groundwater to flow downward between the two units(Table 3-10). Exceptions are upgradient at wells SMW-6 and IMW-6 (west of the site),downgradient at wells W4-C and W-4B and wells SMW-2 and IMW-2 (along the east sideof the tidal marsh), and occasionally at other locations. This suggests that groundwater inthe upper Potomac generally is discharging upward into the Columbia in response to aregion of lower hydraulic head in the unit, probably the tidal marsh and perhaps the lagoon.Although the water-level differences between onsite wells SMW-10 and IMW-10 suggestthe potential for downward flow, the head difference between the two wells typically is lessthan 0.1 foot, indicating that groundwater flow is in the same direction in the two units.

Upward discharge from the upper Potomac to the Columbia probably is enhanced by thefact that the silt layer between the two units under the site appears to pinch out beneath thelagoon and the tidal marsh, leaving the two units hydraulically connected eastward towardthe Christina River (figures 3-3, 3-4, and 3-8).

The silt-and-clay layer confining unit between the upper Potomac and lower Potomac unitsappears to be continuous beneath the site and eastward toward the Christina River. Allavailable water-level measurements in the lower Potomac are lower than those in the upperPotomac, indicating the potential for groundwater flow downward between the two. Thecontinuous nature and thickness of the confining unit (at least 10 to 15 feet) probably limitflow between the two units.

Synthesis. In summary, recharge at the surface of the site infiltrates to the water table inthe shallow sediment and the backfill (Figure 5-1). This infiltration joins groundwatermoving laterally in the shallow sediment and backfill toward the lagoon and the tidal marsh,

5-25

where the shallow groundwater may discharge. Fluctuation of the tidal level modifies therate at which water moves laterally in these units but does not have a significant effect onthe general direction of flow.

Some groundwater leaks downward from the shallow sediment and the backfill into theColumbia water-bearing unit, where it joins groundwater moving laterally toward thelagoon, the tidal marsh, and the Christina River. Some shallow groundwater in theColumbia discharges upward into the surface water in.the tidal marsh and, possibly, thelagoon and the rest flows to the Christina River, discharging upward through the Recentsediment bordering the river. Fluctuation of the tidal level modifies the rate at which watermoves laterally in the unit but does not have a significant effect on the general direction offlow.

In some areas, groundwater in the Columbia unit leaks downward into the upper Potomacunit, whereas in other areas, particularly where the intervening silt-layer confining unit isabsent, it moves upward from the upper Potomac into the Columbia. The groundwater inthe upper Potomac generally moves laterally toward the Christina River, where it dischargesupward through the adjacent Recent sediment into the river.

A limited amount of groundwater may leak downward from the upper Potomac into thelower Potomac. This leakage would likely join groundwater already moving south beneaththe area as it flows toward pumping centers off the site.

5.3.3.2 General Conceptual Model of Contaminant Migration

Contaminant-Migration Rates. The migration velocities of dissolved contaminants rangewidely for different chemicals. The velocities of the contaminants are controlled by twophysical characteristics of the water-bearing units, besides the geochemical factors discussedearlier: advective transport and dispersion. The estimated rates of advective flow arediscussed in subsection 5.3.3.1. Dispersion occurs in moving groundwater because of localvariations in flow velocities caused by the variability of the hydraulic conductivity of theporous media. Typically, the degree of dispersion is greater in the direction of water flowthan in directions perpendicular to it. The concentrations of the chemicals at the center ofthe contaminant plume will decrease as dispersion dilutes the contaminant mass.

The contaminant velocities are controlled largely by the degree of adsorption of thecontaminant. For each contaminant discussed in this chapter, it is possible to calculate aretardation coefficient, an estimate of the degree to which the contaminant is slowed inrelation to the groundwater-flow velocity by adsorption. However, such site-relatedchemicals as ammonia are adsorbed only to a small degree, and the uncertainty inretardation coefficients, particularly for metals, is high. Therefore, only the travel times ofa conservative (i.e., nonadsorbed) chemical in the Columbia and upper Potomac water-bearing units from the eastern edge of the site (i.e., along the tidal marsh) northeast to theChristina River were estimated.

5-26

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A travel distance of 1,500 feet was assumed. Using the velocity data estimated insubsection 5.3.3.1, a nonadsorbed chemical would require from 45 to 600 years (geometricmean 125 years) in the Columbia and 21 to 600 years (geometric mean 70 years) in theupper Potomac to reach the river. Therefore, it is highly likely that site-related chemicalshave affected groundwater quality in the study area east of the site. However, it is unlikelythat even a nonadsorbed chemical from the site has reached the river, and thereforecontamination in wells SMW-12, IMW-12, W5-A, and W5-B probably is derived from thepetroleum coke piles or other operations east of the site.

Upgradient Groundwater. Groundwater flowing laterally under the site from upgradientcontains very low concentrations of some of the chemicals defined in Chapter 4 as beingsite-related. Site-related contaminants found in upgradient wells at low concentrationsinclude sulfide and ammonia in the Columbia unit and ammonia in the upper Potomac andlower Potomac. An exception is the thiocyanate and cyanide detected in upgradient wellSMW-6; this contamination probably was drawn into the vicinity of SMW-6 when off sitewells were pumping in the past, although an unidentified source of thiocyanate may haveexisted west of the site.

Onsite Groundwater. Water infiltrating from the surface leaches contaminants from thesoil at the site and carries them into the shallow groundwater. Because the shallow watertable is within the backfill and the shallow sediment, groundwater also is directly in contactwith contaminated materials and leaches contaminants directly from the materials. Theleaching results in elevated levels of various site-related contaminants in the backfillgroundwater, including arsenic, manganese, thiocyanate, sulfide, and ammonia. Thepotential for this to occur is shown by the mass-balance calculations discussed insubsection 5.3.1.

Contaminated groundwater in the backfill then migrates laterally into the lagoon andperhaps into the tidal marsh and downward into the underlying Columbia water-bearingunit. The organic material at the base of the backfill, when present, would be expected toretain readily adsorbed contaminants, such as metals and SVOCs, depending on such factorsas valence state. The organic material then may serve as a long-term source for retainedcontaminants, along with contaminants contained in the overlying soil. The presence ofhigh levels of arsenic and manganese (both of which are highly mobile under reducingconditions) in the underlying Columbia indicates that these metals have migrated throughthe organic material.

Site-related contaminants, such as arsenic, manganese, thiocyanate, cyanide, sulfide, andammonia, were detected in the Columbia. The organic-carbon content of the Columbiagroundwater is high (Table 5-3), suggesting reducing conditions in this unit and highmobility for these chemicals. Once in the Columbia, the contamination migrates laterallytoward the lagoon and the tidal marsh. Contamination in the upper part of the Columbiamay discharge upward into the tidal marsh (and, less likely, into the lagoon).Contamination in the deeper parts of the unit continues laterally toward the Christina River.

5-29

AR302M I

During this long period of migration, degradation reduces the concentrations of all VOCs,such as carbon disulfide, when they are in a dissolved phase.

During the RI, water samples from the lagoon were analyzed. The samples, especially theone obtained from the southeastern end of the lagoon, nearest the former process plant area,typically contained elevated levels of arsenic, manganese, thiocyanate, cyanide, sulfate, andcarbon disulfide, all of which were detected in the Columbia. The presence of thesechemicals, all of which can be attributed to the site, indicate that contaminants from the sitemigrate both downward into the Columbia and laterally into the lagoon. The levels ofsodium, chloride, and sulfate detected in the surface water in the lagoon and in shallowgroundwater east of the site are much higher than the concentrations of these chemicals inwater from the Christina River, indicating an effect of the site or other sources (e.g., the saltpiles) on surface water and groundwater.

Some contaminated groundwater in the Columbia also migrates downward into the upperPotomac. The migration is indicated by the presence of contamination attributed to the site,such as cyanide, thiocyanate, and ammonia, in onsite wells in the upper Potomac. Reducingconditions continue in the upper Potomac groundwater (Table 4-4).

Although low concentrations of some site-related contaminants such as ammonia andthiocyanate were detected in the lower Potomac, it is unlikely that these contaminants havemigrated vertically downward through the various confining units into this unit. The factthat the hydraulic head drops continuously with depth over most of the site favors themigration of contaminants downward from the surface to the lower Potomac. However, thedegree to which the upper and lower Potomac units interact has not been established andlikely is limited.

Near the former process plant area, carbon disulfide has entered the Columbia, possibly ina DNAPL form, although no direct evidence has been found other than in excavations onthe site. Very high concentrations of carbon disulfide have been detected near well SMW-1and in the process plant drainage ditch. The chemical also has been detected in theunderlying upper Potomac. If a DNAPL mass exists in the subsurface at the site, it wouldmigrate slowly downward under the influence of gravity and independent of advectivetransport. Once it reached low-permeability layers, it would spread laterally until furthermigration is stopped or another vertical passage is encountered.

The silt-layer confining unit between the Columbia and the upper Potomac appears to belargely intact underneath the site, although the presence of carbon disulfide in the upperPotomac suggests some migration vertically downward from the Columbia. A DNAPLmass of carbon disulfide migrating downward from the site (its source being the formerprocess plant area or nearby) would encounter this confining unit and migrate laterally ontop of it. The top of the layer varies in elevation but appears to slope generally to thenortheast (figures 3-4 and 3-6). The silt-layer confining unit also appears to end near thetidal marsh (Figure 3-8). Therefore, a DNAPL mass, if it exists, would be expected tomigrate along the top of the confining unit toward the northeast until it reaches the edge of

5-30

AR302M2

the unit, then migrate downward through the upper Potomac to the confining layer betweenthe two Potomac units. No direct evidence of a DNAPL in the groundwater has beenfound, only concentrations in wells up to 15 percent of the solubility of the chemical.However, droplets of carbon disulfide were observed in excavations on the site.

Downgradient Groundwater. Shallow groundwater in the Columbia unit migratingdowngradient of the site may discharge into the tidal marsh and perhaps into the lagoon.Deeper groundwater in the Columbia and groundwater in the upper Potomac will flownortheastward toward discharge along the Christina River. Groundwater in the lowerPotomac moves southward beneath the site toward pumping-discharge locations.

Concentrations of arsenic, cyanide, and thiocyanate detected in wells along the tidal marshlikely have been derived from the site. The petroleum coke piles east of the site also arepossible sources. Inorganic contamination in wells along Lobdell Canal and the ChristinaRiver probably is from the petroleum coke piles or other sources in their vicinity becausethe travel distances from the site to these wells are long in relation to the expectedmigration rates of site-related contamination, and elevated levels of contaminants are notdetected often in intervening wells. The nature of the contamination also is consistent withpetroleum coke.

The predominant organic contamination downgradient is from chlorinated solvents andBTEX compounds. In particular, vinyl chloride was detected in well W4-C, and toluenewas detected in well SMW-9. Most detections that were not accompanied by blankcontamination were from wells along the tidal marsh. The chlorinated VOC contaminationprobably is from site activities; toluene contamination cannot be attributed to the site withcertainty. Vinyl chloride and 1,1-DCE probably are degradation products of the TCE andPCE detected on the site.

5.3.4 Releases to Surface Water

Contamination in surface soil at the site is free to be transported to the lagoon and the tidalmarsh because little of the site is paved. This transport is by wind erosion and surface-water flow. This load usually is deposited under oxidizing conditions and, therefore,transported chemicals may be oxidized if the transport times are long enough.

Chemicals may be transported into surface water by groundwater. This probably isoccurring from the backfill at the site and from the upper part of the Columbia water-bearing unit into the tidal marsh. Groundwater also may discharge from the Columbia intothe onsite lagoon.

Until recently, much of the contaminated surface water in the lagoon was flushed out twicedaily by the tide through the 1-495 drainage ditch and ultimately into the Christina River.Contaminated surface water in the lagoon also may flow out through the partially crushedculvert beneath the railroad tracks on the northeast side of the facility and into the tidal

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AR302lf 13

marsh. Historical information indicates that other pathways for surface-water flow beneaththe railroad tracks may still exist. The historical source of contamination of surface waterin the tidal marsh was discharge from the onsite lagoon through the pipes beneath therailroad tracks. Other discharges directly from the plant to the tidal marsh may haveexisted. The discharges from the plant caused contamination to be especially significant inthe process plant drainage ditch, the southern reaches of the onsite lagoon, and the northernand central parts of the tidal marsh. The tidal marsh undergoes tidal flushing twice daily.

The depositional environment of chemicals in the sediment of the lagoon is reducingbecause of the amount of decaying vegetation and other chemicals that are undergoingdegradation. The sediment of the tidal marsh is reducing, also primarily because ofdecaying vegetation. These environments will reduce deposited chemicals, which then mayhave long residence times because the sediment is not stirred up often enough to releasechemicals.

5.4 Summary

The physical and chemical properties of the chemicals in the environmental media, thegroundwater and surface-water hydrology, and the potential migration pathways werereviewed to develop a conceptual model of contaminant fate and transport at the site. Thefollowing conclusions were reached:

• Surface and subsurface soil, particularly in the process plant drainage ditch,has been contaminated by discharges from the plant.

• Contamination has leached out of the backfill and the former lagoonsediment at the site into underlying groundwater. The leaching probablycontinues.

• Contamination has been carried with surface runoff into the local surface-water bodies, primarily the onsite lagoon. The process presumably continues.Contamination in the onsite lagoon has entered the tidal marsh,contaminating both surface water and sediment. Surface contamination maybe entrained by wind.

• Groundwater contamination in the backfill migrates both into the onsitelagoon and downward into the Columbia water-bearing unit. Shallow-groundwater contamination in the upper part of the Columbia probablydischarges upward into the tidal marsh (and, less likely, the onsite lagoon)during low tide.

• Groundwater contamination in the Columbia has contributed to toxicity at thelocation of the Bio-7 sampling station in the tidal marsh. Discharge of

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AR302UU

contaminated surface water from the onsite lagoon into the tidal marsh hasalso contributed to contamination at Bio-7. The contamination probably didnot migrate through the subsurface soil from the site.

Site-related groundwater contamination in the Columbia migrates to thenortheast in the direction of the Christina River.

Site-related groundwater contamination .in the upper Potomac primarilymigrates to the northeast in the direction of the Christina River. Somecontamination may migrate downward into the lower Potomac water-bearingunit.

Site-related groundwater contamination in the lower Potomac migrates to thesouth or southeast under the site, toward pumping centers a few miles away.There appears to be little site-related contamination in this unit, but thedirection of the hydraulic gradient in the lower Potomac is conducive tomigration of site-related contamination downward from the upper Potomacinto the lower Potomac and then back under the site as groundwater flowstoward the south.

A DNAPL mass of carbon disulfide probably exists in the Columbia nearwell SMW-1. If it exists, the mass serves as a source of contamination ofgroundwater downgradient in the Columbia and upper Potomac water-bearingunits.

The pattern of groundwater contamination in the Columbia and the upperPotomac suggests that the former process plant area and other parts of thesite have been and continue to be sources of contamination, particularly forcarbon disulfide, ammonia, arsenic, manganese, and thiocyanate. Thecontamination has migrated downgradient at least as far as the east side ofthe tidal marsh.

The petroleum coke piles and perhaps other facilities east of the tidal marshprobably have served as sources of groundwater and surface-watercontamination along the tidal marsh and east of it. The petroleum coke pilesare potential source materials for such contaminants as cyanide, thiocyanate,ammonia, and various metals. Contamination patterns along the ChristinaRiver are consistent with a source near the river.

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5.5 References

Dean, John A., ed. Lange's Handbook of Chemistry. McGraw-Hill Book Company. 1973.

Dragun, James. The Soil Chemistry of Hazardous Materials. Hazardous Materials ControlResearch Institute. 1988.

EBASCO. Final Remedial Investigation Report, Halby Chemical Site; New Castle County,Delaware. USEPA Work Assignment No. 119-3LL7. 1990.

Howard, Philip H., et al. Handbook of Environmental Degradation Rates. LewisPublishers. 1991.

HSDB. USEPA's Hazardous Substance Data Base. 1994.

Lagas, P., J.P.G. Loch, and K. Harmsen. The Behavior of Cyanide in a Landfill and theSoil Beneath It. In Effects of Waste Disposal on Groundwater and Surface Water.Proceedings of the Exeter Symposium. LAHS Publication no. 139. July 1982.

Luthy, R.G., S.G. Brace, Jr., R.W. Walters, and D.V. Nakles. Cyanide and Thiocyanate inCoal Gasification Wastewaters. Journal of the Water Pollution Control Federation, vol. 51,no. 9, pages 2267 to 2277. 1979.

Montgomery, John H., and Linda M. Welkom. Groundwater Chemicals Desk Reference.Lewis Publishers. 1990.

Summers, K., Steve Gherini, and Carl Chen. Methodology to Evaluate the Potential forGroundwater Contamination from Geothermal Fluid Releases. USEPA EPA-600/7-80-117.1980.

Taylor, C.H., S.E. Kesler, and P.L. Cloke. Sulfur Gases Produced by Decomposition ofSulfide Minerals: Application to Geochemical Exploration. Journal of GeochemicalExploration, vol. 17, pages 165 to 185. 1982.

USEPA. Subsurface Contamination Reference Guide. EPA/540/2-90/011. U.S.Environmental Protection Agency, Office of Emergency and Remedial Response. 1988.

Witco Corporation. Progress Report, 2/10/96 through 2/23/96, Response Action PlanImplementation. February 23, 1996.

WDCR1026/006.WP5

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oQ)•a(DO)

i

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AR3Q2M7

Chapter 6Baseline Human-Health Risk Assessment

6.1 Introduction

6.1.1 Background and Objectives

This section briefly describes the Halby Chemical site and the objectives of the risk assessment.The details of the site's setting, operational history, hydrogeology, and nature and extent ofcontamination are presented in other chapters of the report.

The Halby Chemical site, encompassing 14 acres, is in an industrial area in southeasternWilmington, New Castle County, Delaware. The triangular property is bordered by Interstate 495on the northwest, a Conrail railroad track on the east, and Terminal Avenue on the southwest.Figure 1-3 illustrates the site setting and features of interest, including a lagoon on the site, adrainage ditch, and a tidal marsh nearby off the site that runs along the railroad track. The tidalmarsh is connected by Lobdell Canal to the Christina River, which flows to the Delaware River.

The site was used as a chemical-manufacturing facility by Halby Chemical Company and itssuccessor, Witco Chemical Company, from 1948 to 1977. Among the products manufacturedwere ammonium thioglycolate, ammonium thiocyanate, isooctyl thioglycolate, and other sulfurcompounds. During the early years of operation, liquid wastes, including cooling water and acidwastewater, were disposed of in an unlined lagoon on the site. Previous investigations haveindicated significant contamination of soil, surface water, sediment, and groundwater.

In 1977, Brandy wine Chemical Company bought the property and is using it as a storage anddistribution facility.

The site and its vicinity are zoned for industrial use. The site has been used primarily for heavyindustry. Historical data suggest that the site was one of the earliest operating facilities in theport area. Various heavy industries have operated in the area, as noted in Chapter 1. The partsof the site and study area that are undeveloped have always been so. Much of the developed areanear the site was claimed from wetlands, which dominated the area before the 1940s. 1-495 wasconstructed during the 1960s and changed land use north of the site.

Two small residential communities are within 1 mile of the site. The entire city of Wilmingtonis within 3 miles of the site and has a population of 71,529 (U.S. Department of Commerce,1992). The site contains three small trailer residences. Two of the trailer residences wereobserved to be occupied during site activities. The resident population of the site is two and mayvary seasonally. The residents are on the site legally because their residential status predates thezoning of the area as industrial.

The purpose of the baseline risk assessment is to characterize risks to human health from the site.The assessment uses information from earlier assessments, including the Preliminary Health

6-1AR3Q2M8

Assessment for Halby Chemical by ATSDR (ATSDR, January 1989), Public Health Evaluation,Halby Chemical Company Site (EBASCO, September 1990b), and internal USEPA reviews inFebruary and June 1991 (USEPA, Region III, 199la, 1991b). The earlier assessments identifiedelevated concentrations of a number of COPCs and associated cancer and noncancer risks. Therisk assessment incorporates additional data on soil, surface water, and groundwater collected in1993; data on fish and sediment collected in 1994; and soil data collected in 1995 underUSEPA's expedited removal action.

The purpose of the assessment is to identify the sources and pathways that are likely to contributeto elevated risks to human health from current site use and future use as industrial for theexisting industrial parcel and residential for the existing residential parcel. The results of the riskanalysis provide guidance for decisions on remedial action at the site: for either a no-actionalternative or for the development of risk-based remediation goals in combination with otherregulatory standards and criteria.

The original work plan (CH2M HILL, 1993) called for both quantitative and qualitativeevaluations. The qualitative evaluation included screening maximum concentrations for amedium against RBCs. In many cases in the qualitative evaluation, contaminants were detectedat levels above the RBC. The contaminants that exceeded the RBC screening subsequently wereevaluated quantitatively. Fish was the only medium that was evaluated only qualitatively becauseno contaminants were detected above the RBC screen.

6.1.2 Risk Assessment Approach

This risk assessment is based primarily on USEPA guidance, which includes the following:

• Risk Assessment Guidance for Superfund, Volume 1, Human Health EvaluationManual, Part A, Interim Final (USEPA, 1989a)

• Exposure Factors Handbook (USEPA, 1995b)

• Human Health Evaluation Manual, Supplemental Guidance: Standard DefaultExposure Factors (USEPA, 1991b)

• Risk Assessment Guidance for Superfund, Volume 1, Human Health EvaluationManual, Part B (USEPA, 1991c)

• Dermal Exposure Assessment: Principles and Applications (USEPA, 1992a)

• Selecting Exposure Routes and Contaminants of Concern by Risk-Based Screening(USEPA Region III, 1993)

In accordance with USEPA Region III guidance, the assessment ranks and evaluates contaminantsand exposures by a two-step process: (1) initial screening of detected contaminants in eachmedium compared with RBCs and (2) quantitative evaluation of COPCs and associated risks.

6-2AR302U9

The RBCs are calculated by using a hazard quotient of 0.1 or a lifetime excess cancer risk of10"6, whichever results in a lower concentration.

As discussed in section 6.3, "Exposure Assessment," the assessment uses a combination of site-specific observations, professional judgment, and default factors to derive reasonable maximumexposure (RME). The multiple high-end values (e.g., 90th percentile) are far more likely to

Si overstate rather than understate potential exposures. Exposures and risks are quantified for„ current industrial site use and future residential use. Toxicity parameters for COPCs are obtained

)i '• TronTuSEPA's Integrated Risk information System (IRIS), Health Effects Assessment Summary-:f Table (HEAST), or the Superfund Health Risk Technical Support Center (USEPA National

' ;A Center for Exposure Assessment [NCEA]).

The risk assessment is based on the following assumptions:

• No further remedial action will take place at the site.

• The land use could change from industrial to residential. t

• 6.6—Uncertainty Analysis. Major sources of uncertainty, key assumptions, andthe influence of both on risk estimates are outlined.

• 6.7—Summary. Key findings, including major risk drivers, are highlighted toassist in making decisions on how to manage risk.

• 6.8—References.

6.2 Identification of COPCs

The data sources and selection of COPCs are discussed in this section. A chemical detected inthe media at the site is assumed to be a COPC until its risk contribution is quantified, at whichtime it is either eliminated from concern or designated a chemical of concern (COC). Detectedchemical concentrations in the different media at the site—surface and subsurface soil,groundwater, surface water, and sediment—are screened by comparing with corresponding RBCs(i.e., hazard quotient = 0.1 or carcinogenic risk = IxlO"6). The purpose of the screeningprocedure is to identify the chemicals (carcinogens and noncarcinogens), by each medium orpathway, that are most likely to contribute to health risks because of their toxicity or abundance.The data set is reduced further if site concentrations are within the background range, or if thedetected chemicals are essential human nutrients and no toxicity values are available. Thereduction in the number of chemicals makes the quantitative risk assessment more manageableand focuses attention on chemicals that contribute the most risk.

6.2.1 Data Sources

Data collection, validation, and management are discussed in Chapter 2, "Study-AreaInvestigation." The media samples have been analyzed for the suite of chemicals from the TCLand the TAL, and for other parameters, including thiocyanate, weak-acid dissociable cyanide,ammonia, hexavalent chromium, chloride, sulfide, sulfate, and nitrate. The samples have beenvalidated using USEPA protocols.

The first step in data analysis is to combine all relevant and available data for each medium.Table 6-1 lists the date of sampling, the locations, and the number of samples for each medium.Surface soil data used in the risk assessment include the results from samples from the top3 inches, and subsurface data include the results from all the other samples from 3 inches to16 feet. The risk assessment for the soil includes samples that were collected from all parts ofthe site, including the former process plant area and the process plant drainage ditch. Thesamples from the process plant drainage ditch originally were identified by EBASCO in 1990 assediment samples, but the area where the samples were collected is above the low tide for theditch; therefore, the samples were evaluated as soil.

Table 6-1SAMPLING SUMMARYHalby Chemical Site

MediumDate ofSampling Number of Locations and Samples

SampleLocations,BackgroundLocations

SoilSurface Soil(0 to 3 in.and 3 in. to16ft)

SubsurfaceSoil (3 in to16ft)

May-June1993

May-June1993June 1995

119 locations,134 samples(5 duplicates)

97 locations,110 samples(3 duplicates)

SB-02, -06, -13, -16, -18, HCS-03,HAS-7A, -8A, SED-01, -02A, -03A, -26A, SSS-02, SSS-09 to SSS-13, SSS-17to SSS-20, and all samples listed forsubsurface soilHCS-1, -2, HCS-4 to HCS-18, HCS-20,HCS-21, HCS-23 to HCS-25, HAS-1A,-IB, -1C, -2A, -2B, -3A, -3B, -4A, -4B,-5A, -5B, -10BG, -6A, -6B, -6D (andduplicate), -7B, -8B, -9, SB-01 to SB-20(multiple depths) (duplicates of SB-06 andSB- 16), IMW-08, SB-02A, -02B, -02C,SED-02B, -03B, -03C, -26C, SSS-01,-03A, SSS-04 to SSS-06, SSS-14 toSSS-16, SSS-21A to SSS-21D, SSS-22Ato SSS-22D, SSS-23A to SSS-23E,SSS-24A to SSS-24D, SSS-25A toSSS-25E

DNREC,1996

DNREC,1996

Surface WaterOnsite

Offsite

Oct.-Nov.1988, Aug.and Dec.1993Oct.-Nov.1988, Aug.and Dec.1993

6 locations,8 samples

6 locations,7 samples

SW-01 to SW-03, SW-05, C-01, C-10

SW-06 to SW-10, C-02

SW-11

SW-11

SedimentOnsite

Offsite

Nov. 1988,Sept. 1989

Nov. 1988,Sept. 1989

15 locations,16 samples(1 duplicate)

23 locations,25 samples(2 duplicates)

EPA-3, SED-04, SED-05A, SED-05B,SED-06, SED-07, SED-08A, SED-08B,SED-09, SED-27, WES-15016, -15017,15018, T-G-1-01, T-G-1-03EPA-2, -4 to -9, EPA-11, SED-10A,SED-10B, SED-12A, SED 12B, SED-13to SED- 16, SED-20, -21, -24, -25,WES-15015, T-G-1-02, T-G-1-04(duplicate of T-G- 1-02)

SED-22

SED-22

6-5

Table 6-1SAMPLING SUMMARYHalby Chemical Site

MediumDate ofSampling Number of Locations and Samples

SampleLocations,BackgroundLocations

Groundwater (Onsite)

Columbia

UpperPotomac

LowerPotomac

Aug. andDec. 1993(2 rounds)Aug. andDec. 1993(2 rounds)Aug. andDec. 1993(2 rounds)

4 locations,9 samples(1 duplicate)3 locations,7 samples(1 duplicate)1 locations,3 samples(1 duplicate)

SMW-01, -08 (and duplicate), -10,BMW- 10

IMW-01, -08 (and duplicate), -10

DMW-01

SMW-06

IMW-06

DMW-05

Groundwater (Offsite)Columbia

UpperPotomac

LowerPotomac

Fish'

Aug. andDec. 1993(2 rounds)Aug. andDec. 1993(2 rounds)Aug. andDec., 1993(2 rounds)Nov. 1994

6 locations,12 samples

7 locations,14 samples

1 location,3 samples(1 duplicate)Lagoon, 3samples

SMW-2, -9, -11, -12, W4-C, MW-04C,W5-B, MW-05B

IMW-2, -3, -9, -11, -12, W4-B, MW-04B,W5-A, MW-05A

DMW-7

Onsite Lagoon2

SMW-06

IMW-06

DMW-05

UpstreamChristinaRiver3

Note: Background samples have been taken for all of the above media except surface soil for which noappropriate background was found.'Fish were filet samples.2Fish samples collected have different sample identifiers for each analytical method.'Background location for fish sampling was a wetland north of the Halby Ch