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Transcript of Remediation. Topics Covered zClean up standards zSource Control zRisk Assessment zNatural...
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Remediation
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Topics Covered
Clean up standardsSource ControlRisk AssessmentNatural AttenuationPump and TreatLimitations to Pump and TreatAlternatives
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Introduction
Contaminated soil and GW most prevalent problems at HW sites
VOCs/metals most prevalent problem at NPL sites
SVE most widely used innovative techNPL Status ~ 1100 on NPLRCRA - 3000 sites $7.4-41.8 billion. primarily
corrosive/ignitable wastes, HM, org solvents, oil
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General - Cont’d
UST - 161,000, 91% pet, 2% hazardous materials
DOD - 9300 potential petroleum, solvents, metals, pesticides, paints
DOE - 10,500 areas, VOCs and mixed wastes
700 Other federal sites30,000 state sites
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Site Cleanup
Source Control Source removal and disposal Source containment Institutional control
Groundwater/soils cleanup
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Groundwater Contaminant Cleanup
Physical containmentHydraulic containment/treatment (pump
and treat) pump to isolate contaminant zone
Required as long as source exists to meet cleanup standards
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Typical Standards - Numerical Cleanup Criteria
Meet requirements of existing laws, or
Protective of human health or ecological receptors, or
Background concentration, orAnalytical limits
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Fallacy of Numerical Clean up Criteria
Not the only way to protect (paving, capping, fencing)
treatment may reduce some contaminants but not all
May be over protective and too costly
May not be feasible
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Technical Impracticability
Study recognized that pump and treat cannot restore all sites
Recommendations Categorize sites increase incentive to use innovative
technology use experts to determine technical
feasibility charge fee at these sites to fund research
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Relative of Remediation
HydrogeologyContaminant Chemistry
Mobile, Dissolved
Strongly sorbed, dissolved
LNAPL DNAPL
Homogeneous, single layer 1 2 2-3 3
Homogeneous, multiple layers 1 2 2-3 3
Heterogeneous, single layer 2 3 3 4
Heterogeneous, multiple layers 2 3 3 4
Fractured Bedrock 3 3 4 4
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Risk Based Corrective Action
ASTM standards for conducting risk based assessment for corrective action
Risk Assessment Primer
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Risk Assessment
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Topics Covered
DefinitionsRisk Assessment ProcessUncertainty
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Definitions
Risk is the probability that a specific negative outcome will occur
Safety is the complement of risk, or the probability that an adverse effect will not occur
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Risk Values
Activity Annual RiskSmoking 10 cigarettes/day 1 x 10-3
Motor vehicle accidents 2 x 10-4
Manufacturing work accident 8 x 10-5
Pedestrian hit by automobiles 4 x 10-5
Drinking two beers/day 4 x 10-5
Person in a room with a smoker 1 x 10-5
Peanut butter (4 teaspoons/d) 8 x 10-6
Drinking water with EPA limit of Trichloroethene 2 x 10-9
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Factors in Risk Acceptability
Voluntary vs. NonvoluntaryDegree of controlMagnitude of the outcomeAwarenessCatastrophic PotentialGroup involvementCost of alternatives
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Process
H a z a r d
D a t a
I d e n t i f i c a t io n
D o s e - R e s p o n s e
R is k M a n a g e m e n t
W h a t a g e n t s a r e p o t e n t i a l l y h a r m f u l ?
W h o w i l l b e e x p o s e d t ow h a t a n d f o r h o w lo n g ?
H o w i s in t a k e o r d o s er e la t e d t o a d v e r s e e f f e c t s ?
W h a t e f f e c t s a r e l ik e l y o n h u m a n h e a l th a n d e n v i r o n m e n t?
Exposure
Assessment
Risk
Assessment
Characteristics
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Hazard Identification
Toxicity assessment determines whether exposure to a chemical, physical, or biological agent can cause an increase in the incidence of an adverse effect.
Necessary condition for a health or safety riskPhysical, metabolic, and chemical properties of
the agent; Potential routes of exposure; toxicological
effects; results of animal studies; and site characteristics
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Dose-Response Assessment
Relationship between the level of exposure and the extent of injury
Carcinogenic and non-carcinogenic effects
Responses vary from death to tumors, skin irritation, respiratory effects, genetic mutation, and fetal development problems
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Dose-Response Curve
Response
Dose
X
X
X X
No Threshold, Linearat Low Doses
Slope = Cancer Slope Factor
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Dose-Response CurveD
ose
Response
XX
XX
XRfD
NOAEL Threshold
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Noncarcinogenic Reference Dose
SF
NOELRfd
Rfd: mg/kg-d
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Safety Factor
Intrahuman variationsExtrapolation from animalssubchronic to chronicLOAEL to NOELIncomplete database
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Exposure Assessment
Calculates the dose which an exposed individual receives
Delineates the affected population by identifying possible exposure paths
Exposure routes ingestion inhalation dermal absorption
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Dose
Daily Dose (mg/kg-day) = (C)(I)(EF)(ED)(AF) (AT)(BW)C = Concentration, mass/volumeI = Intake Rate, volume/timeEF = Exposure Frequency, time/timeED = Exposure Duration, timeA = Absorption Factor, unitlessAT = Averaging Time, timeBW = Body weight, mass
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Typical Exposure and Intake Rates for Risk Assessment
Variable Value
Soil Ingestion Rate 200 mg/day (children 1 through 6 years old)100 mg/day (age groups greater than 6 years old)
Exposure Duration Adult - 30 years at one residence (national 90thpercentile); Child - 6 years
Body Weight 70 kg adult; 10 kg child
Averaging Time noncarcinogens = exposure durationcarcinogens, 70 years
Water Ingestion Rate Adult - 2 L/d; Child (1-6) 1 L/d
Inhalation RateAdults 0.83 m3/hr; Child 0.46 m3/hr
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Risk Characterization
Risk = Toxicity x Exposure
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Risk Characterization
Probability of adverse incidence occurring under the conditions identified during the exposure assessment.
Carcinogens, the daily dose is multiplied by the CSF Risks are additive for multiple
carcinogenic contaminants.
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Risk Characterization
Non-carcinogens, Hazard index (HI) is calculated by
dividing the daily dose by the RfD Where more than one contaminant is
present a hazard quotient (HQ) is determined by summing all of the HIs
A HI over one indicated an unacceptable risk
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Risk Management
Regulatory actionDecision to mitigate riskAction level
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Uncertainty
The risk assessment process is extremely conservative in nature and utilizes measurements which are uncertain.
insufficient data or information gaps often exist in characterizing the potential risk of an agent, necessitating the need for assumptions or educated guesses.
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Uncertainty
Use computational tools from the field of decision analysis to account for the uncertainties in the process
These tools allow risk to be expressed as a probability distribution rather than a single number which can then be used to make a more informed decision during risk management
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Natural Attenuation
Process whereby concentration and areal extent of contaminant plume is maintained or reduced over time by natural processes
Monitoring onlyNo physical encouragement of processUsually petroleum hydrocarbons
(Oregon permitted PCP to be naturally attenuated)
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NA Considerations
Is the plume increasing, decreasing, or stable?
Has the source been removed? Is there free product?Will the plume affect off site properties?How long will the plume persist?Receptors?What mechanism(s) are controlling plume?
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Mechanisms
Biological processesDispersion/dilutionSorptionVolatilizationChemical transformation
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NA Verification
Plume characterization (delineation)Mass declines over timedecreasing concentration over timePlume stable or decreasing with timePlume movement slower than
predicted
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NA Verification
Geochemical changesLaboratory microcosmsGW modelingBreakdown productsLoss of electron donors/receptors
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NA of Chlorinated Solvent Plumes - Anaerobic systems
Due to biodegradation of anthropogenic organic carbon
drives reductive dechlorinationstrongly reducing conditionsH2 > 5 nanomolarRapid and extensive dechlorination
of PCE, TCE, TCA, and CT
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NA of Chlorinated Solvent Plumes - Anaerobic Systems
Anaerobic conditions result from naturally occurring organic carbon
Coastal or stream/river deposits, shallow aquifers (swamps), natural oil seeps
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NA of Chlorinated Solvent Plumes - Aerobic Systems
Well oxygenated, little organic carbon
no degradation of PCE, TCE; only VC and DCE
advection, dispersion , and sorption
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NA of Chlorinated Solvent Plumes - Mixed Environments
Favors NASequential anoxic-oxic conditions
beneficial for complete degradation of chlorinated solvents
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Pump and Treat System
Location of plume and/or NAPLDesign of capture system
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Design of P/T System
mass reduction occurs when extraction and injection wells positioned in most concentrated portion
minimize distance that contaminant plume pulled to extraction well
design to produce convergent flow toward central extraction location
minimize divergent flow along periphery
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Pump and Treat Systems
Installation of wells/monitoring pointsPumping to contain/remove contaminantTreatment of extracted waterRecharge to subsurface, treatment facility,
or surface water
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Appropriate Site Characteristics
high hydraulic conductivityadequate possible flow rates to create
sufficient capture zonemobile contaminant
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Inappropriate Sites
heterogeneous aquiferlow hydraulic conductivitysorbed or precipitated contaminant,
slowly desorbing, dissolvingimmobile NAPLs contributing to miscible
plume (residual saturation in aquifer)
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Limitations
The need to reach drinking water standards (ppb at times) exacerbates physical problems
Asymptotic behavior
Con
cen t
rati
on
Time
Flushing from large pores
Flushing from smaller pores
Interrupted pumping, additional desorption
Drinking Water Standard
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Limitation - Cont’d
Sorbed compoundsKinetic limits to desorptionEffects of geologic complexity immobile water zonesEffects of fugitive NAPLDesign failure to contain plumeOperational failures
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EnhancementPulsed pumpingsurfactants - increase mobility of contaminant
(more following)Couple P/T with impervious or hydraulic
barriers (injection wells). Impervious more effective but more intrusive.
steam injection and vapor extractionPneumatic or hydrofracturing to increase
permeabilitysolvent mobilization
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Surfactants
enhancement of DNAPL dissolution in water and/or reduction of interfacial tension and decrease capillary forces
Surfactant qualities nontoxic biodegradable low cost decrease interfacial tension mix or emulsify in water
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Surfactant Flushing Technique
inject aquatic/surfactant solution through wells along perimeter of site
recovery well in center of site separate aquatic solution/DNAPL in
treatment tank NAPL disposal, solution reinjection
Field tests reduce interfacial tension from 19.8 dynes/cm to 2-
5 dynes/cm, at 2% concentration. DNAPL removal ranged from 9 to 82 % depending on surfactant
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Alternatives to Pump and Treat for DNAPLs
Conventional bioremediationSequenced anaerobic/aerobic
biodegradationmethanotrophic bacteria air sparging/SVE
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Alternatives to Pump and Treat for DNAPLs
metal catalysts in permeable wallPhytoremediationElectrokineticsInjection of steamIn situ electrodes