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5. GROUNDWATER GEOCHEMISTRY As part of this study, the chemistry of groundwater in the El Toro Planning Area was evaluated to help provide a general understanding of groundwater occurrence and movement, and to assist with assessment of the quality and quantity of available groundwater resources in the study area. This evaluation included the following tasks: Compilation and evaluation of available groundwater chemical data on file for wells in

the El Toro Planning Area. Sampling of groundwater at select wells and analysis for major ion chemistry, stable

isotopes, and other groundwater quality indicators including arsenic and nitrate Available groundwater chemistry data were evaluated to help identify general water quality trends and the nature of their distributions within the study area. Groundwater sampling was conducted to evaluate water quality trends or influences more specifically (e.g. by geologic formation). 5.1 Groundwater Sampling and Analysis One of the objectives of groundwater sampling was to evaluate potential hydraulic communication between geologic formations and the degree of mixing between aquifers. In order to evaluate groundwater mixing, groundwater chemistry was compared between samples collected from different geologic formations. Many wells in the El Toro Planning Area are screened across two or more different geologic formations, but wells that appeared to be screened in a single formation (or in two adjacent formations for mixing evaluation) were targeted for sampling and chemical analyses for this study. Available information, including screen interval and formation--as determined from drillers logs, cross-sections, geologic mapping, and other available data--was used to select wells for sampling. Water samples were collected from a total of 25 wells and analyzed by the Monterey County Health Department Consolidated Chemistry Laboratory using the following methods: Major anions: calcium, magnesium, sodium, and potassium by SM3111B Major cations: chloride and sulfate by EPA Method 300 and total alkalinity by

SM2320B Arsenic and cadmium by EPA 200.8 Nitrate by EPA method 300 Conductivity by EPA 120.1 pH by EPA 150.1

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In addition, samples for stable isotope analyses were sent to Zymax Laboratories in San Luis Obispo, California, and analyzed for isotopes of oxygen (16O/18O) and deuterium/hydrogen (2H/1H). Figure 5-1 shows the locations of wells sampled for this study. Laboratory reports of chemical analyses of water samples collected from wells for this study are confidential and therefore and not included with this report, but have been provided to MCWRA as a supplementary confidential attachment. Major ion chemistry and selected stable isotopes were analyzed and evaluated using various graphical techniques to characterize the groundwater in different geologic units and examine possible mixing of groundwater between geologic units. 5.2 Major Ion Chemistry and Water Quality Major cations and anions were characterized in each sampling location and a summary of the resulting signatures are presented on the Piper and trilinear diagrams shown as Figures 5-2 and 5-3 and the Stiff diagrams shown as Figure 5-4 and 5-5. The general signatures of groundwater compositions can be summarized in terms of their dominant cation and anion. In the El Toro Planning Area, samples are classified as intermediate-composition, in that they do not exhibit both a dominant cation and anion. As shown in Figure 5-3, there is some variability in composition between lithologic units, and samples from individual formations are generally clustered together. In some natural systems, water types may include sodium-sulfate and sodium-chloride groundwaters, which are generally described as high TDS formation waters. These groundwaters typically exhibit TDS concentrations in the several thousands of mg/l. The lack of these groundwaters but presence of moderate to high TDS concentrations in the study area suggests that formation waters in the marine formations have been diluted through groundwater extraction and recharge. The uniformity of TDS values in our samples suggests a substantial hydraulic interconnectivity between lithologic units, as also suggested by groundwater elevations throughout the El Toro Planning Area. While it is useful to group groundwater signatures with respect to the dominant cation and anion, groundwater signatures can also be grouped by the presence or absence of sulfate. Within the study area, all bedrock units contain some sulfate, likely originating from the marine formations. Significant sulfate reduction has not occurred. The Quaternary continental deposits (QTc), which consist of Plio-Pleistocene alluvium, have the lowest sulfate (Figure 5-2 and 5-3), consistent with derivation from non-marine units. Samples from the marine Tsm formation generally have higher sulfate concentrations.

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However, some QTc samples contain significant sulfate, suggesting mixing with waters with higher sulfate content. The relative composition of groundwater signatures noted above suggests a relatively simple groundwater-geochemical system in the El Toro Planning Area. The lack of compositional and TDS variability exhibited by groundwater samples supports some level of mixing between bedrock units. However, samples from each formation have a relatively narrow range of chemical compositions, suggesting that mixing is limited. 5.3 Chemical Impacts to Groundwater In addition to evaluating individual samples, the overall groundwater chemistry of lithologic units was evaluated using past results and periodic water quality data available from MCEHD and DHS files. All wells evaluated as part of this study were assigned to a formation or group of formations based on available data including drillers logs and cross-sections prepared for this study. Each well was grouped by formation, and the most recent available water quality data for each well were used in our evaluation. Groundwater quality in the El Toro Planning Area is generally poor. Based on compilation of groundwater chemistry data from MCEHD and DHS files and analyses of samples collected from 25 wells for this study, primary maximum contaminant levels (MCL) are exceeded in 33% (27 of 82) of wells with available data, and secondary MCLs are exceeded in 78% (64 of 82) of wells. Figure 5-6 shows locations of all wells with water chemistry data that were compiled for this study. The distribution of wells with water quality data that does not meet regulatory drinking water standards is widespread in the El Toro Planning Area. Natural groundwaters in the El Toro Planning Area have intermediate to high total dissolved solids (TDS) concentrations, commonly in the hundreds to thousands of mg/l. In this study, all samples had conductivity values of 885 to 2530 micromho per centimeter (mho/cm), corresponding to TDS values3 of approximately 575 to 1650 mg/l. All but one of the samples exceeded the drinking water secondary maximum contaminant level (MCL) for conductivity. Based on EPA secondary drinking water guidelines, water with TDS above 500 mg/l is not recommended for use as drinking water. Figures 5-7 through 5-10 are plots that show the average and range of concentrations of various chemical parameters for each formation. Dashed horizontal lines in these figures represent primary or secondary MCLs for drinking water established by the California Regional Water Quality Control Board (RWQCB). As shown in Figure 5-7, all formations have conductance ranges above secondary water quality goals. Figure 5-8 shows ranges for chloride and sulfate, and as discussed previously most formations are intermediate in composition. 3 Assuming an average conversion factor of TDS (mg/l) = 0.65 EC mho/cm (e.g. Harter, 2003).

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Groundwater concentration ranges of iron and manganese are shown in Figures 5-9. Nearly all available groundwater chemistry data have iron and manganese values that exceed secondary MCL for drinking water. In addition, many water samples also have concentrations of arsenic that exceed primary MCL for drinking water (10 g/l). Concentration ranges for arsenic and cadmium are shown in Figure 5-10. On average, the concentrations of arsenic in groundwater from wells screened in the QTc (Paso Robles) and in the El Toro Primary Aquifer System (QTc + Tsm) exceed primary MCLs. Available historical data for Ambler Park and Toro Water System show that arsenic concentrations are well above the current MCL (Figure 3-5). Further development of the El Toro Primary Aquifer System as a drinking water source will require groundwater treatment for arsenic. Figure 5-10 shows that some samples of Monterey Formation groundwater contained cadmium in excess of MCLs, but in general cadmium does not appear to be a major concern like arsenic. Transient chemical impacts by nitrate and coliform bacteria are a potential problem in areas with dense concentrations of septic tanks and shallow wells. However, the relatively densely developed portions of the Corral de Tierra and San Benancio subareas have been connected to a sewer system for several years. The sewer system pipes waste water along Hwy 68 out of the El Toro Planning area to the Salinas Valley near Spreckels. Nitrate concentrations in samples collected during this study ranged from non-detectable to 14 mg/l, significantly less than the primary MCL of 45 mg/l. 5.4 Stable Isotope Analysis Isotopes are atoms of the same element that have differing numbers of neutrons. Stable isotopes are those that do not undergo nuclear decay. For example, both hydrogen and oxygen have two stable isotopes (1H and 2H, and 16O and 18O, respectively). Natural hydrologic processes including precipitation segregate these isotopes of hydrogen and oxygen, which makes them ideal tracers of water. Stable isotope geochemistry can provide insight into the origin and age of groundwater. In addition, analysis of stable isotopes may provide information on the degree of mixing of isotopically light and heavy waters, and provide support to mixing models. As part of this study, a total of 22 groundwater wells were sampled and analyzed for hydrogen and oxygen isotopic composition. Stable isotope data are normally reported as values, with units of parts-per-thousand ( or per mil) relative to a standard of known composition. A negative value for a sample indicates that the sample has an isotopic ratio lower than the standard. For example, a 2H value of -49.2 means that the 2H/1H ratio of the sample is 49.2 parts-per-thousand or 4.9% lower than the 2H/1H ratio of the

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standard. values are compared on the basis of high versus low values or more/less positive versus more/less negative values. Figure 5-11 is a scatter plot of the 2H versus 18O for the El Toro wells sampled for this study with the data grouped on the basis of screened geological units. This data was used to estimate a possible local meteoric water line (LMWL; y = 5.806x 8.6305, R2 = 0.83), although no isotopic data from precipitation are available. A LMWL slope of 5.8 is notably lower than both the global meteoric water line (GMWL) and U.S. national MWL (both with slopes of approximately 8.1) but is not atypical of LMWLs found in the western U.S. (Kendall and Coplen, 2001). The data generally plot into a tight group, indicating that groundwater over the sampled region likely originates from the same source, independent of lithlogic unit, and that groundwater mixing between lithologic units may be ongoing. Two exceptions are the wells in Upper Corral de Tierra Valley (Station IDs 25 and 76) which, unlike the other wells included in the sampling, are primarily screened in the basal sandstone geological unit. Samples from both of these wells had significantly lower values of 2H and 18O, indicating that groundwater in the basal sandstone aquifer may be distinct from other groundwater in the El Toro Planning Area. The lower 2H and 18O values suggest that recharge to the basal sandstone is derived from a higher elevation. Recharge to the basal sand aquifer likely occurs in areas of higher elevation on the flanks of Mt Toro above Upper Corral de Tierra Valley. 5.5 Summary of groundwater geochemistry Groundwater quality in the El Toro Planning Area is universally impaired by high

TDS, and locally impacted by specific constituents (i.e., arsenic) General grouping of geochemical data by formation supports limited mixing of

groundwaters between lithologic units; Groundwater from the El Toro Primary Aquifer System generally contains arsenic at

concentrations exceeding the primary drinking water standard of 10 (g/l) , so additional utilization of this resource generally requires treatment.

Isotopic data suggest a relatively common recharge area; however, water produced from the basal sand aquifer likely occurs from higher elevations such as the flanks of Mt Toro above Upper Corral de Tierra Valley.

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Well Locations Sampled for this StudyEl Toro Groundwater StudyMonterey County, California

Figure

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NOTES:This figure was originally produced in color. Reproduction in black and white may result in loss of information.

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LegendIntermittent Stream

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!Location and Station ID of Wells with 2007 Groundwater Sample

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Stiff DiagramsAlluvium - Paso Robles (QTc)and Santa Margarita (Tsm)El Toro Groundwater Study

Monterey County, CA

Figure

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Mg SO4

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Mg SO4

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Cations Anionsmeq/kg0 5 10510

NOTES:Each diagram represents a single groundwater sample collected as part of this study. Each sample was collected froma different well. Diagrams are color-coded to match the formation of origin of the sample (based on well screen interval).

This figure was originally produced in color. Reproduction in black and white may result in loss of information.

Aromas - Paso Robles (QTc) Aromas - Paso Robles + Santa Margarita (QTc + Tsm)

Aromas - Paso Robles + Santa Margarita (QTc + Tsm)

Santa Margarita (Tsm)

Stiff DiagramsMonterey (Tm) and Basal Sand (Tus)El Toro Groundwater Study

Monterey County, CA

Figure

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June 2007

Mg SO4

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Cations Anionsmeq/kg0 5 10510

Cations Anionsmeq/kg0 5 10510

NOTES:Each diagram represents a single groundwater sample collected as part of this study. Each sample was collected froma different well. Diagrams are color-coded to match the formation of origin of the sample (based on well screen interval).

This figure was originally produced in color. Reproduction in black and white may result in loss of information.

Aromas - Paso Robles + Monterey Formation (QTc + Tmd)

Basal Sands (Tus)

Monterey Formation (Tmd, Tm) Monterey Formation + Basil Sands (Tmd + Tus)

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Groundwater Chemistry Showing Locations with MCL ExceedancesEl Toro Groundwater StudyMonterey County, California

Figure

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! Result Exceeds Primary MCLIntermittent Stream

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NOTES:This figure was originally produced in color.Reproduction in black and white may result in loss of information.

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Groundwater Stable Isotope DataEl Toro Groundwater Study

Monterey County, CA

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Local Meteoric Water Line2H = 5.806 * 18O + 8.6305