Groundwater quality in the Middle Sacramento Valley, California
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George L. Bennett
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Groundwater quality in the Southern Sacramento Valley, California
Groundwater provides more than 40 percent of California's drinking water. To protect this vital resource, the State of California created the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The Priority Basin Project of the GAMA Program provides a comprehensive assessment of the State's groundwater quality and increases public access to groundwater-quality information. The Southern Sacramento Valley is one of the study units being evaluated.
Fact Sheet↗
Status of groundwater quality in the Southern, Middle, and Northern Sacramento Valley study units, 2005-08: California GAMA Priority Basin Project
Groundwater quality in the Southern, Middle, and Northern Sacramento Valley study units was investigated as part of the Priority Basin Project of the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The study units are located in California’s Central Valley and include parts of Butte, Colusa, Glenn, Placer, Sacramento, Shasta, Solano, Sutter, Tehama, Yolo, and Yuba Counties. The GAMA Priority Basin Project is being conducted by the California State Water Resources Control Board in collaboration with the U.S. Geological Survey and the Lawrence Livermore National Laboratory. The three study units were designated to provide spatially-unbiased assessments of the quality of untreated groundwater in three parts of the Central Valley hydrogeologic province, as well as to provide a statistically consistent basis for comparing water quality regionally and statewide. Samples were collected in 2005 (Southern Sacramento Valley), 2006 (Middle Sacramento Valley), and 2007–08 (Northern Sacramento Valley). The GAMA studies in the Southern, Middle, and Northern Sacramento Valley were designed to provide statistically robust assessments of the quality of untreated groundwater in the primary aquifer systems that are used for drinking-water supply. The assessments are based on water-quality data collected by the USGS from 235 wells in the three study units in 2005–08, and water-quality data from the California Department of Public Health (CDPH) database. The primary aquifer systems (hereinafter, referred to as primary aquifers) assessed in this study are defined by the depth intervals of the wells in the CDPH database for each study unit. The quality of groundwater in shallow or deep water-bearing zones may differ from quality of groundwater in the primary aquifers; shallow groundwater may be more vulnerable to contamination from the surface. The status of the current quality of the groundwater resource was assessed by using data from samples analyzed for volatile organic compounds (VOC), pesticides, and naturally occurring inorganic constituents, such as major ions and trace elements. This status assessment is intended to characterize the quality of groundwater resources within the primary aquifers of the three Sacramento Valley study units, not the treated drinking water delivered to consumers by water purveyors. Relative-concentrations (sample concentrations divided by benchmark concentrations) were used for evaluating groundwater quality for those constituents that have Federal or California regulatory or non-regulatory benchmarks for drinking-water quality. A relative-concentration greater than 1.0 indicates a concentration greater than a benchmark. For organic (volatile organic compounds and pesticides) and special-interest (perchlorate) constituents, relative-concentrations were classified as high (greater than 1.0); moderate (equal to or less than 1.0 and greater than 0.1); or low (equal to or less than 0.1). For inorganic (major ion, trace element, nutrient, and radioactive) constituents, the boundary between low and moderate relative-concentrations was set at 0.5. Aquifer-scale proportions were used in the status assessment for evaluating regional-scale groundwater quality. High aquifer-scale proportion is defined as the percentage of the area of the primary aquifers that have a relative-concentration greater than 1.0 for a particular constituent or class of constituents; percentage is based on an areal rather than a volumetric basis. Moderate and low aquifer-scale proportions were defined as the percentage of the primary aquifers that have moderate and low relative-concentrations, respectively. Two statistical approaches—grid-based, which used one value per grid cell, and spatially-weighted, which used the full dataset—were used to calculate aquifer-scale proportions for individual constituents and classes of constituents. High and moderate aquifer-scale proportions were significantly greater for inorganic constituents than organic constituents in all three study units. In the Southern Sacramento Valley study unit, relative-concentrations for one or more inorganic constituents with health-based benchmarks (HBBs) were high in 30 percent (%), moderate in 30%, and low in 40% of the primary aquifer. In the Middle Sacramento Valley study unit, aquifer-scale proportions for inorganic constituents with HBBs were high in 24%, moderate in 38%, and low in 38% of the primary aquifer. Arsenic, boron, and nitrate were detected at high relative-concentrations in the Southern and Middle Sacramento Valley study units. In the Northern Sacramento Valley study unit, high, moderate, and low relative-concentrations of inorganic constituents relative to HBBs were 2.1, 12, and 86% of the primary aquifer, respectively. Arsenic was the only constituent detected at high relative-concentrations. The high aquifer-scale proportions for inorganic constituents with non-health-based benchmarks were 32, 27, and 4.6% of the primary aquifer for the Southern, Middle, and Northern Sacramento Valley study units, respectively. The high aquifer-scale proportions for organic constituents with HBBs were less than 1% in the Southern, Middle, and Northern Sacramento Valley study units. Organic constituents were detected at moderate relative-concentrations in about 3% of the Southern and Middle Sacramento Valley study units and in 1% of the Northern Sacramento Valley study unit. Of the 227 organic constituents analyzed for, 86 were detected, and of those detected, 56 have HBBs. Six organic constituents (atrazine, bentazon, chloroform, simazine, tetrachloroethene, and trichloroethene) were detected in 10% or more of the sampled wells in one or more of the three Sacramento Valley study units.
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Groundwater quality in the Northern San Joaquin Valley, California
Groundwater provides more than 40 percent of California's drinking water. To protect this vital resource, the State of California created the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The Priority Basin Project of the GAMA Program provides a comprehensive assessment of the State's groundwater quality and increases public access to groundwater-quality information. The Northern San Joaquin constitutes one of the study units being evaluated.
Fact Sheet↗
Status and understanding of groundwater quality in the northern San Joaquin Basin, 2005: California GAMA Priority Basin Project
Groundwater quality in the 2,079 square mile Northern San Joaquin Basin (Northern San Joaquin) study unit was investigated from December 2004 through February 2005 as part of the Priority Basin Project of the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The GAMA Priority Basin Project was developed in response to the Groundwater Quality Monitoring Act of 2001 that was passed by the State of California and is being conducted by the California State Water Resources Control Board in collaboration with the U.S. Geological Survey and the Lawrence Livermore National Laboratory. The Northern San Joaquin study unit was the third study unit to be designed and sampled as part of the Priority Basin Project. Results of the study provide a spatially unbiased assessment of the quality of raw (untreated) groundwater, as well as a statistically consistent basis for comparing water quality throughout California. Samples were collected from 61 wells in parts of Alameda, Amador, Calaveras, Contra Costa, San Joaquin, and Stanislaus Counties; 51 of the wells were selected using a spatially distributed, randomized grid-based approach to provide statistical representation of the study area (grid wells), and 10 of the wells were sampled to increase spatial density and provide additional information for the evaluation of water chemistry in the study unit (understanding/flowpath wells). The primary aquifer systems (hereinafter, primary aquifers) assessed in this study are defined by the depth intervals of the wells in the California Department of Public Health database for each study unit. The quality of groundwater in shallow or deep water-bearing zones may differ from quality of groundwater in the primary aquifers; shallow groundwater may be more vulnerable to contamination from the surface. Two types of assessments were made: (1) status, assessment of the current quality of the groundwater resource; and (2) understanding, identification of the natural and human factors affecting groundwater quality. Relative-concentrations (sample concentrations divided by benchmark concentrations) were used for evaluating groundwater quality for those constituents that have Federal or California regulatory or non-regulatory benchmarks for drinking-water quality. Benchmarks used in this study were either health-based (regulatory and non-regulatory) or aesthetic based (non-regulatory). For inorganic constituents, relative-concentrations were classified as high (equal to or greater than 1.0), indicating relative-concentrations greater than benchmarks; moderate (equal to or greater than 0.5, and less than 1.0); or, low (less than 0.5). For organic and special- interest constituents [1,2,3-trichloropropane (1,2,3-TCP), N-nitrosodimethylamine (NDMA), and perchlorate], relative- concentrations were classified as high (equal to or greater than 1.0); moderate (equal to or greater than 0.1 and less than 1.0); or, low (less than 0.1). Aquifer-scale proportion was used as the primary metric in the status assessment for groundwater quality. High aquifer- scale proportion is defined as the percentage of the primary aquifer with relative-concentrations greater than 1.0; moderate and low aquifer-scale proportions are defined as the percentage of the primary aquifer with moderate and low relative- concentrations, respectively. The methods used to calculate aquifer-scale proportions are based on an equal-area grid; thus, the proportions are areal rather than volumetric. Two statistical approaches - grid-based, which used one value per grid cell, and spatially weighted, which used the full dataset - were used to calculate aquifer-scale proportions for individual constituents and classes of constituents. The spatially weighted estimates of high aquifer-scale proportions were within the 90-percent confidence intervals of the grid-based estimates in all cases. The understanding assessment used statistical correlations between constituent relative-concentrations and
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Groundwater quality data for the northern Sacramento Valley, 2007: Results from the California GAMA Program
Groundwater quality in the approximately 1,180-square-mile Northern Sacramento Valley study unit (REDSAC) was investigated in October 2007 through January 2008 as part of the Priority Basin Project of the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The GAMA Priority Basin Project was developed in response to the Groundwater Quality Monitoring Act of 2001, and is being conducted by the U.S. Geological Survey (USGS) in cooperation with the California State Water Resources Control Board (SWRCB). The study was designed to provide a spatially unbiased assessment of the quality of raw groundwater used for public water supplies within REDSAC and to facilitate statistically consistent comparisons of groundwater quality throughout California. Samples were collected from 66 wells in Shasta and Tehama Counties. Forty-three of the wells were selected using a spatially distributed, randomized grid-based method to provide statistical representation of the study area (grid wells), and 23 were selected to aid in evaluation of specific water-quality issues (understanding wells). The groundwater samples were analyzed for a large number of synthetic organic constituents (volatile organic compounds [VOC], pesticides and pesticide degradates, and pharmaceutical compounds), constituents of special interest (perchlorate and N-nitrosodimethylamine [NDMA]), naturally occurring inorganic constituents (nutrients, major and minor ions, and trace elements), radioactive constituents, and microbial constituents. Naturally occurring isotopes (tritium, and carbon-14, and stable isotopes of nitrogen and oxygen in nitrate, stable isotopes of hydrogen and oxygen of water), and dissolved noble gases also were measured to help identify the sources and ages of the sampled ground water. In total, over 275 constituents and field water-quality indicators were investigated. Three types of quality-control samples (blanks, replicates, and sampmatrix spikes) were collected at approximately 8 to 11 percent of the wells, and the results for these samples were used to evaluate the quality of the data obtained from the groundwater samples. Field blanks rarely contained detectable concentrations of any constituent, suggesting that contamination was not a noticeable source of bias in the data for the groundwater samples. Differences between replicate samples were within acceptable ranges for nearly all compounds, indicating acceptably low variability. Matrix-spike recoveries were within acceptable ranges for most compounds. This study did not attempt to evaluate the quality of water delivered to consumers; after withdrawal from the ground, raw groundwater typically is treated, disinfected, or blended with other waters to maintain water quality. Regulatory thresholds apply to water that is served to the consumer, not to raw ground water. However, to provide some context for the results, concentrations of constituents measured in the raw groundwater were compared with regulatory and nonregulatory health-based thresholds established by the U.S. Environmental Protection Agency (USEPA) and California Department of Public Health (CDPH) and with aesthetic and technical thresholds established by CDPH. Comparisons between data collected for this study and drinking-water thresholds are for illustrative purposes only and do not indicate compliance or noncompliance with those thresholds. The concentrations of most constituents detected in groundwater samples from REDSAC were below drinking-water thresholds. Volatile organic compounds (VOC) and pesticides were detected in less than one-quarter of the samples and were generally less than a hundredth of any health-based thresholds. NDMA was detected in one grid well above the NL-CA. Concentrations of all nutrients and trace elements in samples from REDSAC wells were below the health-based thresholds except those of arsenic in three samples, which were above the USEPA maximum contaminant level (MCL-US). However, none of these wells were public-supply wells. Concentrations of all radioactive constituents were below health-based thresholds except radon-222, which was detected above the proposed MCL-US of 300 pCi/L in samples from 11 grid wells. Most of the samples from REDSAC wells had concentrations of major elements, total dissolved solids, and trace elements below the non-enforceable thresholds set for aesthetic or technical concerns. A few samples contained iron, manganese, or pH at levels above the SMCL-CA or SMCL-US thresholds.
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Ground-water quality data in the Southern Sacramento Valley, California, 2005 — Results from the California GAMA Program
Ground-water quality in the approximately 2,100 square-mile Southern Sacramento Valley study unit (SSACV) was investigated from March to June 2005 as part of the Statewide Basin Assessment Project of Ground-Water Ambient Monitoring and Assessment (GAMA) Program. This study was designed to provide a spatially unbiased assessment of raw ground-water quality within SSACV, as well as a statistically consistent basis for comparing water quality throughout California. Samples were collected from 83 wells in Placer, Sacramento, Solano, Sutter, and Yolo Counties. Sixty-seven of the wells were selected using a randomized grid-based method to provide statistical representation of the study area. Sixteen of the wells were sampled to evaluate changes in water chemistry along ground-water flow paths. Four additional samples were collected at one of the wells to evaluate water-quality changes with depth. The GAMA Statewide Basin Assessment project was developed in response to the Ground-Water Quality Monitoring Act of 2001 and is being conducted by the California State Water Resources Control Board (SWRCB) in collaboration with the U.S. Geological Survey (USGS) and the Lawrence Livermore National Laboratory (LLNL). The ground-water samples were analyzed for a large number of man-made organic constituents (volatile organic compounds [VOCs], pesticides and pesticide degradates, pharmaceutical compounds, and wastewater-indicator constituents), constituents of special interest (perchlorate, N -nitrosodimethylamine [NDMA], and 1,2,3-trichloropropane [1,2,3-TCP]), naturally occurring inorganic constituents (nutrients, major and minor ions, and trace elements), radioactive constituents, and microbial indicators. Naturally occurring isotopes (tritium, and carbon-14, and stable isotopes of hydrogen, oxygen, and carbon), and dissolved noble gases also were measured to help identify the source and age of the sampled ground water. Quality-control samples (blanks, replicates, matrix spikes) were collected at ten percent of the wells, and the results for these samples were used to evaluate the quality of the data for the ground-water samples. Assessment of the quality-control data resulted in censoring of less than 0.03 percent of the analyses of ground-water samples. This study did not evaluate the quality of water delivered to consumers; after withdrawal from the ground, water typically is treated, disinfected, and (or) blended with other waters to maintain acceptable water quality. Regulatory thresholds apply to treated water that is served to the consumer, not to raw ground water. However, to provide some context for the results, concentrations of constituents measured in the raw ground water were compared with health-based thresholds established by the U.S. Environmental Protection Agency (USEPA) and California Department of Health Services (CADHS) (Maximum Contaminant Levels [MCLs], notification levels [NLs], or lifetime health advisories [HA-Ls]) and thresholds established for aesthetic concerns (Secondary Maximum Contaminant Levels [SMCLs]). All wells were sampled for organic constituents and selected general water quality parameters; subsets of wells were sampled for inorganic constituents, nutrients, and radioactive constituents. Volatile organic compounds were detected in 49 out of 83 wells sampled and pesticides were detected in 34 out of 82 wells; all detections were below health-based thresholds, with the exception of 1 detection of 1,2,3-trichloropropane above a NL. Of the 43 wells sampled for trace elements, 27 had no detections of a trace element above a health-based threshold and 16 had at least one detection above. Of the 18 trace elements with health-based thresholds, 3 (arsenic, barium, and boron) were detected at concentrations higher an MCL. Of the 43 wells sampled for nitrate, only 1 well had a detection above the MCL. Twenty wells were sampled for radioactive constituents; only 1 (radon-222) was measured at activities higher than the proposed MCL. Radon-222 was detected below the threshold in 7 wells and above the threshold in 13 wells. SMCLs have been established for nine constituents or parameters analyzed in SSACV. Six were measured at levels higher than an SMCL: chloride, iron, manganese, pH, specific conductance, and total dissolved solids. Chloride, iron, manganese, pH, and total dissolved solids were measured in 43 wells: 27 wells had no measurements above a threshold and 16 wells had a measurement above a threshold. Specific conductance was measured in 83 wells. In 68 wells, specific conductance was measured lower than the threshold and in 15 wells it was measured above the threshold.
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Source, Distribution, and Management of Arsenic in Water from Wells, Eastern San Joaquin Ground-Water Subbasin, California
Between 1974 and 2001 water from as many as one-third of wells in the Eastern San Joaquin Ground Water Subbasin, about 80 miles east of San Francisco, had arsenic concentrations greater than the U.S. Environmental Protection Agency Maximum Contaminant Level (MCL) for arsenic of 10 micrograms per liter (ug/L). Water from some wells had arsenic concentrations greater than 60 ug/L. The sources of arsenic in the study area include (1) weathering of arsenic bearing minerals, (2) desorption of arsenic associated with iron and manganese oxide coatings on the surfaces of mineral grains at pH's greater than 7.6, and (3) release of arsenic through reductive dissolution of iron and manganese oxide coatings in the absence of oxygen. Reductive dissolution is responsible for arsenic concentrations greater than the MCL. The distribution of arsenic varied areally and with depth. Concentrations were lower near ground-water recharge areas along the foothills of the Sierra Nevada; whereas, concentrations were higher in deeper wells at the downgradient end of long flow paths near the margin of the San Joaquin Delta (fig. 1). Management opportunities to control high arsenic concentrations are present because water from the surface discharge of wells is a mixture of water from the different depths penetrated by wells. On the basis of well-bore flow and depth-dependent water-quality data collected as part of this study, the screened interval of a public-supply well having arsenic concentrations that occasionally exceed the MCL was modified to reduce arsenic concentrations in the surface discharge of the well. Arsenic concentrations from the modified well were about 7 ug/L. Simulations of ground-water flow to the well showed that although upward movement of high-arsenic water from depth within the aquifer occurred, arsenic concentrations from the well are expected to remain below the MCL.
Open-File Report↗
California GAMA program: Ground-water quality data in the northern San Joaquin Basin Study Unit, 2005
Growing concern over the closure of public-supply wells because of ground-water contamination has led the State Water Board to establish the Ground-Water Ambient Monitoring and Assessment (GAMA) Program. With the aid of the U.S. Geological Survey (USGS) and Lawrence Livermore National Laboratory, the program goals are to enhance understanding and provide a current assessment of ground-water quality in areas where ground water is an important source of drinking water. The Northern San Joaquin Basin GAMA study unit covers an area of approximately 2,079 square miles (mi2) across four hydrologic study areas in the San Joaquin Valley. The four study areas are the California Department of Water Resources (CADWR) defined Tracy subbasin, the CADWR-defined Eastern San Joaquin subbasin, the CADWR-defined Cosumnes subbasin, and the sedimentologically distinct USGS-defined Uplands study area, which includes portions of both the Cosumnes and Eastern San Joaquin subbasins. Seventy ground-water samples were collected from 64 public-supply, irrigation, domestic, and monitoring wells within the Northern San Joaquin Basin GAMA study unit. Thirty-two of these samples were collected in the Eastern San Joaquin Basin study area, 17 in the Tracy Basin study area, 10 in the Cosumnes Basin study area, and 11 in the Uplands Basin study area. Of the 32 samples collected in the Eastern San Joaquin Basin, 6 were collected using a depth-dependent sampling pump. This pump allows for the collection of samples from discrete depths within the pumping well. Two wells were chosen for depth-dependent sampling and three samples were collected at varying depths within each well. Over 350 water-quality field parameters, chemical constituents, and microbial constituents were analyzed and are reported as concentrations and as detection frequencies, by compound classification as well as for individual constituents, for the Northern San Joaquin Basin study unit as a whole and for each individual study area. Results are presented in a descending order based on detection frequencies (most frequently detected compound listed first), or alphabetically when a detection frequency could not be calculated. Only certain wells were measured for all constituents and water-quality parameters. The results of all of the analyses were compared with U.S. Environmental Protection Agency (USEPA) and California Department of Health Services (CADHS) Maximum Contaminant Levels (MCLs), Secondary Maximum Contaminant Levels (SMCLs), USEPA lifetime health advisories (HA-Ls), the risk-specific dose at a cancer risk level equal to 1 in 100,000 or 10E-5 (RSD5), and CADHS notification levels (NLs). When USEPA and CADHS MCLs are the same, detection levels were compared with the USEPA standard; however, in some cases, the CADHS MCL may be lower. In those cases, the data were compared with the CADHS MCL. Constituents listed by CADHS as 'unregulated chemicals for which monitoring is required' were compared with the CADHS 'detection level for the purposes of reporting' (DLR). DLRs unlike MCLs are not health based standards. Instead, they are levels at which current laboratory detection capabilities allow eighty percent of qualified laboratories to achieve measurements within thirty percent of the true concentration. Twenty-three volatile organic compounds (VOCs) and seven gasoline oxygenates were detected in ground-water samples collected in the Northern San Joaquin Basin GAMA study unit. Additionally, 13 tentatively identified compounds were detected. VOCs were most frequently detected in the Eastern San Joaquin Basin study area and least frequently detected in samples collected in the Cosumnes Basin study area. Dichlorodifluoromethane (CFC-12), a CADHS 'unregulated chemical for which monitoring is required,' was detected in two wells at concentrations greater than the DLR. Trihalomethanes were the most frequently detected class of VOC constituents. Chloroform (trichloromethane) was the m
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Sources of High-Chloride Water to Wells, Eastern San Joaquin Ground-Water Subbasin, California
As a result of pumping and subsequent declines in water levels, chloride concentrations have increased in water from wells in the Eastern San Joaquin Ground-Water Subbasin, about 80 miles east of San Francisco (Montgomery Watson, Inc., 2000). Water from a number of public-supply, agricultural, and domestic wells in the western part of the subbasin adjacent to the San Joaquin Delta exceeds the U.S. Environmental Protection Agency Secondary Maximum Contaminant Level (SMCL) for chloride of 250 milligrams per liter (mg/L) (fig. 1) (link to animation showing chloride concentrations in water from wells, 1984 to 2004). Some of these wells have been removed from service. High-chloride water from delta surface water, delta sediments, saline aquifers that underlie freshwater aquifers, and irrigation return are possible sources of high-chloride water to wells (fig. 2). It is possible that different sources contribute high-chloride water to wells in different parts of the subbasin or even to different depths within the same well.
Open-File Report↗