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 most frequently detected VOC. None of the 88 VOCs analyzed were detected at concentrations above an MCL, SMCL, NL, HA-L, or RSD5.

Fifteen pesticides and pesticide degradates were detected in ground-water samples within the Northern San Joaquin Basin GAMA study unit. Pesticide and pesticide degradates were detected in 27 of the 70 samples collected for pesticide analysis. Pesticides were most frequently detected in ground-water samples from the Eastern San Joaquin Basin study area and least frequently detected in the Cosumnes Basin study area. Herbicides were the most frequently detected class of pesticide; simazine, a herbicide, was the most frequently detected pesticide. Two herbicides, 1,2-dibromo-3-chloropropane
and 1,2-dibromoethane, were detected at concentrations above their USEPA MCLs. None of the other 120 pesticide or pesticide degradates analyzed were detected at concentrations above an MCL, SMCL, NL, HA-L, or RSD5.

Five wastewater-indicator constituents were identified in ground-water samples collected in the Northern San Joaquin Basin GAMA study unit. Thirteen of the 16 samples analyzed for wastewater-indicator constituents had at least a single detection. Isophorone was the most frequently detected wastewater-indicator constituent. None of the 63 wastewater-indicator constituents analyzed were detected at concentrations above an MCL, SMCL, NL, HA-L, or RSD5.

Twenty-four pharmaceutical constituents were analyzed for in ground-water samples collected at all wells in the Northern San Joaquin Basin GAMA study unit. However, as of the writing of this report, the analytical methods employed in the detection and quantification of the selected pharmaceutical constituents were still in development, and additional quality-control data were needed to verify the results. The results of the pharmaceutical analysis within the Northern San Joaquin Basin GAMA study unit are planned for publication in a future report.

Thirty-four ground-water samples were analyzed for the following constituents of special interest: perchlorate, N-nitrosodimethylamine (NDMA), and 1,2,3-trichloropropane (1,2,3-TCP). Perchlorate, an “unregulated chemical for which monitoring is required,” was detected in two ground-water samples in the Eastern San Joaquin Basin study area at concentrations below the CADHS DLR and NL. NDMA and 1,2,3-TCP were not detected in any ground-water samples.

Eighteen ground-water samples were collected for nutrient analysis, and 10 samples were collected for dissolved organic carbon analysis. Nitrite plus nitrate was detected in 11 of 13 samples (excluding the depth-dependent analyses), whereas nitrite was detected in only two samples; concentrations
of nitrite were not above the USEPA MCL of 1 milligram per liter (mg/L) (as nitrogen). Concentrations of nitrite plus nitrate were not above the USEPA MCL of 10 mg/L (as nitrogen). Dissolved organic carbon was detected in 5 of the 10 samples in which it was analyzed.

Thirty-nine ground-water samples were analyzed for 10 major and minor ions, as well as total dissolved solids (TDS). Eight samples had TDS concentrations above the USEPA recommended SMCL of 500 mg/L. All eight samples with TDS concentrations above the USEPA SMCL were from wells located within the Tracy Basin study area. Three ground-water samples had detections of sulfate above the USEPA SMCL of 250 mg/L, which like TDS, were all located in the Tracy Basin study area.

Thirty-nine ground-water samples were collected and analyzed for 25 different trace elements. Arsenic was detected in all 39 samples; however, it was detected in only three samples above the USEPA MCL of 10 µg/L. Hexavalent chromium, a CADHS “unregulated chemical for which monitoring is required,” was detected in 16 of 39 ground-water samples in which it was analyzed, all of which were at concentrations above the CADHS DLR of 1 µg/L. Iron, detected in 26 of 39 samples, was detected in three samples at concentrations above the USEPA SMCL of 300 µg/L. Manganese, detected in 36 of 39 samples, was detected in eight samples at concentrations above the USEPA SMCL of 50 µg/L. In addition, three samples contained manganese at concentrations above the CADHS NL of 500 µg/L. Boron, a CADHS “unregulated chemical for which monitoring is required,” was detected in all 39 ground-water samples in which it was analyzed. Of those 39 detections, 11 were at concentrations above the CADHS DLR of 100 µg/L, and 5 were at concentrations above the CADHS NL of 1,000 µg/L. Concentrations of boron above the CADHS NL were found only in the Tracy Basin study area. Boron concentrations above the DLR were most frequently found in ground-water samples in the Tracy Basin study area, followed by the Eastern San Joaquin Basin study area, and lastly in the Uplands Basin study area. Vanadium, a CADHS “unregulated chemical for which monitoring is required,” was detected in 38 of 39 ground-water samples, and of those, 33 were above the CADHS DLR of 3 µg/L. However, none of the vanadium detections in ground water were at concentrations above the CADHS NL of 50 µg/L.

USGS laboratories analyzed 70 ground-water samples for stable isotopes of water and tritium. Stable isotopes of water (deuterium and oxygen-18) are used in the assessment of ground-water recharge sources, whereas tritium is often used as an environmental tracer. In addition to the analyses performed by the USGS laboratories, 27 samples were also analyzed for tritium, and 16 were analyzed for noble gases at the Lawrence Livermore National Laboratory. Finally, 13 wells were sampled for radium-226, radium-228, radon-222, gross-alpha radioactivity (72-hour and 30-day count), gross-beta radioactivity (72-hour and 30-day count), and stable carbon isotopes. Tritium was detected in 61 of 70 samples of which all detections showed activities well below the CADHS MCL of 20,000 picocuries per liter (pCi/L). Radium-226 was detected in all ground-water samples in which it was analyzed, whereas radon-228 was detected in only three samples. None of the ground-water samples collected were above the USEPA combined radium-226 and radium-228 MCL of 5 pCi/L. Gross-alpha and gross-beta radioactivity in ground-water samples were in all cases below their USEPA MCLs. Radon-222 was detected in all ground-water samples in which it was analyzed. The proposed USEPA MCL for radon-222 is 300 pCi/L, whereas the proposed Alternate Maximum Concentration Level (AMCL) is 4,000 pCi/L (U.S. Environmental Protection Agency, 2006). Of the 13 samples in which radon-222 was analyzed and detected, only one was below the proposed USEPA MCL, whereas all were below the AMCL. Stable isotope values, reported as ratios of hydrogen and oxygen isotopes, ranged from –81.1 per mill to –43.0 per mill for the hydrogen isotopes and from –11.2 per mill to –5.6 per mill for the oxygen isotopes.

Microbial constituents, including total coliform and Escherichia spp. coliform (E. coli), as well as the viruses F-specific coliphage and somatic coliphage, were analyzed in eight ground-water samples. F-specific coliphage, somatic coliphage, and Escherichia spp. coliform were not detected in any ground-water samples, whereas an estimated three total coliform colonies per 100 milliliters of water were detected in a sample collected in the Uplands Basin study area.