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U.S. GEOLOGICAL SURVEY
Data Series 285 (ver 1.1, August 2018)

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Summary

The Ground-Water Ambient Monitoring Assessment (GAMA) Program is intended to provide a comprehensive statewide assessment of ground-water quality in areas of California where ground-water is a significant source of drinking water. The Southern Sacramento Valley GAMA study unit is the fourth region in which the GAMA Program has implemented its program objectives. The Southern Sacramento Valley GAMA study unit lies within the Central Valley hydrogeologic province and consists of six study areas: the North American, South American, Solano, Yolo, Suisun–Fairfield, and Uplands Basin. A total of 87 ground-water samples were collected from 83 wells in the study unit: 24 in the North American study area, 12 in the South American study area, 13 in the Solano study area, 19 in the Yolo study area, 5 in the Suisun–Fairfield study area, and 15 in the Uplands study area. The well types sampled included public supply, irrigation, domestic, and monitoring.

This report presents the results of analyses for over 350 chemical and microbial constituents in the 87 samples collected between March 2005 and June 2005. Additionally, results were compared with selected Federal and State drinking-water standards. The chemical and microbial data presented in this report characterize the quality of untreated ground-water resources within the study unit, not the treated drinking water that is delivered to consumers by water 
purveyors.

VOCs were detected in 49 out of 83 wells sampled; all concentrations were below MCLs or other health-based thresholds. The frequently detected VOCs (found in more than 10 percent of the wells sampled) were trichloromethane (chloroform), tetrachloroethylene (PCE), carbon disulfide, and trichloroethylene (TCE). Pesticides were detected in 35 of the 83 wells sampled; all concentrations were below MCLs or other health-based thresholds. The frequently detected pesticide compounds were 2-chloro-4-isopropylamino-6-amino-s-triazine (deethylatrazine, a degradate of atrazine), atrazine, and oxamyl. Caffeine was detected in 9 out of 43 wells sampled. The following special interest constituents were detected in this study: perchlorate, chromium(VI), and 1,2,3-trichloropropane; only 1,2,3-trichloropropane was detected at a concentration greater than its California Department of Health Services notification level (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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