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Soil Suitability f Nitrate Loading Study 1rerracon Whitlow Property ■ Stockton, CA November 20. 2019 v Terracon Project No. NA197078 According to the Califomia Department of Water Resources, Groundwater Information Center Interactive Map, depth to groundwater was reported between approximately 90 and 100 feet bgs in Spring 2018 and approximately 130 and 140 feet in Fall 2018. The groundwater flow direction and the depth to shallow, unconfined groundwater, if present, would likely vary depending upon seasonal variations in rainfall and other hydrogeological features. Without the benefit of on-site groundwater monitoring wells surveyed to a datum,groundwater depth and flow direction beneath the site cannot be directly ascertained. 1.4.2 Potential Groundwater Contamination Issues Potential groundwater contamination can occur from a number of sources including historical petroleum hydrocarbon use, agricultural activities, dairies, septic systems, and storm water infiltration. Asper EHD requirements nitrate as N and 1,2-dibromo-3-chloropropane (DBCP)have been included as part of the SSS discussion, Identifying potential groundwater contamination other than nitrates and DBCP is beyond the scope of work for this report. The presence of nitrates is not uncommon in shallow groundwater aquifers in San Joaquin County and other parts of the Central Valley. Nitrate in groundwater occurs as a result of the application of fertilizers, livestock waste, and untreated septic tank waste. Nitrate is mobile and often accumulates in the shallow groundwater zones. In the early 1900s, natural levels of nitrate in groundwater were measured in forty-three (43) wells throughout the Sacramento Valley by Kirk Bryan (1923). It was thought that groundwater at that time was close to "natural" conditions. Based on the work by Bryan, it is estimated that under "natural" conditions, groundwater concentration containing nitrate is no more than about 13.5 milligrams per liter(mgJL) nitrate (NO3) or (3 m9iL-N). Areas having 24.75 mgJL-NO3 (5.5 mglL- N) or more, are those in which nitrate concentrations may be increasing (Hull, 1984). DBCP (dibromochloropropane) is a nematocide and soil fumigant for vegetables and grapes. It is thought to cause health problems consisting of kidney damage, liver damage, and cancer. The use of DBCP was discontinued in 1979. Based on a review of the DWR Water Data Library, in the 1970s, a groundwater well located within 0.36 miles north of the site, was analyzed for various constituents including nitrate. Dissolved nitrate as N was reported in the well between approximately 0.56 and 2.7 mgJL-N below the EPA MCL of 10 mglL-N. Based on a review of the State Water Resources Control Board online map, there were no nitrate impacted wells identified near the site. Based on the historical information, the past agricultural land use and septic systems in the area should be considered potential groundwater contamination sources. Refer to Section 1.3.2 for further discussion for information regarding the site groundwater well. Responsive m Resourceful ■ Reliable 7