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Soil Suitability /Nitrate Loading Study 1 rerracon Eagle's Nest Motorcycle School Stockton, California July 27, 2021 Terracon Project No. NA217040 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. As per 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 (mg/L) nitrate (NO3) or (3 mg/L-N). Areas having 24.75 mg/L-NO3 (5.5 mg/L- N) or more, are those in which nitrate concentrations may be increasing (Hull, 1984). DBCP (dibromo chloropropane) 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 the historical information, the past agricultural land use and septic systems in the area should be considered potential groundwater contamination sources. According to the State Water Resources Control Board website, there were no nitrate impacted wells reported within 2,000 feet of the site. Based on a review of the Department of Toxic Substances Control (DTSC) Envirostor records, for US Chemical and Beaver Chemical Co., (1448 N. Shaw Road), the southern portion of the site was used for a 0.75 -acre industrial waste pond by the northern adjoining facility. Based on a review of a Surface Impoundment Closure Letter dated December 1994, the pond was contaminated with volatile organic compounds (VOCs) and sodium. The Closure letter indicated approximately 2,500 yards of contaminated soil was removed from the site and "confirmation sampling revealed that all materials contaminated with volatile organic compounds and 82% of the sodium found beneath the surface impoundment (pond) were excavated." The Closure letter additionally indicated the pond excavation was backfilled with clean fill dirt. The State Regional Water Quality Control Board granted the former on-site pond regulatory closure status in December 1994. Responsive ■ Resourceful ■ Reliable 8