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from this source. According to an analysis of these maps (Plates 4 and 5), ground water elevation is approximately 20 to 40 feet above mean sea level; the ground-water flow beneath the Site is to the northwest at a rate of approximately 0 to 12 feet per mile. Given that the ground elevation of the Site is approximately 58 to 60 feet above mean sea level, the depth to water below the Site is estimated to be approximately 18 to 40 feet. San Joaquin County experienced its highest recent ground-water levels in 1983 and 1999. Maps of ground-water levels were reviewed for this study (San Joaquin County Flood Control and Water Conservation District, 1983 and 1999); however, the Tracy area was not mapped during these years. Potential Ground Water Contamination Issues The Soil Suitability Study is not intended to be an investigation into ground-water contamination sources, and no such investigation was conducted. Many sources can contribute to ground water contamination, including leaking underground storage tanks, agricultural activities, dairies, septic systems, and storm water infiltration. Agricultural activities and the use of septic systems in the area are known ground water contamination sources with the potential to impact the Site. Two common ground-water contaminants in San Joaquin County are nitrate and dibromochloropropane (DBCP). Live Oak reviewed the San Joaquin County Environmental Health Department's maps of Nitrate — Land Use Data and DBCP— Land Use Data dated February 20, 2019. According to the nitrate map (Plate 6), seven wells within a one-mile radius of the subject Site have been tested for nitrate; it was detected in five of the wells at concentrations between 5.1 and 10.0 mg/L-N, and in two of the wells at concentrations between 0.1 and 5.0 mg/L-N. According to the DBCP map (Plate 7), four wells within a one-mile radius of the Site have been tested for DBCP; no DBCP was detected in any of the wells. The Maximum Contaminant Level (MCL) set by the US EPA for nitrate is 10 mg/L-N; the MCL for DBCP is 0.2 ug/L. The surrounding area consists of orchards, open agricultural land, and occasional rural residences. The Site is located approximately one mile east of the Tracy Defense Depot, a Superfund site which has contaminated ground water in the area with chemicals including trichloroethene, tetrachloroethene, and dieldrin (GeoTracker, 2020). While on-site to run the percolation test, Live Oak spoke with Hugh, a neighbor who lives south of the Site. He indicated that the land north of the Site was a dairy in the past, and that manure ponds had been located northeast of the Site (personal communication, February 21, 2020). Live Oak confirmed the presence of the dairy by viewing historical aerial photographs. Dairies are common sources of elevated nitrate levels in soil and ground water. LOGE 20-08 Page 3