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concentrations no more then about 3 mg/L-N. Nitrate may be increasing in areas with <br />concentrations of 5.5 mg/L-N or more (Hull, 1984). The US EPA has set the Maximum <br />Contaminant Level (MCL) for nitrate at 10 mg/L-N. <br />Although nitrate is a naturally -occurring compound necessary for plant growth, it can <br />cause health problems when present at high levels in drinking water. The most <br />common health effect of nitrates in water is methemoglobinemia, or blue baby <br />syndrome, which results in reduced oxygen supply to vital tissues. Pregnant women <br />and certain others can also develop methemoglobinemia. Symptoms include a bluish <br />color of the skin, as well as headache, dizziness, weakness, and difficulty breathing. <br />Ingestion of high levels of nitrates over time can also lead to gastric problems. <br />DBCP is a nematocide and soil fumigant for vegetables and grapes. It is known to <br />cause male reproductive effects and is classified as a probable human carcinogen. <br />Most domestic use of DBCP was discontinued in 1977 through 1979. The US EPA set <br />the MCL at 0.2 parts per billion (ppb, equivalent to ug/L) because it believes that given <br />present technology and resources, this is the lowest level to which water systems can <br />reasonably be required to remove this contaminant should it occur in drinking water. <br />The Maximum Contaminant Level Goal (MCLG) for DBCP is zero. <br />Soil Profile and Geologic Information <br />The Site is depicted on the Preliminary geologic map of the Tracy quadrangle, San <br />Joaquin County, California (Diblee, 1981) as within a map unit of Holocene alluvium <br />(Plate 8). <br />The surface and near surface soils located at the Site are reported by the USDA Web <br />Soil Survey to be composed of Stomar clay loam (Plate 9) (USDA, 2021). The table <br />below presents some selected properties of this soil: <br />Table 1. Selected Properties of On -Site Soils (USDA, 1992) <br />Soil Name <br />Depth <br />Unified Soil <br />Salinity <br />Soil <br />Perme- <br />Septic tank <br />and Map <br />(in) <br />Classification <br />mmhos/cm <br />Reaction <br />ability <br />absorption <br />Symbol <br />pH <br />(in/hr) <br />fields <br />Stomar clay <br />0-17 <br />Clay loam; CL <br /><2 <br />6.6-7.3 <br />0.2-0.6 <br />Severe: <br />loam, 0-2% <br />17-47 <br />Clay loam, clay; CL, CH <br /><2 <br />6.6-8.4 <br />0.06-0.2 <br />peres slowly <br />slopes, 252 <br />47-60 <br />Clay loam, silty clay <br /><2 <br />7.4-8.4 <br />0.2-0.6 <br />loam; CL <br />Note: The table descriptions do not necessarily reflect site-specific soil properties. See description of the map unit for <br />composition and behavior characteristics of the map unit. <br />LOGE 2204 Page 5 <br />