ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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Arsenic Arsenic levels exceed the MCL of 10 micrograms per liter(gg/1)at 12 of the 16 city wells,which have concentrations of 12 to 19µg/11.Arsenic treatment has been initiated at each well to lower levels to below the MCL, and will be installed in any new wells constructed that exceed the MCL. Manganese There is no primary MCL for Manganese. The city has one well that exceeds the secondary MCL of 50 gg/l. This well has been installed with a filter to bring it into compliance. The secondary MCL addresses the aesthetic characteristics of water(e.g. taste and odor). Organic Chemicals Ethylene dibromide(EDB), dibromochloropropane(DBCP) and perchloroethylene (PCE)have been detected at low levels in seven of the city's wells. EDB and DBCP, agricultural chemicals that have been used in the surrounding area, are below the MCL in all but one well,which has been fitted with an activated carbon filter to remove the EDB and DBCP. PCE has been detected at one well at levels below the MCL. Concentrations of EDB and DBCP are expected to decline over time, as they have not been used for 20 years. PCE is also expected to decline as use of this chemical is more closely regulated. Factors Affecting Groundwater Quality In general,groundwater quality can be affected by unlined WWTP solids storage basins, land disposal sites, on- site wastewater treatment systems(OWTS) such as septage pits, and agriculture,including irrigation, fish,and dairy farming. It is reasonable to suspect that leachate from these sources may contain elevated levels of inorganic ions and salts,nitrogen in the form of nitrate or ammonia, and potentially other contaminants such as trace metals or organic compounds. Soil particles provide an effective barrier to the transmission of many chemical parameters through physical filtration, chemical complexation, or biological decay processes. However,the specific constituents that may be discharged to groundwater, concentrations of constituents in leachate, and fate in the underlying aquifers can cause potential risks to groundwater. Surface Water Quality San Joaquin River Water Quality The water quality of the lower San Joaquin River drainage and the Sacramento—San Joaquin Delta has been heavily affected by human activities.Land use in the Sacramento—San Joaquin Delta is dominated by agriculture (about 538,000 acres),while urban and commercial property account for about 64,000 acres of developed land (DWR 2005). The use of pesticides and fertilizers has altered water quality,with pesticides sometimes reaching levels acutely toxic to sensitive invertebrates. Agriculture and urban/stormwater runoff are the predominant sources of contaminants in the San Joaquin Basin, as shown on Table 4.9-4. Agricultural return flows sometimes contain high concentrations of dissolved solids (i.e. salinity)and trace elements that can degrade water quality. Watersheds that feed the river and Delta contribute pollutants and stressors that produce or exacerbate water quality problems. Construction of the Stockton Deep Water Channel and channeling of the San Joaquin River system have created an area of reduced dissolved oxygen near Stockton. Contaminants on the 303(d)impaired waters list for the lower San Joaquin River drainage and the Sacramento—San Joaquin Delta are shown in Table 4.9-6. Data from the Delta monitoring locations of long-term comprehensive surface water quality evaluation programs shown on Table 4.9-11 were compiled for three different water years for eighteen parameters(see constituents in DER EDAW City of Manteca 4.9-7 Hydrology and Water Quality