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23 October 2020 AdvancedGeo <br /> Project No. 20-6169 An Employee-Owned Company <br /> Page 10 of 15 <br /> beneath the subject property appears to flow to the southwest. Groundwater was not <br /> encountered in the 42-inch deep percolation test hole advanced on 15 October 2020. <br /> 8.2. EXISTING OR POTENTIAL GROUNDWATER CONTAMINATION <br /> The immediately adjacent and outlying land use for the parcel is residential. Potential <br /> groundwater contamination can occur from several sources including leaking <br /> underground storage tanks, septic systems, agricultural activities, dairies, and stormwater <br /> infiltration. <br /> A review of the DTSC EnviroStor Database and the Geotracker Web site did not reveal <br /> any sites of environmental concern within a half-mile radius of the site. Based on the <br /> information reviewed, there is no existing or known groundwater contamination at the site <br /> and the potential for groundwater contamination is de minimis. <br /> 8.3. CURRENT AND FUTURE USE OF GROUNDWATER AT THE SITE <br /> The existing water supply well at 17835 Tretheway Road (site) conforms to DWR and <br /> EHD standards and will continue to be used to supply the subject property with a domestic <br /> water source. <br /> 9.0. IMPACT OF NITRATE TO GROUNDWATER <br /> Due to the planned residential use of the property and the potential for nitrate leaching <br /> into the groundwater from the proposed septic system, a nitrate loading analysis for the <br /> future use of the septic system is required by the EHD. <br /> 9.1 . CURRENT OFF-SITE SOURCES OF NITRATE <br /> Septic systems and agriculture are sources of nitrates in the area of the site. The property <br /> is bordered by agricultural and residential land, which might provide a nitrate source. <br /> There are several residences within 1,000 feet of the site. <br /> 9.2. IMPACT OF RAINFALL CONTRIBUTION TO NITRATE LOAD <br /> The rainfall of the area has been estimated for the nitrate loading analysis. Deep <br /> percolation is estimated by calculating the average monthly rainfall and <br /> evapotranspiration of the site area. If average monthly rainfall exceeds average monthly <br /> potential for evapotranspiration (ETo) there is a potential for deep percolation without <br /> consideration for runoff. Average potential ETo was estimated from data obtained from <br /> the California Irrigation Management Information System (CIMIS) Manteca Station #70. <br />