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IV. CONCLUSIONS AND RECOMMENDATIONS FOR IMPACT MITIGATION From the well water data collected and analyzed, four observations are made: 1.) There is definite and continuing nitrate impact which originated from the Southland Mobile Home Park. This was evidenced by the two wells sampled on the Highway 99 Frontage Road. The well located at 15141 S. Hwy 99 has a higher nitrate concentration (58 ppm) and is directly in downgradient line from the Mobile Home Park according to the calculated groundwater directional flow, than the well located at 15257 S. Hwy 99 (53 ppm). Therefore, the nitrate contaminant plume is theoretically elongated. There may be some influence from the Park to the well at the Lathrop Road Off-ramp (29 ppm). 2.) The well at 15393 Cottage Avenue exhibited high nitrate (47 ppm)presumably from F septic system influence originating from the comparatively high density residential structures in this area. - 3.) The Sephos well at 15444 South Mitchell and the well at 8992 Southland Avenue must be drawing from the same aquifer(s) since both have the same nitrate concentration o_f-1_8 1 ppm _T e Sephos well is perforated from 90 to 170 ft. Well construction details are unknown for the Southland Avenue well. 4.) The well at 15655 had the lowest nitrate concentration at 9 ppm and is perforated from 120 ft to 140 ft. Four assumptions are made from this well water information: 1.) Although the Southland Mobile Home Park was hooked up to Manteca Public Sewer in the mid 1970s, there remains continued nitrate impact. In 1983/84, the nitrate problem originating from the Mobile Home Park was discovered. It cannot be predicted as to when the nitrate concentrations will begin to subside. It is assumed levels remain high due to a.) the massive, original influx of nitrate, b.) the groundwater velocity is comparatively slow in this area, and c.) there are no high capacity wells drawing from the upper aquifers downgradient from the Mobile Home Park. 2.) The three wells with the highest nitrate concentration are older wells, although exact age and construction details are unknown. It is assumed that they are shallow.wells (< 100 ft , deep) and there is either no sanitary seal or a shallow seal of 25 to 50 ft in depth. 3.) Nitrate originating predominantly from septic system effluent appears to be congregating withi�p ers a uifers Ltj thin 80 ft of the surface and im acting shallow wells drawing this water. 4.) It appears there is sufficient clay strata to attenuate the downward migration of nitrate to 7 the deeper aquifers below -100 ft. Vasey Ag Research