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Although perc rates under I min/in are normally deemed unacceptable, it is our opinion that the formation of the biomat will ultimately determine the effluent long-term acceptance rate (LTAR). This will be discussed in Section C of this report. D. GROUNDWATER INFORMATION Static groundwater measurements were taken on November 20, 1998 from an irrigation well directly north of the on-site residence and from the domestic well serving this residence. The static water level in the irrigation well was measured at 42.98 ft from the top of the observation port. The water level at the residential well, which is presumed to be a prior 18" irrigation well was measured at 37.65 ft from the top of the well casing. The elevations of the tops of the wells do not differ by more than two feet. Unfortunately, well logs for these wells could not be obtained and it is assumed they are perforated in different aquifers. The previously referenced geotechnical investigation found relatively moist soil at approximately 22 ft below grade. This may be the capillary fringe of the phreatic zone, due to influence from the Mokelumne River. However, it is our opinion that based upon the static water level well measurements, the sampled soil at the 22 ft depth was probably intermediate vadose zone water and not capillary water. The groundwater under the subject property is being influenced by the Mokelumne River due to the proximity of the project. It is unknown to what extent the river influence continues northwards without the installation of several groundwater monitoring wells and/or piezometers. As with all groundwater tables, there are fluctuations in elevation. The critical issue with regards to the underlying groundwater elevation is that there is sufficient distance between the proposed .� leachlines and the highest anticipated depth to groundwater for wastewater treatment to occur. Due to the topography of the subject property whereby the elevation drops approximately 15 ft from the proposed leachfield area to the water surface of the river, it is assumed the groundwater directional flow is towards the river. The science of river influence on groundwater and aquifers is extremely complex, and beyond the scope of this study. To determine the extent of past nitrate impact to the underlying drinking water aquifers, water samples were taken from the on-site residence and from a residence at the west end of Winery Road, one mile to the northwest of the project. The well at the on-site residence contains a nitrate concentration of 10 ppm and the residence on Winery Road contains <1 ppm. These results indicate there has been virtually no nitrate impact to the respective aquifers. E. SURFACE WATER INFORMATION There is a small body of water directly to the north of the Mokelumne river which is referenced _ as a lagoon. This lagoon fills up when the river reaches an elevation which allows water to gravity flow into it, which is normally in the springtime from snowmelt runoff. If this lagoon retained water year round, it could be used to monitor potential impact from leachfield effluent. 3 va!!ey Ag Rgsearch