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D. GROUNDWATER INFORMATION Groundwater depths and elevations throughout San Joaquin County are illustrated on maps published by the County Flood Control and Water Conservation District. Groundwater lines of equal depth for this area indicate the groundwater depth at less than 10 ft below grade. Along with the percolation test borings,a deep boring of 6 ft was drilled to determine water table depth. The standing water table depth was measured at approximately 5 ft below grade and was observed by EHD sanitarian Ted Tasiopoulos,REHS. A water table depth of 5 ft would preclude the installation of a conventional,or semi-conventional leachfield system. This is due to the required separation distance of five feet that must be maintained between the soil-effluent interface and the highest anticipated depth to the groundwater table. A second critical issue regarding the groundwater elevation in combination with the five-foot separation distance, is that the water table cannot encroach up the sides of the septic tanks,which may cause buoyancy. This is discussed in the"Conclusions and Recommendations"Section of this Report. Since surrounding properties have been intensely farmed for several decades,the potential for nitrate contamination in the underlying groundwater is significant. However,groundwater research in the scientific literature suggests there may be a large denitrification potential within aquifers or saturated zones composed of clay soil fraction. Groundwater Mounding Analysis The document California State Water Resources Control Board-Guidelines for the Design, Installation and Operations of Mound Sewa¢e Disposal Systems states in Section 31.0 that in cases where peak wastewater flows for a commercial system exceed 1,500 gallons per day,an evaluation of hydraulic mounding below the disposal area shall be performed. The encountered depth to groundwater can be considered shallow and may induce a phenomenon known as the"mounding effect'whereby percolating effluent encounters the water table,or a restrictive stratum and cannot disperse laterally fast enough. Consequently,a mound forms under the disposal field possibly creating saturated flow conditions and decreasing the distance the effluent must travel under unsaturated flow for effluent treatment to occur. An equation developed by Finnemore and Hantzsche(1983) is used below to predict the long-term maximum rise of the mound: h=H+Z,a=2 where: h=distance from boundary to mid-point of the long-term mound, in ft H=height of stable groundwater table above impermeable boundary, in ft Z.=estimated long-term maximum rise of the mound, in ft Substituting known and estimated values for the variables,we find the following: H =The height of stable groundwater above an impermeable boundary is estimated to be 4 ft based upon the measured standing water depth in the test boring. Therefore, it will be assumed that a boundary exists at H=5(Highest measured water table depth). Long-term maximum rise of mound is estimated at 0.5 ft. Therefore, h= 1 +(0.5- 2)= 1.25 4