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D. GROUNDWATER 'INFORMATION NLS§ 2.1,._SSS§4:T.: Groundwater depths and elevations throughout San Joaquin County are illustrated on maps published by the County Flood Control and Water Conservation District. However, due to the high degree of variability in groundwater elevations iii the Tracy area, groundwater lines of equal-depth and equal elevation are not drawn for this area. The last map denoting lines'of'equal elevation for the Tracy area was published in 1992, and show the general groundwater flow to be in-a northeasterly direction, toward the San.Joaquin River. However, recent studies of the:groundwater flow under the project site; indicate the flow direction to be in a westerly direction: The on-site-domestic well, which is approximately 175 feet north-northwest from the proposed disposal area, was sounded with the standing water level measured at 9:0 feet from the top of the Vii?casing. This corresponds with the backhoe test pit water table depth determination. The backhoe test pitwas used to measure depth, and sample the water table. The static water. level was determined to'be at 9.5 ft in the proposed location of the effluent disposal area. This.. groundwater depth measurement occurred at the beginning of irrigation season; so the potential may exist for the water table-to rise slightly higher at the end of the irrigation season. With. , concurrence from EHD,a water table depth of 9.5 ft is sufficient depth to install a conventional filter bed leachfield system to a maximum depth of 42 inches. This is due to the required separation distance of five feet that must be maintained between the soil-effluent interface and the highest depth to the groundwater table. Conversations with representatives of Schack& Company anticipate that there will be very little change between the current filter bed area ground elevation and the final elevation after project construction. The following calculations demonstrate the five ft separation distance: 9.5 ft— 114 inches minus 5 ft.(60 inches) = 54 inches. The maximum depth of the filter bed is not to exceed 42 inches; therefore,there:is a 54 - 42,= 12 inch safety factor. -Potential rise in the water . table elevation from "mounding" of the percolating effluent is calculated°to be nonexistent and discussed on the following Page. Another critical issue regarding'the groundwater elevation in combination with the five-foot, separation distance, is that the water table cannot encroach up to the septic tank, which may cause . buoyancy. This is discussed in the "Conclusions and Recommendations" Section of this Report. No well log was found in EHD files for the referenceddomestic well, so underlying lithology could not be determined. In addition, the well.had no electricity to it, so well water could not be sampled and tested to determine nitrate loading impacts from historical on-site and surrounding land uses,or to use in the nitrate loading calculations. NLS§ 2.2,3.1/SSS§ 2 1, 4.2. _Since surrounding properties have been intensely farmed for . - 1. 1, decades,the potential for-nitrate.contamination in the underlying groundwater can be considered to be significant. However, groundwater research:in the scientific literature suggests there may be a large denitrification potential within aquifers or saturated zones composed of a fine clay soil fraction, which was encountered at the.1 I ft depth (See Table 1). Chesney Consulting