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s f k'9 KILEINFELDER t k 1 :. HISTORICAL GROUNDWATER DEPTH In accordance with groundwater reports published by the San Joaquin County Flood Control and Water Conservation District (Spring 1972 to Spring 1999), high groundwater in the project area has been recorded at depths in the range of about 70 to 95 below ground surface. FIELD EXPLORATIONS AND TESTING The subsurface conditions at the leach field were explored on September 6, 2000 by;drilling one boring to a depth of about 15 feet below existing grade. The boring was drilled using a Simco 2400 truck-mounted drill rig equipped with 4-inch O.D. solid-stem auger. Also on that day, two percolation borings were installed to depths of about 3.1 and 9.9 feet below site grade'. Based on our findings, the subsurface soils encountered consisted predominately of moderately plastic silty clays to a depth of about 5 feet, underlain by sandy clays to a depth of about 8 feet. The clays were underlain by sandy silts to the depth explored. The p ercolati on.tests were performed in accordance with accepted guidelines using clear potable water. The procedure calls for first placing about 2 inches of fine gravel at the bottom of the bore hole. The hole is thenp re-soaked with water at least 24 hours prior to performing the test. The test,is performed by first adding 6 inches of water above the gravel. From a fixed reference point, the drop in water level is measured in approximate 30 minute intervals for:',four hours, refilling as necessary to maintain about 6 inches of water over the gravel. Under theirobservation of a sanitarian from the- San Joaquin County Public Health Department, the last 30-minute reading of,the 4-hour period is recorded and used to calculate the percolation rate in!�minutes per. inch. The estimated percolation rates at each test location are as summarized.below. A copy of our field percolation test results is attached. "46 Test;Nii ,�-jgi gjlli;De th` 'ft. ;i l`Percolation Rate (miuJinchF P-1 3.:1a No Percolation { P-2 9.9 15 NI'T'RATE LOADING CALCULATIONS For new developments or property subdivisions, the San Joaquin County Public Health Services I Department (SJCPHS) of the Environmental Health Division requires a study to evaluate if discharge from an on-site sewage disposal system at the property could load or "increase the nitrate levels in groundwater above the state drinking water MCL standard. As you may be aware, the drinking water MCL standard for nitrate as NO3 is 45 mg/l and for nitrate-nitrogen as N is 10 mg/l. For the purpose of our evaluation, we have relied upon-the application of an equation-iritendeff,fo-predict the level of nitrate in groundwater often referred to as the Hantzsche 1/mass,.balanceFeq{$iation lthat is presented below with an explanation for each factor and the values used in our study. ` NOV 27 2000 N,=AVWNw, (I-D) +(NN- r AW+r GRAM f r:uuti6 i rw�ti,i iON CO. STOCKTON Hantzsche N.,Finnemore J (1992),Predicting Ground-Water Nitrate Nitrogen Impacts, Ground Water, Vol. 30,No.4 20-1108-31.G02/200ORI099 November 21,2000 ©2000 Kleinfelder,Inc. Page 2 of 4