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NNW — k'q KLEINFELDER i D – Percent dentnfication of leachfield effluent = 0.3 percent (0.1 to 0.35 percent typical I�, -- per Hantzsche and Finnemore (1992)). 0.3 selected given the interbedded clay strata and the depth to groundwater. .: – R = Annual deep percolation of precipitation (inches) = 11.6 inches (representing 70 percent of the average annual rainfall of approximately 16.5 inches. The remaining 30 percent of rainfall is assumed to drain off-site into natural or manmade drainages r ' , or be lost to evapotranspiration.) _ Nb = Background nitrate nitrogen concentration of rainfall recharge at the water table = 0.5 mg/l (per Technical Memorandum, Hydrologic and Soil Investigation for the Chico Urban Area, page 6, Dames and Moore, April 1996) Our calculation is as follows: N, = 0.8(2)(40}(1 --0.3 + 0.5(1 l.� – 44.8 + 5.8 = 3.8 mgll as N 0.8(2) + 11.6 1.6 + 11.6 8. CONCLUSIONS The results of our percolation tests indicate that a suitable soil percolation stratum exists at depths of about 3 to 3'/z feet below existing site grade at the percolation test locations in Lots 1 through 4, 9, 11, 14, 16, and 17. A suitable soil condition for tests at this depth is defined as a stratum capable of percolating water at a rate of 60 min/inch or faster. Our tests indicate that in the remaining lots (Lots 5 through 8, 10, 12, 13, and 15), suitable ' strata were encountered at the depths of our deeper percolation tests. A suitable soil condition for tests deeper than 3'/z feet is defined as a stratum capable of percolating water at a rate of 30 minlinch or faster. Based on these results, conventional septic tank/leach trench sewage disposal systems designed per SJCEHD criteria should adequately serve the proposed residences constructed on these lots. Any seepage pits should extend into the more sandy soils that were encountered in our borings and in the passing percolation test holes. Based on our results we do not anticipate a need for engineered systems to be installed at the site. As previously discussed, we calculate 3.8 mg/i of nitrate-nitrogen (N) in groundwater resulting from the proposed leach fields, a result that is below the state drinking water standard of 10 mg/l. Based on this level of nitrate-nitrogen, we anticipate that the impact on surrounding wells will be low. Jahant Slough is located immediately adjacent the study area. Due to the depth to groundwater (greater than 60 feet) and the shallow nature of the creek, we anticipate that the impact to surface water will be low. The proposed leach fields should be setback from the slough in accordance to all SJCEHD setback requirements. 52709,GOIIST05R550 May J,2005 ©2005 Kleinfelder, Inc. Page 6 of 7