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k� KLEINFELDER R = Annual deep percolation of precipitation (inches) = 7.5 inches (representing 75 percent of the average annual rainfall of approximately 10 inches. The remaining 25 percent of rainfall is assumed to drain off-site into natural or manmade drainages 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, = 0.03(2)(40)(1 —0.1 + 0.5(7.5) = 2.2 + 3.8 = 0.8 mg/1 as N 0.03(2) + 7.5 0.1 + 7.5 8. CONCLUSIONS The results of our percolation tests indicate that the near-surface soils at the locations of test holes P-1 and P-3 exhibit suitable percolation characteristics. A suitable soil condition for a test between 3 and 3%2 feet is defined as a stratum capable of percolating water at a rate of 60 min/inch or faster. The near-surface soils at the location of test holes P-2 did not meet this criterion. �� „Q��✓ � r As previously discussed, we calculate 0.8 mg/1 of nitrate-nitrogen (N) in ground*ater resulting from the proposed leach fields, a result that is below the state drinking water standard of 10 mg/1. Based on this low level of nitrate-nitrogen, we anticipate that the impact on surrounding wells will be low. Old River is the nearest water body to the site and is adjacent to the property on the south and east. As stated previously, the water table at Union Island is controlled by collecting groundwater in a series of ditches and pumping it into the surrounding rivers and sloughs. Due to this method of groundwater control, some nitrates from septic systems will likely be pumped into the surrounding surface water. However, due to the low level of nitrate loading, we anticipate that the impact to surface water will be low. SJCEHD requires a minimum separation between the bottom of the leach line and the water table of 5 feet. Due to this required separation, conventionalptic tank/leach trench sewage disposal systems will not be permitted at the test locations in rrcels A and B,-)nd depending on the depth of the leach line, possibly not in parcel C. It is our n that mounded sewage disposal systems des jgnper SJCEHD criteria would adequately serve new residences_ at dZ may be.&cQnstructed. s an alternative to a mounded system, building pads are often raised when houses are built in the_delta. raise su icten y-to pf6v-i�de-adequate separation from groundwater, standard septic tank/leach trench sewage disposal systems designed per SJCEHD criteria may be allowed, provided acceptable percolation rates can be achieved in the raised pads. 48067.G01/ST04R1257 Page 6 of 8 ©2004 Kleinfelder,Inc. September 29,2004 `.