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a April 7, 2006 NOA Project Number:E05144B Page 2 too low and 300 gpd per dwelling is about right based on your department's monitoring of in scenarios 300 d per dwelling unit will be used and 40 several houses. In the nitrate loading � mg/L-N will be used for effluent concentration. A denitrification factor of 25 percent would continue to be used. A separate nitrate loading scenario for a primary residence and a primary with SDU for the parcel were performed. A new recharge factor of 5.6 inches representing rain fall and stream infiltration was calculated for the entire San Joaquin County. CALVIN, a new ground water model developed for the State, documents information which could be used to determine an average recharge factor. In the ground-water model, Ground Water Basin 6 includes the area of San Joaquin County where the numbers were derived. I The acreage of the ground water basin,the volume of rain recharge, and volume of stream infiltration(recharge)were used from the groundwater model doculnentatiori: This information was reduced to determine a specific recharge factor for the Hantzshe and Finnemore Equation within the County. The data used to determine the recharge factor is an average of historic records dating from 1920 to 1990 for the basin. The recharge factor, 5.6 inches, would represent rain-related recharge within San Joaquin County as a whole over the long term. The calculation sheet and reference is attached to this letter. Every assumption has positives and negatives. Regarding the new recharge factor,the positive is it is based on historic records and information documented and used in the model. The recharge factor is also based on long term averages. However, since it is a County-wide average and the County is fairly large, the number, when used in the Hantzshe and Finnemore Equation, may underestimate impacts in the Tracy area and may overestimate impacts in the Lodi area due to differing precipitation rates within the County (about 6 inches difference). It is our opinion that both rain fall and stream losses (from rain runoff and snow melt) do occur in the County and should be considered as related recharge. We believe that using 5.6 inches is a reasonable choice for recharge when better information is not available. A summary of the revised nitrate loadiTrg-calcuMtion-lis-presenteFdnexr:-- Surnrnaa of Nitrate Mass Balance Analysis Result Acres Average Daily Flow Effluent Concen- Leachfield Deep Percolation of (Nc) * (A) (Q) (Table 1) tration(Nw) Denitrifi- Rainfall(R)(in.) (mg/L-N) (gpd) (mg/L-N) cation (d) 8.5 2.05 300 40 25% 5.6 Primary Res.only 14.6 2.05 600 40 25% 5.6 Prima Res. and SDU * Maximum contaminant level(MCL) for nitrate in drinking water is 10 mg/L-N.