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FDetermination of the theoretical ni&ogen contribution to the groundwater: % reduction in N (from above): 359 grams N/day - 80%= 72 grams N theoreticallynitrate-nitrogen/day. Dilution water available(avg water use), QAVE=920 gal/day(92 people x 10 gals)+avg gw flow(Q.,,.) Groundwater flow perpendicular to 100 linear ft of leachfield area= k(hydraulic conductivity of sat. zone) x (i - % grade) x 2.5 ft (top mixing layer)X 100 ft= QB,,,=2 X 10-3 ft/min (estimated) x (60 min/hr x 24 hrs/day) 0.5 x 2.5 ft x 100 ft= 360 ft3/day or 2,693 gals/day (10,206 L/day) CALCULATED NITROGEN CONCENTRATION TO GROUNDWATER UNDER LEACHFIELD= 72 grams/QAvE+QB,, = 72 g NO,-N/day_(3,487 L/day+ 10,206 L/day)=0.005 grams NO3 N/L x 1000 = 5.3 mg/L (ppm) NO3-N This assumes no nitrate contribution from the underlying groundwater. If the groundwater is assumed to currently possess a nitrate concentration of 37 ppm NO3(8.2 ppm NO3-N), then the top mixing layer groundwater under the leachfteld will exceed the Maximum Contaminant Level (MCL) of 10 ppm NO3-N by 3.5 ppm NO3-N: 8.2+5.3 = 13.5 ppm NO3-N. However, reincorporating the dilution factor will in all probability dilute this nitrate concentration back down below the MCL due to the tremendous volume of groundwater in comparison to percolating effluent. C. SEPTIC SYSTEM DESIGN/IMPACT MITIGATION The design of a future septic system for the facility is based upon the factors of operation, longevity, and environmental protection. The San Joaquin County Environmental Health Department will stipulate design requirements for environmental protection. SEPTIC TANKS Septic tank designs which affect the sludge, scum and effluent volumes, affect the treatment of nitrogen within the tanks. Therefore, two-2,000 gallon septic tanks in series may be specified by the Environmental Health Department based upon the capacity calculation below. The pump-out interval should be a specified time interval, or periodic inspection either by the Environmental j Health Department or a sewage contractor. CAPACITY: Required septic tank capacity: 2.4(QAvE)x Peaking Factor(PF) =2.4(920 gals/day) x 2.0 = 4,416 gal tank capacity required: Two- 2,000 gallon tanks may be used. DETENTION TIME: Detention Time =4,000 gal tank capaci xty 0.70 (30%volume lost to sludge/scum accumulation) 2 (Peaking factor) x 920 gallon/day Detention Time= 1.5 days. The minimum acceptable detention time is 0.5 day. 6 NaQey Ag Reseamh