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Soil Suitability Nitrate Loading Study Manteca Unified School District 14600 Austin Rd.,Manteca,California Page 9 Table 4 S mmary of Nitrate Mass Balance Analysis Result Acres Average Daily Effluent Leachfield Deep Percolation Parcel' (Nc)* (A) Flow Concentration Denitrification of Rainfall (mg/L-N) (Q)(gpd) (Nw)(mg/L-N) (d) (R)(inches) 1 8.4 16.95 1,361 100 35% 8.31 students/faculty No increase in school population is anticipated as a result of modernization project. Notes: *Maximum contaminant level(MCL)for nitrate in drinking water is 10 mg/L-N; mg/L=milligrams per liter;gpd=gallons per day. Based on the nitrate loading balance calculation,nitrate levels in leachate from the proposed septic tank for proposed Building H for the proposed use of 1,361 gallons per day is 8.4 mg/L, which is below the maximum contaminant level (MCL) of 10 mg/L for nitrate-nitrogen (nitrate-N) in drinking water. The school has an existing student enrollment plus staff of 581,which is not projected to increase as a result of the project. As indicated previously in this report, the Hantzsche and Finnemore method "ignores dispersion, lateral flow, and mixing with groundwater flow from up-gradient areas. These processes would generally contribute to additional reduction of nitrate-nitrogen N concentrations in groundwater to the extent that the nitrate-nitrogen concentration of groundwater flow from up-gradient areas is lower."Thus,the approach is "...a conservative(worst case)first approximation of groundwater nitrate-nitrogen concentration resulting from the combined effect of on-site sewage disposal systems and precipitation." 2.3.6 Calculated Impact of Nitrogen Loading from Site Project to Down Gradient Wells, Rivers, Lakes,Etc. There are no known rivers or lakes near the Site. The nearest known water supply wells are located more than 500 feet downgradient of the proposed leach lines. Since a significant clay layer was logged at a depth of approximately 15 to 17.5 feet bgs, and groundwater exists at a depth of approximately 45 feet bgs, it is unlikely that nitrogen loading will directly impact the wells. Further, the Site is located within an area dominated by agriculture activities, which is the primary source of nitrate impact to groundwater. 2.4 DISCUSSION OF NITRATE LOADING RESULTS RELATIVE TO PROPOSED METHOD OF WASTEWATER DISPOSAL 2.4.1 Total Nitrate Impact from Proposed Project The nitrate MCL set by the US EPA is 45 mg/L-NO3 or 10 mg/L-N.This report concludes that the proposed modernization project at the New Haven Elementary School, if operated at the anticipated capacity of approximately 1,361 gallons per day effluent into the proposed septic system, may result in an estimated nitrate load of 8.4 mg/L nitrate-N,which is below the groundwater drinking water standard of 10 mg/L-N. One must consider that nitrate-N concentrations in groundwater inflow from up gradient areas may also increase over time from other land uses such as agriculture and increased rural residential use. Nitrate in ground water occurs as a result of the application of fertilizers, livestock waste, and untreated septic tank waste. Nitrate is mobile and often accumulates in the shallow ground water zones. The proposed development is located in an area where shallow groundwater currently sits at approximately 45 feet bgs and is separated by septic systems at the Site by a significant clay layer at a depth of approximately 15 to 17.5 feet bgs. Despite the location in an agricultural area, the Site well installed in 2001 has not had any violations with respect to nitrates or coliform bacteria, according to available GAMA records. ;� CONDOR