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UEC Document No.: R3579901 <br /> February 8, 1999 <br /> of the aquifer is calculated as the weighted average of nitrate-nitrogen concentration of percolating rainfall <br /> and waste water, adjusted for expected losses due to soil denitrification. The expression for the resultant <br /> average concentration, nr, of nitrate-nitrogen in recharge water is given by: <br /> n, = [ I *nW *( 1 —d) +R *nb ]/(I +R) <br /> Where... <br /> nW: total nitrogen concentration in waste water [mg/liter] <br /> nb: background nitrate-nitrogen concentration of rainfall recharge at the water table [mg/liter] <br /> d: fraction of nitrogen-nitrate loss due to denitrification in the soil <br /> I: volume rate of waste water entering the soil [inches/year] <br /> R: average recharge rate of rainfall [inches/year] <br /> Of more practical use to the septic system designer /planner is the determination of the carrying capacity of <br /> the land. From the standpoint of groundwater nitrate-nitrogen impacts, the critical minimum gross acreage <br /> per developed lot, G, may be defined as that which would result in a value of nr equal to 10 mg /liter, the <br /> commonly accepted drinking water limit.' By setting I =0.01344 W/A (0.01344 is a conversion factor <br /> having the units of dwelling units per year per gal) and nr =10 mg /liter, and then rearranging the above <br /> expression,the critical minimum gross acreage per developed lot, G, may be calculated as follows: <br /> G = 0.01344 *W *[ nw—d *nw —10]/[ R* (10-nb)] <br /> Where... <br /> W: average daily waste water flow [gallons] <br /> R: average recharge rate of rainfall [inches/year] <br /> The SJC Flood Control District has reported that rainfall data collected at the Eight Mile Road /Bear Creek <br /> data collection station indicates that an average of 21.6 inches of rain has fallen during the past three rainfall <br /> seasons; consequently, R will be set to 21.6 inches /year. The table presented below represents the <br /> _ calculated denitrification zone square footage required to limit the average concentration of nitrate-nitrogen <br /> in recharge water, n,, less than 10 mg/liter. Based on the soil conditions existing at the site and research <br /> data correlated with predicted values in the study performed by Hantzche and Finnemore, the following <br /> values are believed to be representative of the subject site conditions: d =0.20, nw =25 mg/liter, nb =1.0 <br /> 6 <br />