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D. CALCULATED IMPACT FROM NITRATE-NITROGEN LOADING The calculated (or estimated) nitrate impact to the soil and ultimately to the groundwater is extremely difficult to determine due to the number of variables involved. A significant variable found in the Hantzsche/Finnemore equation below is the average number of people that occupy households in San Joaquin County. Found in Appendix 6, is the U.S. Census Bureau data illustrating that the "average household size" for San Joaquin County consists of 3.00 people, which is used in the equation. The following calculations are based on using a semi-conventional type of septic system consisting of two septic tanks in series, a leachfield using Infiltrator® Chambers and seepage pits at the ends of the chambers. HANTZSCHE/FINNEMORE EQUATION TO QUANTIFY THE NITRATE-NITROGEN IMPACT POTENTIAL FROM ON-SITE SEPTIC SYSTEMS INw(1 -d)+R Nb Nr= (I+R) Where: Nr= Resultant average concentration of nitrate-nitrogen in recharge water-(ppm NO3-N) I = Volume rate of wastewater entering the soil averaged over the gross developed area in inches per yr. Nw= Total nitrogen concentration of wastewater in N mg/L (66 mg N/L/day) d = Fraction (%) of nitrate-nitrogen loss due to denitrification in the soil R = Average recharge rate of rainfall - 8 inches per yr Nb= Background nitrate-nitrogen concentration of rainfall recharge, exclusive of wastewater influences (ppm NO3-N) Values and Assumptions 68 galE/?erson/day x 3 people per household x 6 new houses- 18.96 Total Ac x 365 d/ —0.868 in/yr. 1= 43,560 cu ft/Ac ft x 7.48 gal/cu fl x (1 ft/12 in) Nw=66 mg/L - 20%reduction from two septic tanks in series for each new home= 53 mg/L N d 10% from biomat formation and 10% from saturated clay soil conditions under chambers. Nb= Assumed to be 0.1 ppm NO3-N 0.868 in/3(53 mg N/L) (1 - 0.20) + (8 in/)E x 0.1 mg/L) Nr = (0.868 in/yr+ 8 in/yr) Nr = 4.2 ppm as NO3-N or 18.7 ppm as NO3 LL 9 Chesney Consulting