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x 4p -+-eople per.household x 2 houses 1 person/2.08 Ac x 365 d/vr=4.0 in/yr rr7_&ajs/verson/dav 43,560 cu ft/Ac ft x 7.48 gal/cu ft x (I ft/12 in) Nw= 66 mg/L - 20% reduction in each of 2 septic tanks =40%reduction total = 40 mg/L N d = 10% from biomat and 25% from clay soil under leachlines. Nb = Assumed to be 0.1 ppm NO3-N 4.0 in/yr 40 mg N/L I - 0.35 + 13 in/yr x 0.1 m /L Nr = (4.0 in/yr+ 13 in/yr) Nr = 6.2 ppm as NO3-N or 27.6 ppm as NO3 IV. CONCLUSIONS AND RECOMMENDATIONS FOR IMPACT MITIGATION Analyses of soil under the Cole's leachfield reveals apparent denitrification occurring within the soil environment in a relatively short distance: 20%denitrification in 7 ft. It can be assumed that since there was virtually no ammonia-nitrogen(NH4), which is the precursor to nitrate formation, that only the organic fraction of the total nitrogen assessed can contribute to further nitrate formation. This is going to occur only under aerobic, ideal conditions, which are normally not found within the microscopic pore spaces of clay soil. With regard to the nitrate-nitrogen loading to the underlying groundwater, it is important to recognize that when adding one concentration of a solute (e.g.,NO3-1v in recharge water) to another concentration of a solute (e.g.,NO3-N in groundwater), where both solutes are in ppm,the result is not cumulative or the sum of the two solutes. Parts per million is a mass-to-mass ratio. For example, the Nr(the resultant average concentration of nitrate-nitrogen in recharge water in PPM of NO3-N) above, was determined to be 6.2 ppm. This equals 6.2 milligrams of nitrate in 106 milligrams of water(one liter). If this 6.2 ppm concentration is added to the same volume of water (I x 10'milligrams) with the concentration determined in the underlying groundwater from the Cole domestic well, which was 8.5 ppm NO3-N (38 ppm NO3), then the resultant concentration is now 6.2 milligrams+ 8.5 milligrams= 14.7 milligrams in 2 x 106 milligrams (2 liters, or parts per 2 million) of water. Therefore, to convert back to ppm, the numerator and denominator must be divided by 2 with the result of 7.4 milligrams per 1 x 106 milligrams, or 7.4 ppm. On a mass-to- mass ratio basis,there can be a dilution effect of nitrate concentration;from the resultant recharge water(Nr) as long as the underlying aquifer(s) have a higher nitrate concentration than the recharge water. And further degradation to waters of the State do not theoretically occur. To assess nitrate impact to the top aquifer(the water table) using the premise above,the nitrate concentration in the top aquifer under the Cole's residence must be lolown, which it is.not. Pinpoint sampling within the top mixing layer would be difficult to accomplish. The mixing layer is where the resultant recharge mixes with the groundwater. On a mass-to-mass ratio basis, the water table cannot exceed a nitrate concentration of: x+27.6 (Nr)-2 =45 ppm; x= 62 ppm NO3. Therefore,the top aquifer(water table) cannot exceed 62 ppm NO3 in order to stay below the drinking water MCL of 45 ppm NO3. Page -7- Na.Cfey Ag Research