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a IMPACT MITIGATION Nitrate-nitrogen impact from a future septic system expansion must be mitigated by the following design: First,by using two - 1,600 gallon septic tanks in series,there will be a higher percentage of solid organic material trapped within the tanks. Additionally, a filter should be installed on the last tank(e.g. Zabel®or OSIC Filters)before the effluent flows (or is pumped) _ to the leachfield. There is evidence that septic tanks have the capability to denitrify nitrate: As influent enters the first tank, the wastewater has been aerated from flushing action, and from the aerator on a faucet, etc. But as the oxygen is consumed by microbiological processes, the tank becomes anaerobic and the microbes begin to use nitrate as an electron acceptor. When the effluent reaches the second tank, there is virtually no oxygen within the tank contents, thus theoretically promoting denitrification `I . I' ,AC,IA GKtS / 6 - The leachlines should be placed as hif 01.0 _k n' L line trench, preferably 12 inches below grade. Filter beds may I C f r,„ However, there must be a 100% replacement area incorporated i IV. CONCLUSIONS AND Ith 1v11v1r,1N1.ri i IONS It is our opinion the subject site is suitable for its intended use with regard to septic effluent management and that potential environmental impact from future nitrate-nitrogen loading has been mitigated to an acceptable extent. The percolation rates obtained indicate a permeable soil. Effluent management is based on the permeability of the soil eventually reaching an long term application rate (LTAR) of approximately 0.5 to 1.0 gals/ftl/day due to biomat formation. It is imperative that if a repair is made or an additional system installed, the underlying hardpan layer must be completely breached and removed before leachtine emplacement. Sumps should also be used if there is an increase in the number of people using the system, leading to greater wastewater flows than those anticipated in this report. Without sophisticated groundwater monitoring over a long period of time, it is unknown as to the exact groundwater depth at different times of the year, as well as the groundwater directional flow. However, sufficient data has been collected and analyzed to determine that the groundwater table is currently an acceptable distance below grade for there to be adequate biological treatment, according to published scientific literature. Groundwater analyses of the on-site domestic well indicates a medium nitrate impact to the underlying drinking water aquifers. Mounding effects under the leachfields should be of no concern since the saturated zone is at a sufficient depth. 6 Vaffey Ag Rgsearch