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IV. CONCLUSIONS AND RECOMMENDATIONS FOR IMPACT MITIGATION The Hantzsche/Finnemore Formula predicts a resultant average concentration in the recharge water from the proposed project to be 9.8 ppm as NO,. Based upon this Formula, the nitrate impact from the proposed development will be considerably lower than the Maximum Contaminant Level (MCL) of 45 ppm NO,. If it is assumed that the underlying aquifers possess a nitrate concentration of approximately 4 ppm, then the dilution effect will certainly lower the resultant nitrate concentration contributed by the effluent. The percolation test results show the typical leachline depth soils at approximately 42 inches in depth are a tight, compacted hardpan with small porosity. Therefore, seepage pits and sumps are warranted in this area and must be used for this project. The pert test results obtained from the deep perc tests indicate the need for seepage pits. Due to biomat formation, the long term acceptance rate (LTAR) for the seepage pits should stabilize at approximately 0.5 gallons/ft'/day according to the scientific literature. 'Therefore, LTAR for each pit should be: 243 W/3 ft dia. x 25 ft deep pit = 121 gallons/day). Although the shallow percolation tests for each proposed parcel indicated deficient or non- existent percolation rates, leachline trenches can be constructed to take advantage of beneficial properties of the clay surface soils. It is recommended that leachlines trench sidewalls be opened with teeth on the sides of the backhoe bucket. The teeth on the cutting edge of the bucket can be used to scarify and open the bottom of the trenches to the maximum extent possible. This will allow some effluent infiltration and percolation within the leachline trenches and promote denitrification within the anaerobic microsites of the clay soil. If the proposed project is approved, it will be the requirement of PHS-EHD to extend the grout seal in each of the four new wells to at least 200 feet, and possibly 300 feet due to the future uncertainty of leachate emanating from the Harney Landfill. Septic system failure would primarily result from the advanced formation of the biomat or "clogging mat" on the wetted soil/effluent interface. This mat is composed mostly of aerobic and anaerobic bacteria and bacterial products consisting of a slime of polyuronides and polysaccharides. If the soil/effluent interface is well aerated, a variety of soil microfauna such as nematodes and protozoa may digest the clogging mat bacteria. To lessen biomat formation, filters may be installed in the second chamber of each septic tank and consideration should be given to having no garbage disposals in the new houses. The total length of leachline will be dependant upon the number of bedrooms each new home will have. Additionally, the system should be oversized if a number of people, especially children, will inhabit the homes. A 50% replacement area must be incorporated into the design. It is important to note that all septic systems have a lifespan. 'Phis lifespan is dependant on several factors and is therefore difficult to determine. It is recommended that a financial "sinking fund" be initially established to assist in paying for a possible repair/replacement. An accountant can help with establishing this fund. 8