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4_. 056 <br /> agencies or firms with expertise in this field. Detailed inspections of the <br /> soil morphological characteristics to a depth of at least 6 feet below the <br /> infiltrative surface must be conducted and a full evaluation made of both <br /> the vertical permeability characteristics and the horizontal flow potential <br /> of the site. Some of the specific questions which the applicant should be <br /> required to address include: (a) the depth of the restrictive horizon; (b) <br /> the soil permeability above the restrictive horizon; (c) the extent of <br /> mounaing that can be expected to occur. <br /> 2. The preliminary design has been based upon a soil hydraulic loading rate <br /> (HLR) of 0.1 gallon of wastewater per square foot per day. This rate may be <br /> acceptable for subsurface absorption systems serving single family units on <br /> lots which have an assimilative area of one acre or more. Based on <br /> discussions with other parties who design these systems, CXWASA recommends <br /> that an HLR not exceeding 0.075 gallons per square foot per day be utilized <br /> in designing the proposed system, provided there is sufficient documentation <br /> showing that the design loading rate will not result in surface or ground <br /> water contamination problems. <br /> OWASA staff has completed a basic evaluation of the hydraulic loading rate <br /> and nitrogen loading rate for the proposed system. The proposed system <br /> would be designed to receive nearly 2.15 million gallons of wastewater per <br /> year. This equals an application rate of about 59 inches of wastewater per <br /> year over the proposed absorption area. That rate is about 1.2 times <br /> greater than the annual rate of precipitation for this area and would be in <br /> addition to rainfall absorption. <br /> Assuming the wastewater has a Total Nitrogen concentration of 25 parts per <br /> million (the actual concentration will probably be greater) and it is <br /> applied over an absorption field area of about 1.35 acres as proposed, more <br /> than 330 pounds of nitrogen per acre per year would be applied on the site. <br /> Even if it were assumed that the vegetative cover on the absorption field <br /> surface could utilize a substantial amount of that nitrogen, that rate would <br /> greatly exceed the expected rate of uptake by any established cover crop. <br /> Fescue 31, a common grass cover, has an uptake rate of approximately 150 <br /> pounds of nitrogen per acre per year. Even if Tifton 44 Coastal <br /> Bermudagrass is used to provide about the highest nitrogen uptake rate that <br /> could be expected for grass cover crops in the Piedmont (300 pounds per acre <br /> per year) , the nitrogen loading rate would be considerably more than the <br /> plant uptake rate. However, actual plant uptake rates would be :much lower <br /> because subsurface wastewater disposal is utilized. <br /> The excess nitrogen not taken up by plants will be carried away from the <br /> site by surface water runoff and/or seepage into the groundwater table. <br /> Higher nitrogen levels in surface waters can lead to algal growth and <br /> increased eutrophication in University Lake as well as bitter taste in <br /> drinking water. The present drinking water standard for nitrogen is 10 <br /> parts per million. Excessive nitrogen concentrations have been reported to <br /> cause health problems in infants and children. Similar concerns exist <br /> relative to excessive loadings of chlorides and sulphates. <br /> -2- <br />