Agenda - 08-18-1987 - Laserfiche WebLink

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A • The design factors utilized should be based on a detailed site analysis of soils, topography, hydrogeology, long-term acceptable rate of soils, system hydraulic loading, and anticipated agronomic rates of uptake. Prior to the consideration of a permit for the proposed system, a detailed site evaluation must be performed by a registered professional soil scientist/hydrogeologist experienced in the siting and design of large soil absorption facilities, with appropriate assistance provided by the Agricultural Extension Service, the U.S. Soil Conservation Service and other agencies or firms with expertise in this field. Detailed inspections of the soil morphological characteristics to a depth of at least 6 feet below the infiltrative surface must be conducted and a full evaluation made of both the vertical permeability characteristics and the horizontal flow potential of the site. Some of the specific questions which the applicant should be required to address include: (a) the depth of the restrictive horizon; (b) the soil permeability above the restrictive horizon; (c) the extent of mounding that can be expected to occur. 2. It is OHLASA's understanding that the D permissible hydraulic loading rate (HLR) for the receiving soils is 0.2 gallons per day per square foot. The preliminary design is based upon this maximum loading rate. This rate may be acceptable for subsurface absorption systems serving single family units which have an assimilative area of one acre- Or--more; however; it - should not be considered acceptable for a concentrated absorption field area serving 158 dwelling units and located on a public water supply watershed. O ,SA staff has completed an evaluation of the hydraulic loading rate and nitrogen loading rate for the proposed system. The proposed system would be designed to receive nearly 26 million gallons of wastewater per year. This equals an application rate of more than 117 inches of wastewater per year over the propose absorption area. That rate is about 2.5 times greater than the annual rate of precipitation for this area and would be in addition to rainfall absorption. Assuming this substantial volume of wastewater has a Total Nitrogen concentration of 25 parts per million (which may be a low value) and it is applied over an absorption field area of 8.16 acres as shown on the plans, more than 660 pounds of nitrogen per acre per year would be applied on the site. Even if it were assumed that the vegetative cover on the absorption field surface could utilize a substantial amount of that nitrogen, that rate would greatly exceed the expected rate of uptake by any established cover crop. Fescue 31, a common grass cover, has an uptake rate of approximately 150 pounds of nitrogen per acre per year. Even if Tifton 44 Coastal Bermudagrass is used to provide about the highest nitrogen uptake rate that could be expected for grass cover crops in the Piedmont (300 pounds per acre per year) , the nitrogen loading rate would be considerably more than twice the plant uptake rate. However, plant uptake rates would actually be much lower than this because wastewater is applied underground. The excess nitrogen not utilized by plants will be carried away from the site by surface water runoff and/or seepage into the groundwater table. Higher r nitrogen levels in surface waters can lead to algal g g growth and increased eutrophication in University Lake as well as bitter taste in drinking water. The present drinking water standard for nitrogen is 10 parts per million. Excessive nitrogen concentrations have been reported to cause health problems in infants and children. Similar concerns exist relative to excessive loadings of chlorides and sulphates. -2-