Use of a Nitrogen Leaching Model as a Design Criterion for Land Application of Waste Water
West, Larry T.
Cabrera, Miguel L.
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Land application is a viable alternative to conventional waste treatment plants for environmentally safe disposal of liquid wastes. To ensure protection of the state's water resources, guidelines have been established specifying variables that should be considered in design of land application systems. These guidelines also specify maximum levels of hydraulic loading, heavy metals in the soil, and N concentration in water percolating through the soil (Georgia EPD, 1986). Any of these three factors may limit the annual amount of waste applied to a site. For hydrologic loading, the design criteria are based on monthly net precipitation (precipitation - potential evapotranspiration) and soil properties that influence the hydrology of the soil. The maximum monthly rate of waste application is determined by the month in which net precipitation plus monthly waste addition is maximal. Total net precipitation plus waste additions during this month cannot exceed the soil's capacity to transmit the liquid without ponding and runoff. If nitrogen content of the waste is such that limits on soil percolate N concentration will be exceeded with application rates which meet hydrologic loading criteria, size of spray field area is determined based on an annual rather than monthly N balance. The major sink for N considered in design of land application systems is plant uptake. Rather than being uniform throughout the year as suggested by the current practice of using annual values in the system design, crop growth and associated N uptake is cyclic, and depending on the crop or crops growing, periods of plant dormancy or reduced growth may occur when little or none of applied N is being removed from the soil. Thus, during these periods N applied may move quickly through the soil profile and potentially to shallow ground water. This may be especially true in Georgia where warm temperatures promote rapid nitrification and the retentive capacity of the soils for ammonium is low. Models to predict nitrate leaching through soils under different climatic and management regimes are currently available and improved versions are steadily being released. These models simulate N uptake by crops, N transformations such as nitrification, denitrification, and volatilization, and water and nitrate movement through the soil, and offer the opportunity to evaluate the effectiveness of N removal by land treatment under various management, climatic, and soil conditions. The objective of this study was to use a N leaching model to evaluate the soil N balance under different soil and management conditions for a hypothetical land application system sized by current design criteria.