Transport and fate of nitrogen from earthen manure storage effluent seepage
Fonstad, Terrance Alden
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The primary focus of this project was determination of the potential for nitrogen transport and mobility from earthen manure storage (EMS) facilities - the key issue in regulations, guidelines, and management practices. Specific objectives, addressed through an integrated set of laboratory, column and batch test experiments and detailed field monitoring, were to: 1. Measure effluent source chemistry within EMS used in pork production operations and interpret this chemistry in light of the potential for contaminant transport and groundwater contamination; 2. Characterize geochemical conditions within an EMS effluent plume and summarize factors controlling mobility of species of concern; and 3. Develop a method of establishing nitrogen mobility based on effluent and/or soil characteristics. EMS solutions contained, on a molar basis, 36% ammonium, 36% bicarbonate, 8% potassium, 6% chloride, 5% sodium plus sulphate, calcium, magnesium and other nutrients. Additionally, the solution contained ~6,000 and 9,000 mg/L organic and inorganic carbon, respectively, and had a near neutral pH. The high biological demand results in a solution with a low Eh causing nitrogen to remain in the ammonium form. Conditions within the effluent plume were slightly reducing due to naturally low oxygen concentrations and the chemistry of the EMS effluent. Ammonium and potassium transport in the effluent plume was attenuated by ion exchange; the release of magnesium and calcium from the soil exchange complex produced concentrations in excess of 29 and 7 times their concentration respectively in the background groundwater. This hard water front or "hardness halo" offers a method to provide early indications of EMS plume advance. Organic carbon transport was similar to chloride (assumed to be non reactive) and further promoted reducing conditions. Nitrogen remained as ammonium with the exception of the leading edge and upper fringe of the plume where both oxidation and reduction of nitrogen occurred depending on chemical conditions and time of year. The variability of the retardation of cations during contaminant transport may be caused by variations in ion adsorption due to variability in their selectivity by the soil exchange complex. Preliminary modeling of EMS effluent transport using varying selectivity coefficients illustrates the potential for enhancement of current models by incorporation of subroutines to accommodate variable selectivity. Selectivity coefficients for ammonium, as referenced to sodium, varied from 0.23 to 2.2 and distribution coefficients for ammonium varied from 0.03 to 0.8L/kg. The ability of ammonium to replace ions on the exchange sites was a function of the ratio of monovalent to divalent cations in solution rather than the concentration of only the ammonium in solution. The ability of ammonium to replace ions on the exchange sites increased with an increasing ratio of monovalent to divalent cations in solution, and became significant above a ratio of 0.9. The retardation factor for ammonium was determined to be less than 3, an order of magnitude less than assumed in previous studies; nitrogen transport in EMS effluent plumes may therefore be much greater than originally assumed by some regulators, developers and engineers. Furthermore, sorbed ammonium will likely release into solution once the source is removed and natural waters begins to leach the contaminated soil.