Investigation of the Flow and Fate of Nitrate in Epikarst at the Savoy Experimental Watershed, Northwest Arkansas
Many karst aquifers are at high risk of nitrate contamination due to a combination of vulnerable geology characterized by thin soils and conduit flow, and excess inputs of nutrients from animal feeding operations. One zone that is present in many karst regions and could play an important role in nitrate attenuation due to properties such as increased residence time and matrix-water contact is the upper, weathered portion of karst, the epikarst. However, the understanding of this role is lacking, and the objective of this dissertation was to elucidate it. The fate of nitrate in the epikarst was traced along a hydrologic gradient using a multi-faceted geochemical approach based primarily on concentration and stable isotope composition of the reactants and products of denitrification. In addition, dye-tracing tests were conducted to assess the flow, solute transport and aquifer characteristics of the epikarst system. The study found multiple lines of evidence for denitrification which is spatially and temporally highly variable and can remove up to 33% of nitrate along the studied flowpaths. Dissolved organic carbon and dissolved oxygen appear to control denitrification levels, and both in turn appear to be controlled by hydrologic conditions (saturation). However, the most significant agent of nitrate attenuation is dilution, decreasing nitrate concentration by upwards of 50%. Transport of water and solutes in the epikarst can be relatively fast (up to 2.2 m/h) and involves preferential flowpaths. However, transport of a point-source solute located in the upper epikarst depends on saturation, and the transported mass is likely to be negligible in the short term (weeks-months) under the normal weather pattern. Overall, the results indicate that the epikarst can be an important buffer against potential groundwater contaminants.
Biogeochemistry, Denitrification, Epikarst, Groundwater Contamination, Karst Hydrology, Stable Isotopes, Geology, Hydrology
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