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Technical Report

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Aquifer, Ground water, Contamination


The presence of an extensive network of solution channels in the fractured carbonate bedrock and a thin permeable regolith in northwest Arkansas makes aquifers in this region susceptible to contamination. Because of these conditions, there is concern about nitrate contamination of the ground water from land applied animal wastes, commercial fertilizers, rural septic systems and municipal sewage systems. In response to these concerns a survey was conducted of the nitrate concentration in rural water wells in the carbonate aquifers of a 420 mi2 area of northern Madison county during "wet" and "dry" seasons in 1990. Information from well owners, drilling records, Mg/Ca (meq/L) ratios as well as other chemical parameters were utilized to determine the primary aquifer source. Thirty-one samples were collected from the mostly unconfined, shallow, more commonly used Springfield Plateau aquifer (Boone-St. Joe aquifer). Seventeen wells were completed in the deeper, confined Ozark aquifer. Another sixteen wells were determined to be from undifferentiated shallow non-carbonate aquifers. The deeper Ozark aquifer is much less susceptible to nitrate contamination than the overlying springfield Plateau aquifer. Comparison of seasonal mean nitrate plus nitrite (mg/L as N) values suggests that more nitrate is introduced to the Springfield Plateau aquifer during the wet (2.89 mg/L) season than in the dry ( 1.79 mg/L) season. The deeper Ozark aquifer seasonal mean nitrate values were slightly higher in the dry (0.16 mg/L) season compared to the wet (0.13 mg/L) season. overall, nitrate levels in ground water in northern Madison County are generally below the 10 mg/L as N standards for drinking water.

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