Evaluation of Groundwater Nitrate Contamination in Major Aquifers in Texas

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Nitrate is the most widespread groundwater contaminant in Texas and in the U.S. There are many potential adverse health implications of elevated groundwater nitrate, including methemoglobinemia and cancer risks. There are a variety of sources of nitrate, including natural sources, inorganic and organic fertilizers (manure), output from concentrated animal feeding operations (CAFOs), septic tanks, and leaking sewer systems. Natural sources result from nitrogen fixation by legumes, mineralization of organic matter (nitrification), and natural geologic sources.

Many previous studies have been conducted on groundwater nitrate contamination in Texas. The early studies focused on source identification using nitrogen isotopes, mainly distinguishing between nitrate from fertilizers and septic tanks. Groundwater nitrate levels were expected to be high in the Ogallala Aquifer beneath playas adjacent to concentrated animal feeding operations; however, many studies showed that nitrate levels were reduced by denitrification attributed to high levels of organic matter. A recent study suggested that nitrate contamination has been increasing in the state over the past several decades and identified the Seymour aquifer in the Rolling Plains as a hotspot of groundwater nitrate contamination. A study evaluated controls on groundwater nitrate contamination using logistic regression, indicating that precipitation, percent of agricultural land, low-density residential land, and soil organic matter were the dominant explanatory variables. Unsaturated zone sampling was used to link land surface processes to groundwater nitrate levels and suggested that much of the elevated nitrate levels in the Ogallala and Seymour aquifers could be attributed to high levels of natural nitrate prior to cultivation that was oxidized during cultivation and mobilized into the underlying aquifer.

The current study examined the distribution of groundwater nitrate in major and minor aquifers in the state using approximately 10,000 analyses from major aquifers and approximately 2,000 analyses from minor aquifers. Approximately 70% of the samples in the major and minor aquifers exceeded the detection levels for nitrate. The majority of the samples are from rural domestic and irrigation wells. A total of 5.5% of the samples from the major aquifers exceeded the MCL, with the highest level of contamination in the Seymour Aquifer (61% of sampled > MCL), followed by the Pecos Valley Aquifer (11%), Ogallala Aquifer (9%), Edwards Trinity High Plains (6%), and the remaining major aquifers < 2%. Groundwater nitrate concentrations generally decreased with depth.


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