Evaluation of Groundwater Nitrate Contamination in Aquifers in Texas

Nitrate is the most widespread groundwater contaminant in Texas and the U.S. Elevated levels of groundwater nitrate can have serious health implications, including methemoglobinemia and increased cancer risks. Sources of nitrate contamination include both natural and anthropogenic sources such as inorganic and organic fertilizers (including manure), output from concentrated animal feeding operations (CAFOs), septic tanks, and leaking sewer systems. Natural sources of nitrate arise from nitrogen fixation by legumes, mineralization of organic matter (nitrification), and natural geological processes.

Numerous previous studies have investigated groundwater nitrate contamination in Texas. Early studies focused on identifying sources using nitrogen isotopes, particularly distinguishing between nitrate from fertilizers and septic tanks. While groundwater nitrate levels were anticipated to be high in the Ogallala Aquifer beneath playas adjacent to CAFOs, many studies found that nitrate levels were reduced due to denitrification facilitated by high levels of organic matter. Recent research suggests that nitrate contamination has been increasing in Texas over the past few decades, with the Seymour aquifer in the Rolling Plains identified as a hotspot of groundwater nitrate contamination.

Studies have also assessed the factors influencing groundwater nitrate contamination, with logistic regression indicating that precipitation, percentage of agricultural land, low-density residential land, and soil organic matter are significant explanatory variables. Unsaturated zone sampling has helped link land surface processes to groundwater nitrate levels, suggesting that much of the elevated nitrate in the Ogallala and Seymour aquifers may be attributed to high levels of natural nitrate prior to cultivation, which was oxidized during cultivation and subsequently mobilized into the underlying aquifer.