Spatio‐temporal Variability of Groundwater Nitrate Concentration in Texas: 1960 to 2010

Nitrate (NO 3 ) is a major contaminant and threat to groundwater quality in Texas. High‐NO 3 groundwater used for irrigation and domestic purposes has serious environmental and health implications. The objective of this study was to evaluate spatio‐temporal trends in groundwater NO 3 concentrations in Texas on a county basis from 1960 to 2010 with special emphasis on the Texas Rolling Plains (TRP) using the Texas Water Development Board's groundwater quality database. Results indicated that groundwater NO 3 concentrations have significantly increased in several counties since the 1960s. In 25 counties, >30% of the observations exceeded the maximum contamination level (MCL) for NO 3 (44 mg L −1 NO 3 ) in the 2000s as compared with eight counties in the 1960s. In Haskell and Knox Counties of the TRP, all observations exceeded the NO 3 MCL in the 2000s. A distinct spatial clustering of high‐NO 3 counties has become increasingly apparent with time in the TRP, as indicated by different spatial indices. County median NO 3 concentrations in the TRP region were positively correlated with county‐based area estimates of crop lands, fertilized croplands, and irrigated croplands, suggesting a negative impact of agricultural practices on groundwater NO 3 concentrations. The highly transmissive geologic and soil media in the TRP have likely facilitated NO 3 movement and groundwater contamination in this region. A major hindrance in evaluating groundwater NO 3 concentrations was the lack of adequate recent observations. Overall, the results indicated a substantial deterioration of groundwater quality by NO 3 across the state due to agricultural activities, emphasizing the need for a more frequent and spatially intensive groundwater sampling.