Kartik Venkataraman and John W. Lozano report that arsenic contamination of groundwater in the Southern Gulf Coast Aquifer of Texas is a critical public health concern as much of the area is rural in nature with decentralized water supplies. Previous studies have pointed to volcanic deposits as the regional source of arsenic but no definitive or reliable predictors of arsenic maximum contaminant level (MCL) exceedance have been identified. In this study, we have studied the effect of various hydrogeochemical parameters as well as soil and land-use variables on arsenic MCL exceedance using logistic regression (LR) techniques.
The LR models display good accuracy of 75% or higher but suffer from a high rate of false negatives, highlighting the challenges in capturing the spatial irregularities of arsenic in this region. Despite not displaying high statistical significance, pH appears to be an important variable in the LR models—its effect on arsenic exceedance is not clear and warrants further investigation. The results of the study also show that groundwater vanadium and fluoride are consistently the only significant variables in the models developed; the positive coefficients for both these elements indicates a common geogenic source for arsenic, fluoride and vanadium, corroborating the findings of earlier studies.
Arsenic occurrence at elevated levels in the Southern Gulf Coast Aquifer of Texas is an urgent public health concern considering the rural nature of this area. Water supplies in many of these areas are decentralized and the need to identify arsenic ‘hot spots’ using surrogate measures is critical. The LR models developed in this study show that vanadium and fluoride exert the largest control on arsenic occurrence in the area and that fluoride on its own can be used as a reliable predictor. As such, volcanic deposits appear to be the regional source of groundwater arsenic, but on smaller spatial scales other factors may be responsible for localized arsenic enhancement.
Considering the differences in the performances of the three models developed, it is recommended that arsenic occurrence in the Evangeline formation, where most of the wells are screened, be treated separately. We have found the highest arsenic concentrations to occur in the Catahoula, pumping from which appears to be common along the western boundary of the study region where it outcrops. We recommend that exploitation of this formation for drinking-water purposes be avoided.
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