Hydrogeochemical processes driving elevated Arsenic and Fluoride in the Texas Gulf Coast Aquifer

Dissolved arsenic (As) and fluoride (F) concentrations frequently exceed the United States Environmental Protection Agency (US EPA) drinking water standards in groundwater wells screened within the three aquifers (Chicot, Evangeline, Jasper) that comprise the Gulf Coast Aquifer (GCA) in Texas. The primary objective of this study is to identify the hydrological and geochemical processes that mobilize dissolved groundwater As and F in each aquifer across a north-south climate gradient from humid to semi-arid. Comprehensive chemistry was analyzed from 1,164 water samples taken from 534 wells between 2002 and 2022 and reported in a public database of the Texas Water Development Board (TWDB). Surface and geologic sources of As and F were tested by observing their spatial correlation to land use and geologic layers. Multivariate analysis revealed that in the Northern and Central climate zones As co-varies with dissolved iron (Fe) and/or manganese (Mn), whereas F varies with bicarbonate (HCO 3 ), sodium (Na), and pH. In the Southern climate zone As and F covary with vanadium (V). Plausible chemical reactions that generated As and F were analyzed with geochemical plots and stoichiometric inverse modeling. The dissolved products of silicate hydrolysis and cation exchange were evident along all flowpaths. The removal of Ca from solution through cation exchange drives calcite and fluorite dissolution, which is inferred to be the primary source of F to groundwater in the Northern and Central climate regions, whereas the reductive dissolution Fe- and Mn-oxides releases dissolved As. Arsenic and F are released to Southern aquifers primarily by silicate weathering of the Catahoula Formation volcanic ash layers. This study reveals the changing drivers of As and F release across a 650 km wide region with varying geology and climate. Understanding these drivers is important for deciding the best strategy for avoiding consumption of the polluted groundwater or making the contamination worse through land use or groundwater pumping. • Silicate weathering drives fluorite dissolution and fluoride release in the GCA • In the semi-arid region, volcanic ash is an important source of arsenic and fluoride • In the humid region, reductive dissolution of metal oxides releases arsenic • 23 shallow private wells have much higher As and F than evident in public database