Arsenic concentrations and speciation along a groundwater flow path: The Carrizo Sand aquifer, Texas, USA

Groundwater samples for arsenic (As) concentration and speciation analysis were collected from production wells along a flow path in the Carrizo Sand aquifer in southeastern Texas during June 2004. The study was conducted as part of a larger investigation of trace metal and metalloid concentrations, speciation, and geochemical behavior along a groundwater flow path in the relatively pristine, Carrizo Sand aquifer. Additional sample aliquots were collected and analyzed for Fe(II) and Fe(III) concentrations, H2S concentrations, pH, and alkalinity. Oxidation-reduction potential (Eh) and dissolved oxygen concentrations were also monitored in groundwaters at each well head. Dissolved inorganic As [i.e., As(III) and As(V)] species in Carrizo Sand aquifer groundwater samples were separated during sample collection using anion-exchange chromatography. Arsenic concentrations were subsequently determined by inductively coupled plasma mass spectrometry. Total dissolved arsenic, AsT [i.e., As(III) + As(V)], ranges from ∼0.37 to 2.5 nmol kg− 1 in groundwater from the Carrizo Sand aquifer. Arsenate, which is the dominant form of As in Carrizo groundwaters collected from the first ∼15 km along the flow path, ranges from below the method detection limit (0.36 nmol kg− 1) to 2.0 nmol kg− 1. Arsenite concentrations are relatively constant along the flow path compared to As(V), ranging from 0.29 to 0.87 nmol kg− 1. Arsenic concentrations and speciation exhibit systematic changes along the Carrizo Sand aquifer flow path that correspond to changes in dissolved Fe(II) and Fe(III) concentrations, dissolved sulfide concentrations, pH, and alkalinity. The data are consistent with a combination of physiochemical reactions and inferred microbial processes (e.g., ferric iron reduction, adsorption, pH related desorption) playing significant roles in speciation, mobility, and cycling of As in the Carrizo Sand aquifer.