SPECIATION AND DISTRIBUTION OF ARSENIC ALONG A GROUNDWATER FLOW PATH IN THE UPPER FLORIDAN AQUIFER, FLORIDA, USA

We present arsenic (As) concentrations and speciation data in a well characterized groundwater flow path in the Upper Floridan (carbonate) aquifer in west-central Florida in order to investigate the biogeochemical “evolution” of As in this relatively pristine, uncontaminated aquifer. Collected groundwater samples (July 2004) were speciated (i.e., arsenite [As(III)], and arsenate [As(V)]) in the field using an anion-exchange chromatography method and were subsequently analyzed by inductively coupled plasma mass spectrometry. In situ measurements of ferrous and ferric iron concentrations, dissolved sulfide concentrations, pH, alkalinity, oxidation-reduction potential (Eh) and dissolved oxygen concentrations were determined to establish the oxidation-reduction conditions and solution chemistry along the studied flow path. Measurements of As indicate that distribution of As(III) and As(V) in these groundwaters appears to be primarily governed by the major geochemical reactions such as, dedolomitization, ferric hydroxide dissolution, sulfate reduction, and pyrite precipitation reactions occurring in this aquifer. The redox conditions along the flow path varied from relatively oxic to anoxic (i.e., sulfidic). Concentrations of total dissolved As range from 19 nmol kg-1 to 0.72 nmol kg-1 and exhibit relatively higher concentrations in groundwater nearest to the recharge area compared to the down gradient waters. Analytical results indicate that in general, the distribution of As species along the flow path are consistent with the theoretical thermodynamic behavior, with As(V) dominating in more oxidizing waters near the recharge area, and As(III) concentrations predominating in the increasingly reducing and sulfidic groundwaters beyond the recharge area. Furthermore, the data suggest that in the reducing groundwaters from the lower reaches of the aquifer, As mobilization is controlled via sorption onto or coprecipitation with sulfide minerals.