ARSENIC AND ANTIMONY IN GROUNDWATER FLOW SYSTEMS

Groundwater samples were collected along flow paths in three different aquifers to study the evolution of arsenic (As) and antimony (Sb) concentrations and speciation as a function of changing groundwater composition and oxidation-reduction potential. Aquifers studied included the Oligocene – Eocene Upper Floridan aquifer (south-central Florida), the Eocene Carrizo Sand aquifer (southeasten Texas), and the Paleocene Aquia aquifer (coastal Maryland). The Upper Floridan aquifer is a carbonate aquifer, whereas the Carrizo Sand and Aquia aquifers are unconsolidated, Coastal Plain aquifers of subaerial and marine (i.e., greensands) origins, respectively. All samples were collected and using ultra-clean techniques. Arsenic species [i.e., As(III) + As(V)] were separated in the field by anion exchange chromatography and measured by high resolution (magnetic sector) inductively coupled plasma mass spectrometry (HR-ICP-MS). Total Sb concentrations were measured (HR-ICP-MS) in Upper Floridan and Carrizo Sand groundwaters, whereas Sb(III) and Sb(V) were determined in Aquia groundwaters using the anion exchange separation, followed by HR-ICP-MS analysis. Although the geochemistry of As in Upper Florida and Carrizo Sand groundwaters were discussed previously, the Sb data has not been presented for these aquifers. Moreover, the Sb speciation data for Aquia aquifer are among the first reported for groundwaters. Antimony concentrations in Upper Floridan and Carrizo Sand groundwaters range from ~ 6 to 1460 pmol/kg (mean 173 pmol/kg) and from 16 to 200 pmol/kg (mean 60 pmol/kg), repectively. Antimony concentrations are weakly to moderately correlated to As in these groundwaters (r = 0.78 and -0.57 for the Upper Floridan and Carrizo, respectively). Antimony is moderately to strongly correlated with As in Aquia groundwaters (r = 0.78), and both Sb(III) and As(III) predominate. Sequential extractions of Aquia aquifer sediments indicate that elevated As concentrations in the groundwaters (As = 1072 nmol/kg; 80 ppb) reflect reductive dissolution of Fe(III) oxides/oxyhydroxides in the aquifer sediments. The moderate correlations between As and Sb in the Aquia (and to a lesser extent the Upper Floridan and Carrizo Sand aquifers) suggests that similar biogeochemical reactions control Sb concentrations and speciation in groundwaters.