GEOCHEMISTRY OF TOXIC AND POTENTIALLY TOXIC OXYANION-FORMING METALS AND METALLOIDS IN GROUNDWATERS

Arsenic (As) and antimony (Sb) are designated as priority pollutants by the US EPA, and chronic exposure to As from drinking water has been linked to skin, lung, and bladder cancers, type 2 diabetes, and heart disease, among others. Antimony is known to be a genotoxin, and like As it is a recognized clastogen. Much less is known about the toxicity of tungsten (W), although evidence is accumulating that it may be carcinogenic. Indeed, a number of childhood leukemia clusters in three western US states have been tentatively linked to the proximity of these sites to W-bearing ore deposits and associated active or inactive smelting operations. Groundwaters from one of these locations in western Nevada have both elevated As and W concentrations, with W exhibiting concentrations ranging from 1.47 nmol/kg up to 4036 nmol/kg (0.27 to 742 ppb). Arsenic and W are strongly correlated in these western Nevada groundwaters (r² = 0.63, p < 0.05), suggesting similar mechanisms of mobilization. We measured As, W, and Sb concentrations in groundwaters from West Bengal, India to investigate whether W and Sb were also elevated in these high As groundwaters. Total dissolved As concentrations range from 12.6 nmol/kg to 61.7 μmol/kg (< 1 to 4622 ppb), and As(III) predominates accounting for between 54% to >98% (mean = 71%) of As in solution. Antimony concentrations range from 158 pmol/kg to 1670 pmol/kg (19 to 203 ng/kg), and W concentrations range from 821 pmol/kg to ~ 6 nmol/kg (0.15 to 1.1 ppb). Arsenic is not correlated with either Sb (r² = 0.03, p > 0.2) or W (r² = 0.08, p > 0.2) in Murshidabad groundwaters. An important difference between the Murshidabad and the western Nevada groundwaters is that the Murshidabad groundwaters are neutral (mean pH = 7.14), whereas those from Nevada are alkaline (pH > 8). Surface complexation modeling indicates that As and W desorb from mineral surface sites at high pH and under oxidizing conditions. However, in reducing waters of neutral pH, As is more mobile than W and possibly Sb. We hypothesize that the differences in mobility between As and W reflects the predominance of the uncharged H₃AsO₃⁰ form of As(III) in the Murshidabad groundwaters, compared to the more surface reactive tungstate oxyanion, WO₄^2-.