NATIONAL TO LOCAL SCALE CYCLING OF ARSENIC IN GROUND WATER
Alan H. Welch 1 and Joseph D. Ayotte 2
U.S. Geological Survey 1
333 W. Nye Lane, Carson City, NV 8970 2361
Commerce Way, Pembroke, NH 03275

Widespread high arsenic concentrations in potable ground water are most commonly caused by release from phyllosilicate, iron oxides, and sulfide minerals. Similarities between the geologic and geochemical characteristics of New England and other high-arsenic regions suggest that the latter two sources are important in the cycling of arsenic. A strong association between arsenic and weathered biotite in arsenic-rich ground water of Bangladesh has recently been demonstrated. Although weathering of biotite in a warm tropical delta is likely very different than weathering in the New England climate, the association between arsenic and biotite is worth consideration. Oxidation of pyrite and other less common sulfide minerals, such as arsenopyrite and cobaltite, can release arsenic to ground water. Pyrite commonly contains arsenic in at least trace amounts, with arsenic concentrations exceeding five percent in some cases. Molecular oxygen is quantitatively the most important oxidant in ground-water systems, although nitrate from agricultural activities also can oxidize sulfide minerals. Sulfide mineral oxidation is commonly limited by the amount of molecular oxygen contained in the water during recharge. Sulfate concentrations in most New England ground water are low (generally < 30 mg/L), suggesting that sulfide mineral oxidation is not much greater than could be attributed to oxygen in equilibrium with the atmosphere. In examples from other parts of the United States, exposing sulfide minerals to the atmosphere through lowering of ground-water tables can greatly increase oxidation. The resulting low pH ground water containing sulfate concentrations ranging greatly in excess of a few hundred mg/L, is not typical of ground water of New England. Arsenic can be released to ground water by desorption from, and dissolution of, HFO (hydrous ferric oxide) and other iron oxides. Desorption from iron oxide is an important process affecting arsenic concentrations in alkaline, oxic ground water because iron oxide commonly contains arsenic as an impurity. Desorption of arsenic can be promoted by an increase in either pH or the concentration of a competing ion, such as phosphorus. Sodium exchange for calcium can increase calcite dissolution (because of the lowered aqueous calcium concentration), thereby producing ground water with high pH and arsenic such as in the central Oklahoma aquifer. This scenario may be responsible for some of the high arsenic ground water of coastal New England where aquifer materials were affected by seawater either from their original depositional environment or from sea-level rise associated with Pleistocene deglaciation. As(III) is less readily sorbed onto HFO than As(V) within the pH range of most ground water. Because As(III) is present in moderate to high concentrations in some ground water from bedrock of New England, the lower adsorption may be an important factor affecting arsenic mobility. Dissolution of arsenic-bearing HFO, and other iron oxides, is an important source of arsenic in some ground water. High arsenic ground water in New England generally does not contain high iron concentrations, suggesting that oxide dissolution is not a major factor releasing arsenic; however, desorption may be a more important mechanism. While HFO is likely present in these aquifer materials, geochemical conditions generally are not favorable for dissolution of HFO and other iron oxides.