National Water-Quality Assessment (NAWQA) Program
Citation: Welch, A.H., Nov. 20-23, 2001, Arsenic cycling in ground water--Processes leading to widespread high concentrations: In Arsenic in the Asia-Pacific Region, Adelaide, Australia, p. 89-90.
Widespread high concentrations generally result from natural processes, although human activities locally increase arsenic. The most prevalent causes of widespread high concentrations in potable ground water are release from iron oxide and sulfide mineral oxidation. These seemingly simple processes are actually relatively complex and many details are poorly or only broadly understood.
Sources of arsenic in ground water:
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 either an increase in pH or the concentration of a competing ion, such as phosphorous. Sodium exchange for calcium can increase calcite dissolution, 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 along the New England coast where aquifer materials were affected by seawater. High pH, high arsenic ground water in felsic volcanic rocks have been well documented in parts of the western U.S. and Argentina.
Reduction of As(V) to As(III) promotes desorption from HFO within the pH range of most ground water. Recent laboratory studies demonstrate that a wide variety of microorganisms can reduce aqueous As(V) or adsorbed on HFO or Al(OH)3 which can be a detoxifying mechanism or a source of energy for growth. Although the laboratory studies clearly demonstrate that reduction can be microbially mediated, the abundance and importance of these organisms in the natural environment is unknown, particularly in ground water with typical arsenic levels of < 10 µg/L.
A well-documented, biologically mediated reaction that can release arsenic from iron oxide, which is commonly referred to as dissimilatory iron reduction, involves organic carbon and iron oxide. Deposition of Fe-coated sediment along with organic matter can lead to the dissolution of the oxide coating with consequent release of arsenic to ground water. Introduction of synthetic organic compounds into aquifers also can lead to reductive dissolution of iron oxide and arsenic release.
Pyrite commonly contains arsenic in trace amounts, with arsenic concentration exceeding five percent in some cases. In ground water the extent of sulfide-mineral oxidation is limited by the supply of an oxidizing agent, most commonly molecular oxygen, although nitrate from agricultural activities also can oxidize sulfide minerals. In most ground water, sulfide mineral oxidation is limited to the amount of molecular oxygen contained in the water during recharge. However, exposing sulfide-bearing material through lowering of ground-water tables can greatly increase oxidation. The resulting high sulfate, low pH ground water commonly reacts with other minerals producing high concentrations of other solutes. An example of high arsenic resulting from ground water development is the St. Peter sandstone in southeastern Wisconsin.
Areas of needed research:
Although cycling of arsenic in ground water is broadly understood, some aspects are poorly understood. A few areas that need field efforts include understanding of arsenic adsorption on iron oxide, particularly with regard to the effect of individual competing ions such as CO32- as well as the multiple species. A comprehensive model of arsenic adsorption would be useful for prediction of the fate of arsenic released to the environment and in removal from drinking water.
Arsenic adsorption on silicate minerals has been examined in laboratory settings. The importance of this association in ground water systems containing high arsenic concentrations is not well studied, however. Recent studies of aquifer materials containing arsenic-rich ground water in Bangladesh suggests that weathering of biotite can lead to significant arsenic adsorption.
The long-term fate of both historic uses of inorganic arsenic in agriculture and current uses in wood preservation and pesticides applied to agricultural plants and as food additives for poultry and swine has not been widely studied from a field perspective.