ABSTRACT
Typically, when it comes to contamination of drinking water by Cr(VI), anthropogenic contamination is much more significant compared to natural Cr. Dissolved concentrations of total Cr in groundwater from natural processes are typically below 10 µg L−1 (Richard and Bourg, 1991). In contaminated areas, Cr(VI) concentrations are commonly 300 to 500 µg L−1 (CRWQCB, 2000; Maxwell, 1997) and have been reported to reach 14 g L−1 (Palmer and Wittbrodt, 1991). A yellow color is imparted to the water at about 1 mg L−1 Cr(VI) (Palmer and Wittbrodt, 1991). Kharkar et al. (1968) reported an average total Cr value in river water of 1.4 µg L−1, while Richard and Bourg (1991) reported total dissolved Cr concentrations from zero to 208 µg L−1 for unpolluted waters, with typical values given of 0.5 µg L−1 for rivers and 1.0 µg L−1 for groundwaters. While it is often the case that Cr(VI) in drinking water is evidence of anthropogenic contamination, over the last decade there have been many reports of naturally occurring Cr(VI) in groundwater and at very high levels. Cr(VI) of natural origin has been found in the groundwater of numerous alluvial basins in Arizona and adjacent parts of California, New Mexico, and Nevada at concentrations of up to 220 mg L−1 Cr(VI) (Robertson, 1975; 1991). In the remote Cadiz Valley in the Mojave Desert of southeastern California at the location of a proposed water storage project between Metropolitan Water District of southern California (MWD) and Cadiz, Inc., concentrations of 15 to 26 µg L−1 were found in the native groundwater (MWD and Bureau of Land Management, 2001). A combined field and laboratory study of chromite bearing oxidized serpentinite rocks in India indicated the possibility of Cr mobilization from chromite ores to water bodies (Godgul and Sahu, 1995). The authors observed that serpentinization is an intensely oxidizing process that creates alkaline pore waters that would promote oxidation of Cr(III). The study suggested that mining practices enhance the rate and intensity of Cr mobilization (Godgul and Sahu, 1995). Cr(VI) in groundwater has also been documented in the vicinity of UC Davis in the Sacramento Valley (Chung et al., 2001). The Table 18.1 (adapted from AWWA Reports) summarizes the reported instances of naturally occurring Cr(VI) in groundwater.
