In this past Sundays Chicago Tribune (August 7, 2011), the lead article was about high levels of hexavalent chromium in Chicagos drinking water. The article was titled Toxic metal stays in drinking water. Why is this news? After all, the U.S. Environmental Protection Agency has a drinking water standard for total chromium (100 ppb), thus public water supplies are required to meet this standard.
The problem is that the state of California has adopted a “Public Health Goal” for hexavalent chromium in drinking water of 0.02 ppb, 5,000 times lower than the current standard. This value was determined based on epidemiological studies showing hexavalent chromium can be carcinogenic. Note that the goal is specifically for hexavalent chromium, one form of chromium, while EPA’s standard is for total, or all forms, of chromium. (If hexavalent chromium rings a bell, it might be because it was the contaminant in "Erin Brockovich.") There are two primary forms of chromium (Cr) in the environment, hexavalent [Cr(VI)] and trivalent [Cr(III)]. Hexavalent chromium is the oxidized form, trivalent the reduced form. In Illinois groundwater, which is predominantly reducing, we would expect most of the chromium to be Cr(III). For surface water, which is directly exposed to atmospheric oxygen, we’d expect Cr(VI) to predominate.
Chromium is similar to arsenic in that the more oxidized form tends to be more mobile in groundwater. Cr(VI) tends to form negatively charged species in water, such as H2CrO72-, HCrO4-, and CrO42-, while Cr(III) tends to form positively charged or neutral species in water. Because most mineral surfaces are negatively charged, negatively charged aqueous species tend to be repelled by the surfaces and are thus more mobile than positively charged species, which are attracted to negative surfaces. Thus shallow aquifers under oxidizing conditions, such as karst regions or areas with thin soils, would be more vulnerable to chromium contamination. The town of Hinkley, CA, in the Mojave Desert (i.e., very thin/no soils), has the highest recorded levels of total Cr measured in the U.S., 580 ppb, due to leakage from a Pacific Gas and Electric facility.
Chromium in water can come from both natural mineral sources and human activities. Lake Michigan may be particularly vulnerable to chromium contamination due to the large steel mills that discharge waste into it. The Tribune reported that two of the mills dumped 2,350 pounds of chromium into Lake Michigan and its tributaries in 2010. The source of hexavalent chromium in Chicago’s drinking water has not been definitively determined yet.
Here at the Water Survey, our Public Service Lab routinely analyzes water samples for total Cr (among many other elements). The current detection limit for their instrument is 5.8 ppb, well above the 0.02 ppb California goal. (New instrumentation using mass spectrometry can detect down to 0.002 ppb.) Getting levels of any compound down to sub-ppb levels will be a challenge. The technology exists (primarily using reverse osmosis), but the costs may be extremely high. While conflicts over issues such as this often are portrayed as big business vs. concerned citizens (or environmental extremists vs. mom and pop businesses), it’s a complicated issue. How much money, especially during difficult economic times, should be spent on keeping our water resources clean? As with any contaminant, it makes much more sense, both economically and environmentally, to prevent chromium from getting into our water resources rather than having to clean it up.