James M. King, Ph.D.
Wilcox Environmental Engineering, Inc.
December 7th, 2021
Hexavalent chromium shot into the public consciousness in 1993 when Erin Brockovich began campaigning against Pacific Gas & Electric, which had used hexavalent chromium compounds as a rust inhibitor at a natural gas compressor station until 1966. Over time, hexavalent chromium contaminated the groundwater supply of the southern California town of Hinkley and plagued the town’s residents with a bevy of chronic health problems. Brockovich was instrumental in building the case against PG&E, which was settled in 1996. The plight of Hinkley was captured in the 2001 film “Erin Brockovich” starring Julia Roberts, which continued to keep hexavalent chromium in the public eye. Since then, hexavalent chromium, also known variously as hex chrome, chromium 6, chromium (VI), Cr6+, or Cr(VI), has stayed in the spotlight as a nasty player when it comes to human health.
Cr(VI), one chemical state of the natural metallic element chromium, is appearing more frequently as a contaminant at Wilcox project sites. Trivalent chromium, or Cr(III), and Cr(VI) are the two most common forms, and they behave quite differently. Cr(III), fortunately the more abundant form in nature, has low toxicity, aqueous solubility, and mobility in the environment. Cr(VI), on the other hand, is toxic, highly soluble, and much more mobile
Cr(VI) compounds are used in a wide range of industrial applications – for example, in electroplating, stainless steel production, protective metal coatings, leather tanning, dye and paint production, and wood preservation. Not surprisingly, most common exposures tend to be occupational, though the case of Hinkley, California, is an alarming example of residential exposure. Exposure to Cr(VI) can occur by inhaling it, ingesting it in food or water, and by direct skin contact. Cr(VI) is listed as a known human carcinogen, and its compounds have been shown to cause lung cancer in humans when inhaled, particularly among workers who were exposed to Cr(VI) in workplace air. Other adverse health effects include nasal and sinus cancers, kidney and liver damage, nasal and skin irritation and ulceration, and eye irritation and damage.
Screening levels published by the Indiana Department of Environmental Management highlight the stark differences in toxicity between Cr(VI) and total chromium, which is typically comprised mostly of Cr(III). The Residential Groundwater Direct Contact screening level for total chromium is 100 µg/L (or parts per billion), whereas the same screening level for Cr(VI) is 0.35 µg/L, 285 times lower. Because of its inhalation risk, residential and commercial indoor-air screening levels for Cr(VI) are among the lowest of any metal, including 2,600 times lower than for residential exposure to vapor from elemental mercury. Total chromium, on the other hand, has no indoor-air screening levels. The residential, commercial, and excavation soil direct contact screening levels exhibit similar contrasts between Cr(VI) and total chromium and Cr(III) as insoluble salts. Differentiating them further, the Cr(VI) screening levels are for carcinogenic (cancer-causing) endpoints, whereas the screening levels for total chromium are for non-carcinogenic endpoints.
Adding to its mischievousness, Cr(VI)’s chemical properties make it tricky to characterize during environmental investigations. Cr(VI) is sensitive to changes in its environment that can transform it to Cr(III) in environmental samples, necessitating a bit more involved groundwater sampling protocols in the field. As noted earlier, Cr(VI) is very soluble and does not significantly interact with soil matrices, traits that give it high mobility in the subsurface. In fact, the distribution (adsorption) coefficient (Kd) of Cr(VI), expressed in L/Kg, is only 1/100,000 that of Cr(III) for the most common pH ranges. These characteristics allow Cr(VI) to readily leach from surface soil into groundwater and then move at essentially the same velocity as the groundwater with little chemical interaction with organic carbon in the aquifer matrix that would slow it down (chemical retardation). Consequently, it’s common to find Cr(VI) at greater down-gradient distances than other contaminants released from the same source.
Fortunately, Cr(VI)’s proclivity for transformation also facilitates its remediation. Cr(VI) in groundwater can be readily reduced to Cr(III) using many of the same methods that rely on electron-donating compounds that create reducing conditions for in-situ chemical reduction (ISCR) and enhanced reductive dechlorination (ERD) at sites contaminated by chlorinated solvents. Under reducing conditions, Cr(VI) reacts with other subsurface minerals to form less toxic, insoluble, and immobile Cr(III) hydroxide precipitates. The precipitates are stable, solid, and integrated with the soil matrix, so they generally stay put – no need to remove them. Under naturally reducing subsurface conditions, Cr(VI) can also be chemically or biologically attenuated.
From all appearances, Cr(VI) will continue to emerge more often as a primary and secondary contaminant at environmental investigation and remediation sites. Our extensive experience with Cr(VI) and with remediation methods that rely on establishing reducing conditions in the subsurface has positioned Wilcox to provide a full range of high-value services to clients affected by Cr(VI) in soil and groundwater.