Chromium concentrations in groundwater and surface water often exceed screening criteria but may be naturally elevated. In regulated settings, these elevated concentrations are often conservatively assumed to represent the presence of hexavalent Cr, even when speciation has not been performed to confirm the presence of the more toxic and more mobile hexavalent form. This assumption can be tested using standard analyses obtained during typical environmental investigations, including paired unfiltered and filtered metals analyses, elemental ratios (Cr/Al, Cr/Fe), total suspended solids analyses, and field readings (ORP, DO, and pH). Contaminated samples containing hexavalent Cr are distinguished from uncontaminated samples containing naturally occurring concentrations using this forensic technique, which is based on multiple independent lines of evidence and considers the effects of sorption on suspended particulates, pH, redox, salinity, and well-construction materials. Case studies are drawn from sites across the U.S., where the technique was successfully used to confirm the need for remediation or further investigation at some sites, and to achieve "No Further Action" decisions at other sites where the elevated Cr concentrations were demonstrated to be natural.

Chromium in aqueous media:  How to determine source and speciation with limited data

255th National Meeting (2018)

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