A site that has entered the Maryland Department of the Environment Voluntary Cleanup Program (MDE/VCP) typically gets analyzed for the Priority Pollutant list (PPL) of metals, as well as, various organic analytes. The PP list of metals includes total chromium. However, if previous site use indicates that chromium is likely to be present or if prior investigation reveals total chromium concentrations in excess of the acceptance criteria, then speciation of chromium is required. Sampling and analysis for chromium VI will then be requested by MDE. This enables an examination at the site of the most toxic form of chromium.
Chromium (Cr) is a steel-grey lustrous metal. It is used in industry in a diversity of products and processes. In the environment, chromium is found in two forms - the naturally occuring Cr III and Cr VI which is a by product of industrial processes. While Cr III is an essential trace element for human health, there is evidence of Cr VI carcinogenicity in humans. Thus, the strong interest in Cr VI cleanup at sites. The more toxic nature of chromium VI is also reflected in the much lower cleanup standards for Cr VI than those for Cr III.
MDE/VCP Cleanup Standards for Chromium
Source: MDE Cleanup Standards for Soil & Groundwater, August 2001 update
Cr VI presents a challenge in that transformation from Cr VI to Cr III occurs rather easily in the presence of organic matter. Water samples contain dissolved organic matter thus rapid conversion of states is an issue. The EPA has designated 24 hours as the maximum holding time before analysis for Cr VI in water samples. This is so that the laboratory test results accurately represent the true field conditions.
To improve analytical results, the 1996 update to SW846 added method 3060A as a preparation method for chromium VI in soils. It is an alkaline digestion method that extracts both the soluble and insoluble forms of Cr VI unlike the previous techniques laboratories were using. Prior to method 3060A, laboratories extracted soils with deionized water. The use of deionized water only extracted the soluble Cr VI forms.
The EPA has not designated a holding time for Cr VI in soil. According to method 3060A, Cr VI has been shown to be quantitatively stable in field-moist soil samples for 30 days from sample collection. Additionally, Cr VI has also been shown to be stable in the alkaline digestate for up to 7 days after extraction from soil.
To address the problem of conversion of Cr VI to Cr III, researchers at Virginia Tech are developing a technique of capturing Cr VI at the sampling site. This will enable the laboratorys test results to better represent the conditions in the field. The specific types of chromium will be absorbed and stabilized on a plastic membrane. This technique would likely be used in conjunction with existing sampling technology and should not require additional training.
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