Theme: 08. Metals

Topic Area: 8a. Managing Chromium-Contaminated Sites

Formatted Title:

Determination of a Site-Specific Background Concentration of Hexavalent Chromium in Groundwater

Background/Objectives:

Problematic concentrations of hexavalent chromium [Cr(VI)] are a common occurrence in groundwater in regions where serpentine soils are derived from ultramafic parent rock, such as the Sacramento Basin of California. The origin of Cr(VI) is thought to be related to dissolution of Cr(III) within mineral phases, the oxidation of Cr(III) to Cr(VI) on Mn(IV) oxides, and the subsequent transport of Cr(VI) in groundwater; these are complex redox- and pH-dependent processes. Groundwater in the Sacramento Basin is recharged by precipitation leaching vertically through soils and laterally through the parent material that outcrops at the eastern periphery of the basin. Cr(VI) leaches directly from parent material and in soil is mobilized through geogenically and biogenically mediated reactions involving manganese oxides.

Approach/Activities:

A site-specific background concentration for groundwater was proposed on behalf of the responsible party (RP) for a site in the basin. The estimate was based on a q-q plot analysis of the ensemble of total Cr concentrations in multiple groundwater wells at the site. A q-q plot estimate can be subjective, however, so additional analyses were requested of the EPA Office of Research and Development (ORD). ORD used k-means clustering to investigate the locations of wells producing concentrations deemed acceptable by the q-q plot method. ORD also triaged wells by the slope in the data through time using the robust Theil-Sen regression method and subjected the results to cluster analysis.

Results/Lessons Learned:

K-means cluster analysis indicated that wells having data identified as background by the q-q plot method were not concentrated in areas most likely to be representative of background. Cluster analysis also indicated that wells without significant trends in time were not concentrated in areas likely to represent background concentrations. Based on these results the EPA remedial project manager for the site elected to apply the California MCL for groundwater of 50 µg/L.

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Patrick Kelly