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Chromium(VI) concentrations in groundwater sampled from three contaminant plumes in aquifers in the Mojave Desert near Hinkley, Topock and El Mirage, California, USA, were as high as 2600, 5800 and 330 mu g/L, respectively. delta Cr-53/52 compositions from more than 50 samples collected within these plumes ranged from near 0 parts per thousand to almost 4 parts per thousand near the plume margins. Assuming only reductive fractionation of Cr(VI) to Cr(III) within the plume, apparent fractionation factors for delta Cr-53/52 isotopes ranged from epsilon(app) = 0.3 to 0.4 within the Hinkley and Topock plumes, respectively, and only the El Mirage plume had a fractionation factor similar to the laboratory derived value of epsilon = 3.5. One possible explanation for the difference between field and laboratory fractionation factors at the Hinkley and Topock sites is localized reductive fractionation of Cr(VI) to Cr(III), with subsequent advective mixing of native and contaminated water near the plume margin. Chromium(VI) concentrations and delta Cr-53/52 isotopic compositions did not uniquely define the source of Cr near the plume margin, or the extent of reductive fractionation within the plume. However, Cr(VI) and delta Cr-53/52 data contribute to understanding of the interaction between reductive and mixing processes that occur within and near the margins of Cr contamination plumes. Reductive fractionation of Cr(VI) predominates in plumes having higher epsilon(app), these plumes may be suitable for monitored natural attenuation. In contrast, advective mixing predominates in plumes having lower epsilon(app), the highly dispersed margins of these plumes may be difficult to define and manage. Published by Elsevier Ltd.