Isotope evidence of hexavalent chromium stability in ground water samples

- Cr(VI) reduction modified strongly Cr isotope ratios within 24 h after sampling.
- On-site preservation recommended by commonly available methods is not sufficient.
- Water samples should be processed on-site through anion column chemistry.

Chromium stable isotopes are of interest in many geochemical studies as a tool to identify Cr(VI) reduction and/or dilution in groundwater aquifers. For such studies the short term stability of Cr(VI) in water samples is required before the laboratory analyses can be carried out. Here the short term stability of Cr(VI) in groundwater samples was studied using an isotope approach.Based on commonly available methods for Cr(VI) stabilization, water samples were filtered and the pH value was adjusted to be equal to or greater than 8 before Cr isotope analysis. Based on our Cr isotope data (expressed as δ53CrNIST979), Cr(VI) was found to be unstable over short time periods in anthropogenically contaminated groundwater samples regardless of water treatment (e.g., pH adjustment, different storage temperatures). Based on our laboratory experiments, δ53CrNIST979 of the Cr(VI) pool was found to be unstable in the presence of dissolved Fe(II), Mn(IV) and/or SO2. Threshold concentrations of Fe(II) causing Cr(VI) reduction range between 10 mg L−1 and 100 mg L−1and less than 1 mg L−1 for Mn. Hence our isotope data show that water samples containing Cr(VI) should be processed on-site through anion column chemistry to avoid any isotope shifts.

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