The fate of Cr(VI) in contaminated aquifers 65 years after the first spillage of plating solutions: A delta Cr-53 study at four Central European sites
Abstrakt
Chromium isotope systematics and concentrations of dissolved Cr were studied in shallow aquifers at four industrial sites in the Czech Republic, Central Europe. Chromium plating baths and wastewaters started to contaminate groundwater in the late 1940s.
Since plating shops at all sites still operate, it was possible to determine the delta Cr-53 signatures of the plating baths. The mean delta Cr-53 values of the Cr plating baths, representing contamination sources, were 0.0 parts per thousand at Zlate Hory (ZH), 0.5 parts per thousand at Loucna nad Desnou (LD), 0.3 parts per thousand at Letnany (AV), and 0.1 parts per thousand at Velesin (JI).
At all sites, groundwater Cr was isotopically heavier, compared to the contamination source, reflecting natural in-situ Cr(VI) reduction to insoluble Cr(III). The magnitude of the average positive isotope shift from the plating bath Cr(VI) to groundwater Cr-aq, mostly residual Cr(VI), was 1.1 parts per thousand, 1.3 parts per thousand, 3.3 parts per thousand and 3.3 parts per thousand at ZH, LD, AV, and JI, respectively.
The mean concentration of Cr-aq was 0.43, 0.47, 2.38 and 4.32 mg L-1, respectively. AV and JI, the sites with lower residual Cr-aq concentrations in groundwater, had higher delta Cr-53 values, suggesting higher rates/efficiency of Cr(VI) reduction, and of toxicity removal.
Simple Rayleigh models indicated that as much as 60% of Cr(VI) has been removed from the groundwater by spontaneous reduction at AV and JI. At ZH and LD, the same models indicated a 30% Cr removal from the solution.
Even if uncertainties in model application and Cr fractionation factors are considered, it appears that natural attenuation at Cr(VI)-polluted sites can improve groundwater quality by tens of percent in a relatively short period of time of several decades.