The fate of Cr(VI) in contaminated aquifers 65 years after the first spillage of plating solutions: A δ53Cr study at four Central European sites

- Natural attenuation of Cr(VI)-contaminated groundwater was studied using δ53Cr values.
- A δ53Cr shift to higher value from plating bath to aquifer indicated Cr(VI) reduction.
- Across the sites, lower residual Craq concentrations accompanied higher δ53Cr values.
- Rayleigh models suggested a large (30 to 60%) chromium removal as insoluble Cr(III).
- Uncertainty caused by variable Cr isotope fractionation factors was about 30%.

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 δ53Cr signatures of the plating baths. The mean δ53Cr values of the Cr plating baths, representing contamination sources, were 0.0‰ at Zlate Hory (ZH), 0.5‰ at Loucna nad Desnou (LD), 0.3‰ at Letnany (AV), and 0.1‰ 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 Craq, mostly residual Cr(VI), was 1.1‰, 1.3‰, 3.3‰ and 3.3‰ at ZH, LD, AV, and JI, respectively. The mean concentration of Craq was 0.43, 0.47, 2.38 and 4.32 mg L− 1, respectively. AV and JI, the sites with lower residual Craq concentrations in groundwater, had higher δ53Cr 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.