Temporal changes in Cr fluxes and δ53Cr values in runoff from a small serpentinite catchment (Slavkov Forest, Czech Republic)

Abstract

Headwater catchments underlain by ultramafic bedrock may release toxic amounts of hexavalent chromium [Cr(VI)] into runoff. Redox cycling of Cr during weathering, pedogenesis and runoff generation can be constrained by studying 53Cr/52Cr isotope systematics. We report a 12-month time-series of δ53Cr values of runoff from a small serpentinite-dominated catchment in Central Europe, complemented with a 5-year monitoring of Cr and DOC concentrations. δ53Cr values of bulk bedrock, rock-forming minerals, and bulk soil were also determined. Cr(VI) concentrations in runoff exceeded the 10-ppb maximum contaminant level by 70%. Concentrations of Cr(VI) in runoff were positively correlated with water discharge and DOC concentrations. Chromium export flux in winter was significantly higher than in summer. Runoff δ53Cr exhibited a temporal trend, with negative values (−0.6‰) in winter and positive values (0.5‰) in summer. On an annual basis, the mass-weighted δ53Cr value of runoff was −0.4‰. Dissolved Cr exported in winter was isotopically lighter than the within-site Cr sources: δ53Cr of bulk bedrock ranged from −0.2 to 0.0‰, and δ53Cr of soil ranged from −0.2 to 0.3‰. Chromium isotope analysis of seven rock-forming minerals separated from serpentinites, tremolite/actinolite schists and amphibolites revealed a much wider range of δ53Cr values, from −0.4‰ (hornblende) to 1.7‰ (albite). Most of the variability in runoff δ53Cr can be explained by incongruent oxidative weathering, with a contribution of isotope-selective processes related to pedogenesis. Isotopically light Cr was preferentially mobilized from weathered bedrock and soil in cold, water-rich periods and exported via stream water. In these periods, precipitation water more rapidly penetrates into the soil than in summer months, characterized by lower rainfall and larger evapotranspiration, and more efficiently leaches the accumulated Cr(VI).