Behaviour Of Cr In Runoff From Two Catchments Underlain By Felsic Bedrock

Chromium isotope and chemical compositions in runoff from two felsic rock catchments, the Lysina (LYS; Czech Republic) and the Neglinka (NEG; northwest Russia), were studied. Three types of water were identified: Na-Ca-SO4 type for the LYS, Mg-Ca-Cl-HCO3 type for the NEG downstream, and Mg-Ca-Cl to Mg-Ca-HCO3 type for the NEG upstream. The Cr (III) liberated during soil/bedrock weathering oxidized to Cr (VI), which was the main dissolved Cr species in both catchments. Variations in delta Cr-53 values for LYS runoff (-0.07 parts per thousand to +0.37 parts per thousand) reflected a sole Cr source, seasonality of weathering and hydrologically controlled leaching/export processes. In the NEG catchment, back-reduction of Cr (VI) to Cr (III) during riverine transport resulted in a decrease in Cr concentrations (to 10-30%) and in an increase of delta Cr-53 values (+0.09 to +0.42 parts per thousand for the upstream to +0.72 to +1.73 parts per thousand for the downstream). Groundwater discharge could have influenced the chemical composition of the downstream NEG runoff to some extent.