A preliminary study on the effects of rainfall-related conditions on chromium increase in ultramafic-hosted springs:...

This study investigates the impact of intense rainfall on chromium concentrations in five springs discharging from ultramafic rocks in the Northern Apennines (Italy), which are used for drinking water supply through integration into the local water network. Total chromium concentration increased significantly in response to heavy rain, exceeding the WHO drinking water guideline value (up to 80 μg/L) in one spring and the forthcoming 2036 EU target of 25 μg/L in all the springs. This increase could be attributed to a synergistic combination of factors: i) the reduction of Cr(VI) to Cr(III) by natural organic matter (NOM) in soil and transport as NOM-Cr(III) colloids and/or during the oxidation of magnetite to ferrihydrite in the aquifer; ii) the abundance of detrital ultramafic material in the study area, which may store Cr(III)-bearing colloids too; iii) a triggering effect of first intense rainfall after a 20 dry consecutive days period (wet-dry cycle). Moreover, the persistence of a high Cr(III) concentration in the aquifer even a month after the intense rainfall event aligns with previous laboratory studies on NOM-Cr(III) colloidal stability, which showed that such colloids are highly stable and can persist in solution for at least 20 days.