Emerging investigators series: frontier review: occurrence and speciation of chromium in drinking water distribution systems
Chromium can exist as both trivalent Cr(III) and hexavalent Cr(VI) in typical drinking water conditions, with Cr(VI) being of particular concern due to its high toxicity and new regulatory perspectives. This study critically reviewed the occurrence and speciation of chromium in drinking water distribution systems in the U.S., based on a detailed analysis of the up-to-date EPA Unregulated Contaminant Monitoring Rule (UCMR) database. Data show that the nationwide Cr(VI) levels in drinking water ranged from non-detectable to 97 μg L−1 at the entry point to drinking water distribution systems, and between non-detectable and 67 μg L−1 at the maximal residence time of distribution systems. Geographic distributions show that higher drinking water Cr(VI) concentrations were observed in Southwest U.S. and were closely associated with the dependence on groundwater as a drinking water source. While Cr(VI) and total chromium concentrations showed either an increase or a decrease between the entry point and the maximal residence time of drinking water distribution systems, they suggested complex physicochemical processes that can act as both sources and sinks of chromium in distribution systems, including the oxidation of Cr(III)-containing solids by residual disinfectants and the adsorption of Cr(VI) by corrosion scales. Time-dependent data analysis revealed that a considerable fraction of Cr(III) at the entry point to distribution systems was inadvertently converted to Cr(VI) by the time the drinking water reached the maximal residence time of distribution systems, and this conversion was positively correlated with the presence of residual disinfectants. Data analyses suggest that the reactivity of downstream water distribution should be managed to control chromium in tap water.