Geogenic and Anthropogenic Chromium Contamination in Groundwater in an Ophiolitic Area, Northeastern Iran

Abstract

Geology of Iran shows an ophiolitic belt around the central Iran micro-continent. One of the main ophiolite suites, with ca. 100km long and 15km wide, located northeastern Iran as Sabzevar ophiolite suite. Ultramafic rocks of this ophiolite suite, display a high concentration of chromium (1000-3000 ppm ) as a compatible element, especially in pyroxene group minerals. Average Chromium content of this ophiolite suite in peridotites is 2558 ppm , with maximum 4525 ppm (in pyroxenite) and minimum 832 ppm (in dunite). Dunite layers lie underneath of these ultramafic rocks, containing chromite lenses (FeCr 2 O 4 ) with 20.56wt% Cr 2 O 3 . Today, about 10 active mining sites excavate ultramafic rocks for chromite ore mineral and altered ultramafic rocks, serpentinite, dump as unconsolidated gangue materials along stream pathways. There is an unconfined aquifer just southern of this ophiolite range containing detrital altered ultramafic rocks with the high concentration of chromium minerals. In this study 23 groundwater samples, collected from unconfined serpentinite alluvium aquifer that shows cumulative increasing Cr towards south because of increasing residence time, and much more water-rock interactions. Total Cr concentrations in this aquifer are from 12 to 61µg/l, higher than normal level of Cr mentioned by WHO (2µg/l). On the other hand, discharging of chromite mine and mineral processing site, contaminate one of drinking well at Forumad village up to 61µg/l of total chromium.Although trivalent Cr is an essential nutrient, in oxidation conditions with increasing Eh and pH, it changes to chromate (CrO 4 −2 ) and dichromate (Cr 2 O 7 2-) as dissolved anions which will be toxic and carcinogenic in groundwater. Based on California EPA Office of Environmental Health Hazard Assessment (OEHHA), 7.2% of total Cr is hexavalent. Recent information indicates that hexavalent chromium varies from 50% to 90% of the total chromium in many water supplies.