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 (1000-3000ppm) as a compatible element, especially in pyroxene group minerals. Average Chromium content of this ophiolite suite in peridotites is 2558ppm, with maximum 4525ppm (in pyroxenite) and minimum 832ppm (in dunite). Dunite layers lie underneath of these ultramafic rocks, containing chromite lenses (FeCr2O4) with 20.56wt% Cr2O3. 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 (CrO4-2) and dichromate (Cr2O72-) 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.
Highlights
Serpentinites or altered ultramafic rocks are distributed around central Iran and always associated with ophiolite complexes
Chromium in these ultramafic rocks exists as a trace element especially in pyroxene group minerals, olivine, and chlorite, and Cr as a major element exists in chromite, and spinel minerals
The purpose of the present study is to find out total chromium level and Cr(VI) concentration in water resources of Davarzan area which is the important area because of proximity several active chromite mine, serpentinite and peridotite aquifers alluvium, large watershed basin, and populated area with several village and a big town, Davarzan (Figure 1)
Summary
Serpentinites or altered ultramafic rocks are distributed around central Iran and always associated with ophiolite complexes. 200km long and maximum 20km wide and 2100km areas (Figure 1) Chromium in these ultramafic rocks exists as a trace element especially in pyroxene group minerals, olivine, and chlorite, and Cr as a major element exists in chromite, and spinel minerals. Alteration and weathering of these rocks generate serpentinite soil and sediments containing high-level Cr as alluvial fans in southern and northern plains of this highland. Drainage basins wash both sides of this ophiolite suite (Figure 1) and leach Cr into groundwater. At very low or no oxygen levels, Cr(III) is the dominant species, which will be in cationic (Cr+3, CrOH+2, or Cr(OH)2+) or neutral (Cr(OH)3) form depending on the pH [5,7]. Cr(III) tends to be extremely insoluble (< 20 μg/L) between pH 7 and pH 10, with minimum solubility at pH 8 of about 1 μg/L[5]
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