CHROMIUM CHEMISTRY AND SPECIATION IN NATURAL WATERS ASSOCIATED WITH MAFIC ROCKS, ICELAND

Natural waters in Iceland associated with mafic rocks were collected and analyzed for chromium concentration and speciation (Cr^III, Cr^VI and Cr_TOT). The water sampled included non-thermal surface and spring water, surface geothermal water, and single and two-phase geothermal well discharges with temperature ranging from 0 to 184 °C, pH of 2.01-9.58, and total dissolved solids (TDS) of 35 to 4030 ppm. The total Cr concentration was between <0.01 and 660 ppb with highest concentrations with the waters associated with the lowest pH. At pH>4 the measured Cr^III concentration is low, generally <1 ppb but with decreasing pH elevated Cr^III were observed reaching values of hundreds of ppb. At pH<6 no measurable Cr^VI was detected whereas in neutral to alkaline waters measured Cr^VI concentrations were as high as 3 ppb, frequently dominating over Cr^III. The Cr chemistry in natural waters associated with mafic rocks in Iceland is largely influenced by the water pH. At low pH Cr-containing minerals are unstable and Cr leaches into the waters as Cr^III. Possible oxidation of Cr^III to Cr^VI is minimized thermodynamically and the mobility of Cr^VI is further reduced by surface complexation reactions. At neutral to alkaline pH the primary Cr-containing mineral phases like titanomagnetite and chromite may be stable limiting Cr-rock leaching. Solubility of secondary Cr^III-Fe^III-mineral phases may further reduce and/or limit the Cr^III availability. In contrasts, Cr^VI mobility is enhanced at pH >8 associated with decreasing importance of mineral surface complexation. Hence, Cr^VI becomes increasingly dominant form of dissolved Cr with increasing pH above 7-8. Many groundwater drinking supplies associated with mafic rocks are characterized by moderately alkaline pH result in Cr^VI concentrations of few ppb.