RARE-EARTH ELEMENT GEOCHEMISTRY OF COLOR LAKE, AN ACIDIC FRESH-WATER LAKE ON AXEL-HEIBERG-ISLAND, NORTHWEST-TERRITORIES, CANADA

A lake column profile was collected in 24 m of water from Colour Lake, Axel Heiberg Island, Northwest Territories, Canada, in early June of 1991 beneath 1.8 m of lake-ice. The rare-earth element (REE) concentrations of the acidic, fresh waters of Colour Lake were analyzed, along with the major solute chemistry, in order to investigate REE distribution and speciation in the low-pH waters. Previous studies indicate the lake is dominated by Ca2+, Mg2+ and SO42-, and has no measurable carbonate alkalinity. Shale-normalized REE patterns for the lake waters are all enriched in the middle REE's (MREE's) as compared to the light REE's (LREE's) and heavy REE's (HREE's). Dissolved REE concentrations in Colour Lake (e.g., 2.3-22 nmol kg(-1) for Nd, mean value of 20 nmol kg(-1)) are typically less than values obtained for the acid, saline waters of the Lake Tyrrell playa system in Australia as well as the slightly acidic groundwaters of the Carnmenellis region of England but greater than REE concentrations measured in the alkaline, saline waters of Mono Lake, California, U.S.A. The speciation of the REE's in Colour Lake was modelled based on the major solute chemistry, using a combined specific ion interaction and ion pairing model. These calculations suggest that in the relatively high-sulfate, low-pH waters of Colour Lake, the REE's exist as either free metal, SO42-, or F- complexes. Our calculations indicate that SO42 complexes dominate over the free metal and fluoride species for the majority of the REE's in the lake water and that, in general, for acidic, high-sulfate waters, sulfate complexes with the REE's may increase the dissolved concentrations of the REE's over natural waters where relatively low sulfate concentrations prevail.