ARSENIC VARIABILITY IN GROUND WATER, ESTER DOME, ALASKA

Ester Dome, located along the western margin of Fairbanks, AK, is characterized by high dissolved As concentrations in ground water and intrusion-related gold lode deposits. Ester Dome is primarily composed of biotite schists that were domed by the intrusion of ca 90 Ma granite. NE-trending, shear zones cut the dome and host arsenopyrite-, stibnite-, and gold-bearing quartz veins. A hydrogeochemical investigation is being conducted to determine baseline conditions, to identify hydrologic pathways, and to identify processes controlling the variability of arsenic in the ground water. An understanding of As distribution and mobility is important for land use decision-making. Elevated As levels in domestic well waters have been of concern since the 1970’s when concentrations as high as 10,000 µg/L were reported. Twelve monitoring wells and sixteen domestic wells were sampled quarterly from November, 2000 to September, 2001 to determine the spatial and temporal variability in water chemistry. Field parameters, major and trace element concentrations, and hydrogen, oxygen, sulfur, and strontium isotopic compositions are being determined. The circumneutral, Ca-HCO₃-dominated water has a range of dissolved As concentrations from < 3 to 850 µg/L with a median of 150 µg/L. One well, located at the top of the dome, exhibits the greatest temporal variability in As concentrations, which correlates inversely with the static water level. Spatially, dissolved As concentrations are also quite variable, with wells located a few hundred meters apart having < 3 and 300 µg/L. Although redox chemistry and elemental ratios suggest that a number of processes likely affect the dissolved As concentrations, proximity to mineralized shear zones appears to be the primary control on As concentrations in the ground water.