A uranium mill, located on the White Mesa in San Juan County, Utah, began operating in 1980 to extract yellowcake (U₃O₈) and vanadium from uranium ores.  Potential air and ground-water exposure pathways of uranium and other metals to tribal members include (1) airborne dust from ore storage pads, from trucks delivering ore to the mill, and from the mill’s drying ovens, (2) dissolution of airborne dust deposited on the soil, and (3) leakage from the tailings ponds to the groundwater aquifer, which flows from the mill toward the reservation.An evaluation of the concentration of major ions and metals in the groundwater up- and down-gradient of the mill reveals complex spatial variations in (1) the concentration of uranium and other metals in bedrock, soils, and groundwater, (2) the geochemical conditions favorable for either uranium solubility or precipitation in ground water, and (3) geologic conditions that can influence groundwater residence times in White Mesa.  This spatial variability makes it extremely difficult to assess the environmental impact of the mill by using concentration data alone.  In this study we supplement the ground-water concentration data by (1) age dating groundwater to determine the time required for groundwater to flow from the mill to the reservation, (2) analyzing ground-water samples for uranium, oxygen, and sulfur (S and O in sulfate) isotopes to determine if the source of uranium in groundwater is natural or is a result of extraction processes, (3) using geochemical modeling to determine the mobility of uranium and other metals in the groundwater in White Mesa, and (4) trace-element analysis of <-200 mesh stream sediments from ephemeral drainages surrounding the mill.  This talk will focus primarily on how the results from this supplemental monitoring are helping to determine the effects of the mill on ground-water and surface-water chemistry in White Mesa.