Groundwater flow, hydrochemistry, and uranium deposition in the Powder River Basin, Wyoming

The relation between regional groundwater flow, hydrochemistry, and uranium distribution in the Powder River Basin indicates that uranium was deposited during the Tertiary Period in groundwater recharge areas where the groundwater changed from a sulfate-bicarbonate water to a bicarbonate-rich water. The regional recharge and discharge areas of present-day groundwater-flow systems have about the same location as the recharge and discharge areas of the Tertiary groundwater-flow systems. The present-day groundwater is recharged in the eastern, western, and especially in the southern margins of the basin and is discharged in the valley of the Powder River, especially in the north. Flow nets for the groundwater were constructed on the basis of piezometric data from existing water wells in the Powder River Basin. The Tertiary groundwater-flow systems had a larger longitudinal flow component than present-day groundwater-flow systems. The present-day topography causes a large lateral and vertical groundwater-flow component toward the large river valleys. The topography during the Tertiary was a result of deposition and had little relief, whereas, the present-day topography is a result of erosion and has large relief. The groundwater chemistry of this area during the Tertiary was probably similar to that of today because the groundwater flowed through the same sediment as present-day groundwater. Anions in the present-day groundwater undergo the following sequence of hydrochemical changes along the regional flow path from the southern recharge area to the northern discharge area: HCO3- + SO4-- à SO4--+ HCO3-àHCO3-. An abrupt decrease in sulfate concentration occurs where the sulfate is removed from solution in the transition zone between the SO4-- + HCO3-and HCO3- facies. The most likely cause of this decrease is a strong reducing environment, which may be caused by sulfate-reducing bacteria. Cations in the present-day groundwater undergo the following sequence of hydrochemical changes along a flow path from shallow to deep groundwater in recharge areas: Ca++ + Na+ àNa+ + Ca++ àNa+. The decrease in calcium and increase in sodium with depth is caused by base exchange and the precipitation of calcite cement. Major unoxidized uranium deposits in the Powder River Basin occur near the transition zone between the SO4-- + HCO3-and HCO3- facies. The uranium is transported in solution by groundwater in the HCO3- + SO4-- and the SO4-- + HCO3- facies and precipitated in the transition zone between the SO4-- + HCO3-and HCO3- facies. Precipitation occurs where strong reducing conditions exist around abundant organic material in which sulfate-reducing bacteria may live and multiply