Tracing Water Sources Along the Brazos River Alluvial Aquifer with 234U/238U Activity Ratios and Uranium Concentrations

Abstract

The combined use of 234U/238U activity ratios and uranium concentrations can be used to trace water sources within a hydrological system. Additionally, the specific uranium concentration and isotopic signature of each source can be applied to mixing calculations to estimate the relative contribution of each source in a river. 234U/238U activity ratios and uranium concentrations were measured over a 6-month period in the Brazos River watershed along the Brazos River Alluvial Aquifer to determine where water in the Brazos River is sourced from and to estimate groundwater discharge to the Brazos River. Results from this study indicate that lithology within the Brazos River watershed affects 234U/238U activity ratios and uranium concentrations along the studied stretch of river as there is a change from carbonate rocks to siliciclastic rocks downstream and an associated change in 234U/238U activity ratios. Rain was found to have a negligible effect on 234U/238U activity ratios of sources within the area, and only dilutes the concentration of uranium in these sources. 234U/238U activity ratios and uranium concentrations suggest water in the Brazos River near Bryan/College Station, Texas is sourced from Lake Whitney, groundwater, and tributaries. Groundwater appears to discharge from the alluvial aquifer to the river at a steady rate with slight increases in groundwater contributions as discharge increases in the Brazos River. This study demonstrates the utility of uranium as a natural tracer of water sources.