TRACING WATER PROCESSES IN THE CENTRAL RUN OF THE BRAZOS RIVER: EVIDENCE FROM δ18O, δD, AND CONDUCTIVITY

To examine hydrologic processes influencing water in the central run of the Brazos River, bi-weekly water samples were collected in Brazos County, Texas and analyzed for δ¹⁸O, δD, conductivity, and temperature from January 2012 through September 2012. The same measurements were performed on water sampled by Niskin bottle at varied depths from Lake Whitney on August 3, 2012. δ¹⁸O and δD in Brazos River water are correlative and highly variable, ranging from -5.70 to -0.50‰ and -38.3 to -8.0‰ respectively. Rather than following the trend expected for meteoric water, the Brazos River δ¹⁸O-δD data fall along an evaporation trend that intersects and surpasses values for Lake Whitney (δ¹⁸O = -1.6 ± 0.3‰, δD = -12 ± 1‰) collected during the same period. In contrast, δ¹⁸O-conductivity data for the Brazos River do not intersect data for Lake Whitney, but rather are lower in salinity and suggest mixing with an evaporated water source of lower conductivity than Lake Whitney water. Discharge, conductivity, δ¹⁸O, and δD show strong seasonal trends with highest discharge and lowest conductivity, δ¹⁸O, and δD during January-March; lowest discharge and highest conductivity, δ¹⁸O, and δD during July-September; and intermediate values from April to June. This reflects higher precipitation and lower temperature in winter versus summer, with enhanced evaporation in summer. Based on a Rayleigh distillation (closed-system) modeling, the water in Lake Whitney has experienced ~20% evaporation since falling as precipitation, whereas water in the Brazos River during low flow conditions has experienced ~30% evaporation since falling as precipitation.