QUANTIFYING WATER EXCHANGE BETWEEN THE BRAZOS RIVER AND THE BRAZOS RIVER ALLUVIAL AQUIFER USING HIGH TEMPORAL AND SPATIAL RESOLUTION MEASUREMENTS

Accurate evaluations of water supplies in the American West are vital to the development of water management plans that will be fair and effective during times of drought. Most management plans in place today do not account for the exchange between surface and groundwater sources, resulting in allocation of more water than is available. In this study we investigate the use of high frequency differential stream gaging and hydraulic gradient monitoring, combined with radon radioisotope (Rn²²²) tracer mass balance calculations, specific conductance measurements, and chemograph separation to find this missing variable for the Brazos River watershed, Texas. Current high resolution measurements allow us to understand the exchange patterns between the Brazos River and its alluvial aquifer with greater detail than has been found in past studies of groundwater-surface water exchange.

To perform differential stream gaging the river level is recorded at 20-minute intervals at 5 stations (creating 4 reaches) along a 136 km river stretch from Hearne, TX, to Navasota, TX. Rating curves were developed at each gaging station using discharge measurements from a canoe-mounted Acoustic Doppler Current Profiler (ADCP). These gages allow high temporal resolution estimates of river water gains or losses in between stations. Paired synoptic, same-day discharge measurements during the development of the rating curves in May-July, 2015, indicate the first reach (40 km long) lost 186,590 m³km^-1d^-1, while the second reach (24.3 km) gained 58,347 - 94,368 m³km^-1d^-1. The third reach (22 km) gained 50,479 m³km^-1d^-1, and the fourth (50 km) gained 171,904 m³km^-1d^-1. High frequency hydraulic gradient and river level data show an inverse relationship between gradient and river level, always indicating groundwater flow towards the river. During a one day longitudinal survey (July 31, 2015) along the second reach, specific conductance increased 26 µS/cm, indicating groundwater inflow of 13,982 m³km^-1d^-1_. This is 24% of the groundwater discharge indicated by the most recent differential gaging (July 23, 2015), reflecting the higher river discharge on July 23 (1.5e7 m³d^-1) vs. July 31 (7.7e6 m³d^-1). Future plans include calculating and comparing exchange volumes from chemograph separation and a Rn²²² mass balance.