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

The Brazos River is the second largest river in Texas and a vital water supply for municipal, industrial, and agricultural uses in the state. The Brazos River Alluvial Aquifer (BRAA) is a minor aquifer supplying water mainly for agricultural use along 563 km of the Brazos River. Its connection to the Brazos River is well established, although the connection has not been studied using a range of physical and chemical methods at high temporal resolution. As population grows and water supply declines in the area, both aquifer and river will become ever more important resources. Fully understanding interactions between the Brazos River and the BRAA will be a vital part of effectively and fairly managing them in a more water-scarce future.

To better understand the connection between these two water resources we measured groundwater discharge (Q_gw) to a 24.3 km stretch of the Brazos River using high temporal frequency differential gaging (measurements taken every 20 minutes) from May 2015 - Feb. 2016. On 11 different occasions we also used two different mass balance methods to estimate Q_gw based on specific conductance and endmember-mixing analyses (EMMA). Estimated Q_gw obtained using the chemical methods consistently aligned with differential gaging estimates. We found that the Brazos River recharges the BRAA during rain events, and that Q_gw is at its maximum (up to 50 m³/s) immediately following rain events. The chemical tracers identified this water as low in dissolved solutes and, therefore, short-term bank storage. Q_gw decreased steadily with time since a rain event. The unconfined groundwater table remained high throughout both wet and dry conditions, whereas Q_gw decreased to zero after 6-8 weeks of dry conditions. This implies that the river may be more connected to rechargeable bank storage reservoirs rather than the broader aquifer. Further work is needed to characterize the large scale connectivity between the Brazos River and the broader (10 km wide) BRAA.

This study is an important step in understanding temporal fluctuations in amount and direction of water exchange between a lowland river and its alluvial aquifer. Understanding such processes is invaluable in aiding managers of ground and surface water bodies to coordinate to efficiently appropriate limited water resources.