Storage dynamics of the upper Nueces River alluvial aquifer: Implications for recharge to the Edwards Aquifer, Texas

The karstic Edwards Aquifer is a crucial water resource in south-central Texas, serving as the primary water source for over two million people in the greater San Antonio area. The Nueces River basin is the largest contributor of recharge to the Edwards Aquifer, and recharge has traditionally been measured as the difference between river discharge at the upstream and downstream ends of the Edwards Group outcrop (Edwards Aquifer Recharge Zone, EARZ). This study investigated the extent to which groundwater in alluvial terraces and younger, near-channel alluvium deposits impact the timing and magnitude of recharge from the Nueces basin. Estimates of alluvial storage derived from geologic maps and sparse groundwater data suggest that over 21,000 acre-feet (25.9 x 106 m3) of groundwater are stored in the upper Nueces River alluvial aquifer, with an estimated maximum capacity of over 75,000 acre-ft (92.5 x 106 m3). However, the dynamics of alluvial bank storage and drainage from the alluvial aquifer and their impacts on downstream recharge are unknown. In this study, river water storage and transport in alluvium were investigated using differential gaging, dye tracer testing, baseflow recession analyses, and floodplain groundwater mass balances. Field investigations were made at a gaged, alluvium-lined reach of the Nueces River where the river partially supplies a major tributary that maintained baseflow during the 2011 drought (whereas river flow ceased). Significant streamflow losses in the study area are largely the result of storage in high conductivity gravels adjacent to the channel, with some recharge to the alluvial aquifer and discrete recharge into bedrock. The baseflow contribution from the upper Nueces River alluvial aquifer contributes up to 100% of river flow during low flow conditions in the basin. The magnitude and shape of baseflow recession is dynamic between the growing and dormant seasons. These findings have implications for groundwater pumping from the upper Nueces River basin alluvial aquifer and the management of recharge to the Edwards Aquifer.