STREAM/ALLUVIUM INTERACTION IN KARST AQUIFER RECHARGE: EDWARDS AQUIFER - NUECES RIVER SYSTEM

The karstic Edwards Aquifer is the primary source of drinking water in south-central Texas. The greatest recharge into the aquifer occurs in the Nueces River Basin system. Aquifer recharge is a key input parameter in management decision models. Current models for resource allocation calculate recharge based on sparse gauging station data and may underestimate recharge in the basin. There are a limited number of gauges in the recharge zone of the Edwards Aquifer. Synoptic gain/loss studies conducted in 2012-2013 in the Nueces River show that: 1) that recharge is difficult to estimate without multiple gauges; 2) recharge is not always occurring over the designated recharge zone of the Edwards Aquifer; and 3) some reaches can be either gaining or losing over time. We focus on a 64-km reach of the Nueces west of Uvalde, TX, that includes the recharge and contributing zones of the Edwards Aquifer. It is estimated that 40% of aquifer recharge occurs in this segment of the river which has intermittent reaches with significant gains and losses, but aquifer recharge does not account for all losses. A significant amount of this variation is controlled by bank storage and underflow (including hyporheic flows) in the channel deposits and adjoining fill terraces. Recharge is controlled by water table elevation and alluvium permeability, not discrete karst features (e.g., dolines). Surficial transport and deposition of alluvial sediments in this system may change aquifer recharge rates. Currently research is being conducted using hydrochemical, geophysical, stream discharge, tracer test, and weather data to evaluate the sources of stream discharge, alluvial flow paths, and rates of recharge in specific areas.