RECHARGE POTENTIAL THROUGH TRIBUTARIES TO AN UNCONFINED ALLUVIAL AQUIFER

Recharge is one of the most difficult aquifer characteristics to quantify. In the Brazos River Alluvium Aquifer (BRAA), an unconfined alluvial aquifer in central Texas, precipitation is generally considered the primary source of recharge. However, potential recharge contributions from tributaries entering the Brazos River floodplain are not well documented or understood and may be important. This study seeks to determine if tributary streams provide recharge to the northern segment of the BRAA and quantify their contribution. Tributary streams could recharge the aquifer as losing streams crossing the floodplain or from lateral groundwater flow through tributary floodplain deposits. Tributaries crossing the BRAA were mapped with ArcGIS and categorized as ephemeral or perennial using existing USGS maps. Representative streams were selected for instrumentation with mini-piezometers and seepage meters to determine their interactions with the underlying sediment. Infiltration into dry stream beds was quantified with a Guelph Permeameter and the Soil and Water Assessment Tool (SWAT) was used to estimate annual flow of selected tributaries. Stream flow over the alluvium calculated by SWAT combined with infiltration into the aquifer from instrumented stream channels will provide an estimate of recharge from selected tributary channels to the BRAA. The results will then be extrapolated to estimate recharge contributions from tributaries in the northern segment of the BRAA to aid groundwater management entities.