title: Estimation of Groundwater Recharge to the Gulf Coast Aquifer in Texas, USA
Creators: name: Scanlon, Bridget R.
name: Reedy, R. C.
name: Strassberg, Gil
name: Huang, Yun
name: Senay, Gabriel
Publisher: name: The University of Texas at Austin
Year: 2011
Subjects: subject: Groundwater recharge
subject: Gulf Coast Aquifer
subject: Texas
subject: water resources

description: Quantifying groundwater recharge is essential for managing water resources in aquifers. The objective of this study was to quantify spatial variability in recharge in the outcrop zones of the Gulf Coast aquifer in Texas. Regional recharge was estimated using the chloride mass balance approach applied to groundwater chloride data from the TWDB database in 10,530 wells, which represented the most recent samples from wells located in the region. Regional groundwater recharge was also estimated using streamflow hydrograph separation in 59 watersheds using USGS unregulated gage data. Recharge was also estimated by applying the chloride mass balance approach to unsaturated zone chloride data from 27 boreholes that represented a range of precipitation, land use, and soil texture settings in the central and southern Gulf Coast regions. Groundwater chloride concentrations generally decrease from the southern to the northern Gulf Coast, qualitatively indicating increasing recharge in this direction with increasing precipitation. Ratios of chloride to bromide are less than 150 to 200 throughout most of the Gulf Coast, suggesting a predominantly meteoric source for groundwater chloride. Recharge rates based on the chloride mass balance approach range from less than 0.1 in/yr in the south to 10 in/yr in the north, correlated with increasing precipitation. Streamflow ranges from ephemeral in parts of the southern Gulf Coast to perennial throughout the rest of the Gulf Coast based on flow duration curves. Hydrograph separation using Base-Flow Index (BFI) showed that recharge increased from south to north, similar to increases in recharge based on groundwater chloride data. Unsaturated zone profiles show high local variability in chloride concentrations, with mean concentrations below the root zone ranging from 7 to 10,200 mg/L. Resultant percolation rates below the root zone based on the chloride mass balance approach range from less than 0.1 to 6.8 in/yr. In some areas, variations in percolation rates are related to differences in soil texture, whereas in other regions, they are related to differences in land use. However, there is no systematic variation in percolation rates throughout the region, unlike the trends in recharge with regional precipitation from groundwater chloride data and stream hydrograph separation. Recharge rates based on groundwater chloride data can be considered to provide a conservative lower bound on actual recharge because many processes can add chloride to the system, resulting in lower recharge rates whereas there are no processes that can remove chloride from the system in the Gulf Coast. Stream hydrograph separation provides recharge rates in contributing basins that do not cover the entire Gulf Coast region. Recharge estimates from the chloride mass balance applied to groundwater and perennial stream hydrograph separation are highly correlated (r = 0.96), and differences between these two sets of recharge estimates can be used to evaluate uncertainties in recharge rates in contributing basins to the stream gages. Recharge rates from groundwater chloride and streamflow hydrograph separation can be used to provide a range of recharge rates for future groundwater models of the Gulf Coast aquifer.