KARST MATURITY OF THE BARTON SPRINGS SEGMENT OF THE EDWARDS AQUIFER IN CENTRAL TEXAS

With increasing karst maturity, conduits and catchment areas are expected to increase, which results in proportionately greater discrete groundwater flow and recharge. The karst maturity of the Barton Springs Segment is examined through its recharge and groundwater characteristics is quantified by the portion of rainfall that directly infiltrates on the aquifer outcrop as well as the percent of aquifer outcrop area contained in internal drainage basins. The autogenic recharge of rainfall (direct rainfall on outcrop areas of the Barton Springs segment between major creek channels) ranges between about 29-35%. This range is based on single 1.4 year period upland water balance interval of 20% higher than average rainfall utilizing eddy covariance evapotranspiration-climate tower and a second four-year water balance of residual from Barton Springs flow after subtracting measured major creek channel flow loss and pumpage. The pitting index is the percent of land surface covered by internal drainage basin. At least 10% of the outcrop area of the Barton Springs Segment is mapped as internal drainage basin. The degree of internal drainage basins filled in by urbanization is unknown.

The general characteristics of groundwater flow within the Barton Springs Segment include: 1) Groundwater flow localizes along a few preferential groundwater flow paths that dominate flow within three groundwater basins; 2) Rapid (first arrival) transport velocities exceeding 5 miles/day, decreasing to about 1 mile/day or less during low flow (drought) conditions; 3) Relatively low natural attenuation, where Peclet numbers, or ratio of advection to dispersion and/or diffusion, exceed 1,000 based on results from 5 analyzed tracer tests; and 4) Hydraulic conductivity measured in wells measured across the Barton Springs Segment varies with distance to a few mapped preferential groundwater flow paths as follows: K= 340 L^-1.04, where K = hydraulic conductivity [m/day] and L = distance from flow path (m). This characterization suggests that the Barton Springs Segment is a relatively mature karst aquifer.