Complexities of the Flow System System Edwards Aquifer

Complexities of the Flow System, System Edwards Aquifer Susan Hovorka, Bureau of Economic Geology with data from: Geary Schindel and Steve Johnson, Edwards Aquifer Authority J. P Nicot, Bureau of Economic Geology Steve Worthington, Worthington Associates Adrien Lindley and Thandar Phu, University of Texas at Austin

Purpose • What is the scale, distribution , and hydrologic significance of heterogeneity in the Edwards aquifer? • How much information can be extracted from existing data? • Where are productive areas for future research to better answer this question?

Techniques • Thin section petrography • Measurement of porosity and permeability core plugs from cores and outcrops • Calibration of wireline logs using core plug data and porosity/permeability transforms • Core examination • Fracture aperture measurement • Outcrop photograph digitization • Cave trend analysis • Calculation of permeability from specific capacity tests • Well hydrograph recession analysis • Pseudosynoptic analysis of troughs in the water level • Calcite saturation and other geochemical indicators

Facies Cross Section

Based on log analysis

Variation in Water Level

High Permeability Deep in the Aquifer

Pore Types

Permeability in Carbonate 1 mm Dissolution of dolomite during diagenesis Randolph FM 1604 core, 924 ft

Impact of Gypsum on Permeability Calcitized gypsum = vuggy, zone, moderate to low permeability

Conduit Flow Dominates • To match springflow, models must upscale transmissivity several orders of magnitude above estimated transmissivity from wells High transmissivity areas of Maclay and Land overlain on well-test-derived permeability mode SWRI, 2002

Conduit Flow Dominates • In wells with estimated transmissivity, matrix flow accounts for about 1% of total flow. Mace and Hovorka, permeability from core compared to permeability calculated from specific capacity

Fractures and Caves are Obvious in Outcrop

From G. Schindel

Evolution of Karst, Confined Zone Hovorka and Yeh

Karst features are found in the confined aquifer Missing section, terra rosa, Off-scale caliper 5 cm

Effects of faulting on permeability Fracturing focuses development of karst Offset decreases crossfault transmissivity Fractures and brecciation increase permeability parallel to the fault

Topographic Expression of Balcones Fault Zone, Austin

Mixing Zone Dissolution – Matrix and Karst

Tracer tests to define conduits in Barton Springs segment Brian Smith BSEACD Nico Hauwert COA

Uranine Tracer Test Results in the Landa Park Area Uranine NBU Well 5 Eosine Panther Canyon Well LCRA Well Schindel, Johnson and others, 2002

Conduit Interpretation (Steve Worthington) San Marcus Sp. Comal Sp. Base permeability from SWRI, based on statistical handling of well tests

Synoptic Potentiometric Surface of San Antonio Segment Esquilin, 2003 provisional synoptic

Trough in Water Level Interpreted as Conduits Brian Smith, Barton Springs Edwards Aquifer CD; Nico Hauwert City of Austin

Conduits Interpreted from Pseudosynoptics

Interpreted Conduits Compared to Other Indicators

Conclusions • Matrix permeability is inadequate to describe Edwards flow system • Numerous lines of evidence suggest that conduit flow dominates