Complexities of the Flow System, Complexities of the Flow System, Edwards AquiferEdwards Aquifer

Susan Hovorka, Bureau of Economic Geologywith data from:

Geary Schindel and Steve Johnson, Edwards Aquifer Authority

J. P Nicot, Bureau of Economic GeologySteve 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

Hovorka and Yeh

Intergranular pores

Intercrystalline pores

Separate vugs (molds)

Touching vugs

QAa4613c

BureauofEconomic

Geology

Pore Types

Permeability in Carbonate

Dissolution of dolomite during diagenesis

Randolph FM 1604 core, 924 ft

1 mm

Impact of Gypsum on Permeability

Calcitized gypsum =vuggy, zone, moderateto 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 modelSWRI, 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 topermeability 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

5 cm

Missing section, terra rosa,Off-scale caliper

Effects of faulting on permeability

QAa8500c

BureauofEconomic

Geology

Offset decreases cross-fault transmissivity

Fractures andbrecciation increasepermeability parallel to the fault

Caves

Fracturing focusesdevelopment of karst

Topographic Expression of Balcones Fault Zone, Austin

Mixing Zone Dissolution – Matrix and Karst

t2 t3 t4 t5

t1

Tracer tests todefine conduitsin Barton Springssegment

Brian Smith BSEACD Nico Hauwert COA

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Uranine Tracer Test Results in the Landa Park Area

LCRA Well

Panther Canyon Well

NBU Well 5Eosine

Uranine

Schindel, Johnson and others, 2002

Conduit Interpretation(Steve Worthington)

Comal Sp.

San Marcus Sp.

Base permeability from SWRI, basedon 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