A multidisciplinary evaluation of the potential for cross-contamination of the Edwards aquifer from dissolved contaminants in the shallow alluvial aquifer in the vicinity of Kelly Air Force Base found that although hydrologic heads in the Edwards aquifer are generally above heads in the alluvial aquifer, the volatility of water levels in the Edwards aquifer as compared with those in the alluvial aquifer results in downward gradient during lowest aquifer stages. Hydrologic heads in the highly permeable Edwards aquifer vary.

Cross-contamination at these times therefore depends on the permeability existing between the water-bearing units. Our assessment suggests that incorrectly completed wells have a significant potential of resulting in cross-contamination. The risk of leakage along natural pathways, such as fractures, has been assessed as small because structural evaluation shows that faults beneath the contaminant plume do not completely offset the thick shales of the Navarro Formation and Midway groups. Local data, as well as analogy, suggest that in the absence of opening pressure and below the weathered zone such shale-against-shale fractures are likely to have low permeability.