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.
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.