ABSTRACT

The distribution of water in the Edwards aquifer was assessed using a core and log-based study. Porosity distribution reflects both depositional fabric and subsequent diagenesis. Vertical facies stacking patterns influence the depositional porosity as well as dolomitization and diagenetic porosity modification. Subtidal facies deposited during sea-level highstands are generally undolomitized and exhibit low porosity (5 to 10 percent); platform grainstones commonly have high depositional porosity and significant solution enhancement (20- to 42-percent porosity). Dolomitized subtidal facies in tidal-flat-capped cycles have very high porosity (20 to 40 percent) because of selective dolomite dissolution in the fresh-water aquifer. Porosity in former evaporite beds is high in some areas because of dissolution and collapse, but it is low where gypsum was replaced by calcite cement. Low-energy subtidal and evaporitic units in the Maverick Basin have porosities that are generally less than 15 percent. The overlying basinal packstones and grainstones have solution-enhanced porosities of 25 to 35 percent.

Mapped average porosity also shows nonstratigraphically controlled variations. Diagenesis associated with fluctuations in water chemistry near the saline/fresh-water interface may be one cause. Other complex patterns of high and low porosity are attributed to structurally and hydrologically controlled porosity enhancement and cementation.

Three-dimensional mapping of porosity trends provides data for improved aquifer management. Only about 3 percent of the water stored in the aquifer during a year of average water level lies above the water table at which natural spring flow is diminished. An average specific yield of 42 percent in the unconfined aquifer is determined from total porosity, changes in the water-table elevation, and changes in estimated recharge and discharge. Average storativity of 2.6 10^-4 in the confined Edwards is estimated using average porosity and barometric efficiency calculated from comparing water-level hydrographs and atmospheric-pressure changes.