In cooperation with the Bexar-Medina-Atascosa Counties Water Control and Improvement District No. 1
Geologic Framework and Hydrogeologic Characteristics of the Outcrops of the Edwards and Trinity Aquifers, Medina Lake Area, Texas
By Ted A. Small and Rebecca B. Lambert
U.S. Geological Survey
Water Resources Investigations Report 97–4290
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Methods of Investigation
Porosity and Permeability
|1.||Map showing hydrogeologic subdivisions of the outcrops of the Edwards and Trinity aquifers, Medina Lake area, Texas|
|1.||Map showing location of the study area|
|2.||Generalized hydrogeologic section A–A' of the Edwards aquifer, Medina Lake area, Texas|
|3.||Generalized hydrogeologic section B–B' of the Edwards aquifer, Medina Lake area, Texas|
|1.||Summary of the lithologic and hydrologic properties of the hydrogeologic subdivisions of the outcrops of the Edwards and Trinity aquifers, Medina Lake area, Texas|
|2.||Description of composite stratigraphic section of the Edwards aquifer outcrop, Medina Lake area, Texas|
The hydrogeologic subdivisions of the Edwards aquifer outcrop in the Medina Lake area in Medina and Bandera Counties generally are porous and permeable. The most porous and permeable appear to be hydrogeologic subdivision VI, the Kirschberg evaporite member of the Kainer Formation; and hydrogeologic subdivision III, the leached and collapsed members, undivided, of the Person Formation. The porosity of the rocks in the Edwards aquifer outcrop is related to depositional or diagenetic elements along specific stratigraphic horizons (fabric selective) and to dissolution and structural elements that can occur in any lithostratigraphic horizon (not fabric selective). Permeability depends on the physical properties of the rock such as size, shape, and distribution of pores.
The Edwards aquifer has relatively large porosity and permeability resulting, in part, from the development or redistribution of secondary porosity. Lithology, stratigraphy, diagenesis, and karstification account for the effective porosity and permeability in the Edwards aquifer outcrop. Karst features that can greatly enhance effective porosity and permeability in the Edwards aquifer outcrop include sinkholes, dolines, and caves.
Field observations in the Medina Lake area confirm the findings of previous investigators that Medina Lake mostly overlies rocks of the upper member of the Glen Rose Limestone. The channel downstream of Medina Dam to the upper end of Diversion Lake also overlies the upper member of the Glen Rose Limestone. Most of Diversion Lake overlies a thin section of the Edwards aquifer—hydrogeologic subdivision VIII (basal nodular member) and the basal part of hydrogeologic subdivision VII (dolomitic member). Hydrogeologic subdivisions VIII and VII might be hydraulically connected to Medina Lake at high lake stages.
The Trinity aquifer, which crops out in the northern part of the Medina Lake area and underlies the Edwards aquifer in the southern part, is much less permeable and productive than the Edwards aquifer. Where the Trinity aquifer underlies the Edwards, the Trinity acts as a lower confining unit on the Edwards.
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Last modified: Friday, September 16 2005, 04:24:22 PM