Abstract

Geophysical surveys (resistivity, natural potential [self-potential], conductivity, magnetic, and ground penetrating radar) were conducted at three locations across the Mount Bonnell fault in the Balcones fault zone of central Texas. The normal fault has hundreds of meters of throw and is the primary boundary between two major aquifers in Texas, the Trinity and Edwards aquifers. In the near surface, the fault juxtaposes the Upper Glen Rose Formation on the Edwards Plateau, consisting of interbedded limestone and marly limestone, against the Edwards Group, which is mostly limestone, on the eastern down-thrown side (coastal plain). The Upper Glen Rose member is considered to be the Upper Trinity Aquifer and also a confining zone underlying the Edwards Aquifer. However, recent studies have documented a hydraulic connection between the Edwards and Upper Trinity aquifers. The uppermost portions of the Upper Trinity and the Edwards aquifers, in some places, operate as a single aquifer system, while the lowermost units of the Upper Glen Rose are confining layers between the Edwards Aquifer and the Middle Trinity Aquifer. Geophysical data, which include resistivity, natural potential, magnetic, conductivity, and ground penetrating radar, indicate not only the location of the fault but additional karstic features on the west side (Upper Glen Rose Fm.) and on the east side (Edwards Group) of the Mount Bonnell fault. Resistivity values of the Glen Rose and Edwards Group do not appear to have significant lateral variations across the fault. In other words, the Mount Bonnell fault does not appear to juxtapose different resistivity units of the Edwards Group and Upper Glen Rose member. Thus, the fault may not be a barrier for groundwater flow. With the abundant karstic features (caves, sinkholes, fractures, collapsed areas) on both sides of the fault, determined by the geophysical data, one can conclude that lateral groundwater flow (intra-aquifer) between the Edwards Aquifer and Upper Trinity is likely.

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