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January 2016 Bulletin

Beyond the Bad-Water Line: A Model for the Occurrence of Brackish Water in Upper Coastal Plain Aquifers in Texas

Alan Dutton
Amy Shelton and V.H. McNutt Distinguished Professor in Geology
The University of Texas at San Antonio


There is increased interest in developing brackish-water zones in aquifers for diversified public-water supplies. Data on brackish water zones, however, are limited typically to borehole geophysical logs, from which elevations of tops and bottoms of brackish-water intervals can be picked and from which water salinities can be calculated. There are little data on hydraulic head and hydrochemical composition of brackish water. Data from the groundwater industry for aquifers seldom extend to great depth or beyond the freshwater zone, whereas most data from hydrocarbon exploration and production activities are for the deeper saline zone where most oil and gas fields occur. Such data sets rarely overlap and require significant interpolation across the brackish-water zone.

Brackish water in upper coastal plain aquifers in Texas might occur in a convergent mixing zone between (a) fresh meteoric groundwater moving downdip by gravitational drive and (b) saline water driven updip from great depth from geopressured formations. This model seems correct for the Carrizo-Wilcox aquifer in central Texas, and probably applies to other upper coastal plain aquifers, including but not limited to the Edwards Balcones Fault Zone aquifer in south-central Texas.

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