STRATIGRAPHIC EVALUATION OF TRINITY AQUIFERS IN HAYS AND WESTERN TRAVIS COUNTY AND IMPLICATIONS FOR GROUNDWATER AVAILABILITY

Trinity Aquifers are very important groundwater resources in central Texas. However, they show signs of hydrologic stress and depletion in some areas. This study focused on detailed stratigraphic evaluations to better understand hydrologic variations in the Trinity Aquifer system. The study consisted of seven wells located throughout the Western Travis and Northern Hays County area spanning XX miles. All the wells had cuttings and geophysical logs. Two of the wells are multiport monitor wells providing detailed hydrologic data. Geophysical logs, cuttings, thin sections (where available), and outcrop descriptions were compiled, integrated and correlated. Stratigraphic cross sections were constructed and hydrologic data for the wells were plotted and compared to geologic data.

Results suggest that there are stratigraphic variations in the Middle and Lower Trinity units that may influence groundwater availability. From stratigraphic top to bottom, the Upper Glen Rose is a regionally consistent carbonate interval. The Middle Trinity Aquifer is composed of the Lower Glen Rose, Hensel and Cow Creek limestone. The discontinuous nature of the biostrome/reef intervals in the Lower Glen Rose appears to correlate to poor water quality and poor yield where the reef is absent. The Hensel, which transitions from a predominantely clastic and water bearing interval in the updip areas, to a thinner silty, dolomitic aquitard internval downdip. The underlying Cow Creek indicats a facies transition from an updip grainstone to dolomite in the downdip areas with higher yields. The underlying Hammett is a regionally consistent clay and aquitard unit for the Middle Trinity.

In addition to high levels of pumping, groundwater availability appears to be influenced by facies changes within the Middle Trinity units of the study area. Understanding the detailed straigraphy will provide insights into these important groundwater resources.