Tiffany Noel Kocis
Published: 2014
Total Pages: 148
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The Edwards and Trinity Aquifers supply over 700 million gallons per day (2.6 x 109 I/day) to the public; therefore, it is crucial to understand how water is lost from the Trinity and exchanged into the Edwards. This thesis addresses the following questions in the scope of Hays County, TX: How are gains and losses temporally and spatially distributed along the Blanco River? What controls the distribution of spring discharge contributing to gains along the Blanco River? Finally, what does this spatial and temporal distribution of gains, losses, and joints mean for available water resources? From a time-series analysis of gain-loss on the Blanco River, it is evident that coarse resolution gain-loss studies are not accurate enough to capture the flow dynamics of the river or to understand flow paths along the river, particularly after storm events. The coarse resolution studies miss out on spring discharge zones and on smaller, but significant recharge zones located within net-gain reaches. The detailed gain-loss study from November 2013 was compared to a detailed study conducted in January 1955; the comparison suggests that that gain-loss conditions change depending on flow conditions and that regions that serve as aquifer recharge zones during low flow conditions serve as discharge zones during high flow conditions, which may serve to offset water level declines in the aquifer at the beginning of a drought because recharge into the aquifer is sustained by flow in the Blanco River. Furthermore, when comparing the present method for estimating recharge (estimated as loss between two USGS gauges), using only the loss estimated by the gauges instead of a detailed gain-loss study is a significant underestimate (by 5 times) of the total amount of recharge entering the Edwards-Trinity system along the Blanco River. Finally, the structural analysis of fracture orientations suggests that the spring discharge fracture networks are actually joint networks controlled by both topography and the development of the Balcones Fault Zone. To conclude, both parts of this study have important implications for groundwater resources: understanding gain-loss dynamics provides an important dataset for Groundwater Availability Models and for conservation districts who must allocate water resources, and understanding the joint networks through which springs discharge could allow drillers to target high water yield fractures.
