Streamflow from the Blanco River to Barton Springs
An informative report regarding the subject issue is presented by Hunt and others (2017). The report thoroughly examines the geology and hydrology in the Onion Creek and Blanco River basins and identifies potential for hydrologic communication between the two areas.
In an effort to verify that the Blanco River contributes flow to Barton Springs during low-flow conditions, water-traceable dye has been injected into the Blanco River and sampled at Barton Springs. These studies have documented, during low-flow conditions, recharge from the Blanco River to move south to San Marcos Springs and north to discharge at Barton Springs. However, the magnitude of the Blanco River contribution to Barton Springs discharge is unknown. Nevertheless, many reports have speculated the Blanco River to be a major source of Barton Springs discharge during low-flow conditions. For example, A 2011 report regarding this issue is hosted online by the Save Barton Creek Association. As part of the report, values for discharge are noted to be comparable between Barton Springs and the Blanco River. The report concludes "Thus, it appears that the Blanco River might be directly sustaining a significant portion of Barton Springs flow under drought conditions". An additional report published in the proceedings of a Karst Conservative Initiative in Feb. 17, 2011, concludes "Analysis comparing Blanco River recharge to Barton Springs discharge indicates that the Blanco River has a major role in sustaining Barton Springs flow during droughts." Finally, based on hydrograph comparisons between discharges for Barton Springs and the Blanco River, a 2012 report by the City of Austin concludes, during dry conditions, "...most of the Blanco recharge flows toward Barton Springs and not toward the San Marcos Spring".
However, discharges for many streams outside the Edwards aquifer also are comparable to those for Barton Springs. In an effort to identify additional information regarding this issue, this short informal report presents simple comparisons of basic data for Barton Springs and the Blanco River. In order that the three illustrations might be independent from the report, a summary conclusion is included on each illustration.
Data for Barton Springs and the Blanco River during the severe drought of 1954-56 represent perhaps the best analyses of base flow conditions for Barton Springs. During this period, discharge for the Blanco River at Wimberley gage represented total recharge within the Blanco River basin to the Edwards aquifer. As shown in figure 1, Barton Springs maintained a base flow of 10-12 ft3/s for most of 1956. However, the discharge for the Blanco River at Wimberley declined from January to August to a flow of less than 1 ft3/s in July and August--a value less than 10 percent of that for Barton Springs, assuming all Blanco River recharge moved to Barton Springs and none to San Marcos Springs. A steady-state source of inflow maintained Barton Springs discharge at 10-12 ft3/s, but the source was not the Blanco River. These data suggest that Blanco River recharge is not the major source of base-flow discharges for Barton Springs.
Dissolved-solids concentrations thus specific conductance values increase as Barton Springs discharges decrease. During low-flow conditions, conductance values for Barton Springs typically exceed 700 μS/cm (fig. 2). However, based on 114 values from 1962-2008, specific conductance values for the Blanco River at Wimberley range from 370-530 μS/cm. Therefore, conductance values for the Blanco River are too low to cause increasing conductance values for Barton Springs, thus indicating that the Blanco River might not be the major source of low-flow discharges for the springs. The likely source for the increased dissolved solids values is the Bad Water zone east of Barton Springs. However, due to its large concentrations of dissolved solids, even a relatively small contribution of water from the Bad Water zone could dominate specific conductance values for Barton Springs and possibly conceal relative contributions of water from the Blanco River.
As Barton Springs discharge decreases, its water quality changes from that representing only the Edwards aquifer to that representing a mixture of water from the Edwards and another source (fig 3), Black dots on the illustration present the water quality for various discharge conditions of Barton Springs. The number beside each black dot represents the value for Barton Springs discharge during sample collection. During high-flow conditions, Barton Springs calcium-bicarbonate water is indicative of that from Edwards aquifer wells remote from the Bad Water zone (red data point). However, as springflow decreases, sodium, chloride, and sulfate concentrations substantially increase, thus suggesting its water to be a mixture of water from the Edwards and Bad Water zone. The water quality in Edwards wells proximate to the Bad Water Zone (green data points) also appear to be a mixture of water from the Edwards and Bad Water zone. The relative locations for the wells are presented at the bottom of figure 3 and additional data for this analysis is presented by Slade and others (1986).
None of these basic-data analyses provide insight regarding the magnitude of contribution from the Blanco River to low flow at Barton Springs. However, all three analyses suggest that the Blanco River might not be the major source of low flow for Barton Springs, and two of the analyses suggest that the Bad Water zone might represent the major contribution to Barton Springs during low-flow conditions.
References
Hunt, B. and others, 2017, Surface-water and Groundwater Interactions in the Blanco River
and Onion Creek Watersheds: Implications for the Trinity and Edwards Aquifers of Central Texas: Bulletin of the South Texas Geological Society. https://bseacd.org/2017/01/surface-water-and-groundwater-interactions-in-the-blanco-river-and-onion-creek-watersheds-implications-for-the-trinity-and-edwards-aquifers-of-central-texas/
Slade, R.M., Jr., Dorsey, M.E., and Stewart, S.L., 1986, Hydrology and water quality of Barton Springs and associated Edwards aquifer in the Austin area, Texas: U.S. Geological Survey Water-Resources Investigations Report 86-4036, ll7 p.
https://pubs.er.usgs.gov/pubs/wri/wri864036
Illustrations follow
