Update on Continued Studies Related to Preservation of Balmorhea-area Springs, Trans-Pecos Texas

Presenter

Rebecca C. Smyth
Hydrogeologist, M.A., P.G.
Bureau of Economic Geology
Jackson School of Geosciences
The University of Texas at Austin

Description

Springflow along the northeastern edge of the Davis Mountains (southernmost Delaware Basin) – the Balmorhea Spring Complex – is in decline.  The region remains in exceptional drought.  Many of the springs are currently not flowing even though the “usual” monsoonal season has just ended.

Between 1940 and ~2017, springflow to the Balmorhea State Park (BSP) pool in Toyahvale, Texas decreased from over 60 cubic feet per second (cfs) (27,000 gallons per minute [gpm]) to ~20cfs (9,000 gpm).  In nearby Phantom Cave springflow decreased from over 60 cfs in the 1940s to dry in the late 1990s.  Phantom has recently stopped flowing again.  These declines are due to both long-term drought, AND regional and local groundwater withdrawals for public and industrial water supply.

Beginning with the U.S. Geological Survey in the 1930s, numerous studies have collectively identified sources of Balmorhea-area spring water: (1) regional flow from the westerly Salt Basin (baseflow) and (2) less geographically definitive chemical hydrogeologic methods (e.g., Uliana and Sharp, 2001; Nicot et al., 2022) More physical hydrogeologic data (water levels [WL]) are needed.

Since the start of BEG water level monitoring in 2019, the only significant rainfall in the region was in Fall 2022.  What I mean by significant rainfall is the resulting amount of stormflow needed to affect brief, but measurable “freshening” in the BSP pool.  An example of freshening of pool water from >2,000 milligrams per liter total dissolved solids (mg/L TDS), almost down to 500 mg/L TDS, came after multiple 1- to 3-inch regional rainfall events between August and November 22.  Observable changes in pool chemistry and associated increases in springflow provide clues about sources of groundwater recharge.

The arid-region hydrogeology on the north side of the Davis Mountains is complex, especially given the regional structural deformation and five overlapping aquifers!  Ultimately, groundwater flow modeling will be needed to interpret the surface water / groundwater processes; that is how springflow responds to geographically variable, major rainfall events.  However, we still need more WL data over a wider geographical area, and rainfall(!) in order to generate realistic model output

I’ll present selected results from ongoing research to define specific recharge areas and flow paths for groundwater discharging to the Balmorhea-area springs.  Our ultimate objective is to identify locations where industrial-scale groundwater withdrawal might have less of an impact on springflow; hence increasing the chances of spring preservation.