Download evaluation of sustainable yield of the

GROUNDWATER AVAILABILITY MODELING OF THE
BARTON SPRINGS SEGMENT OF THE EDWARDS AQUIFER, HAYS AND TRAVIS
COUNTIES, TEXAS
Brian A. Smith, Ph.D., P.G. and Brian B. Hunt, P.G.
ABSTRACT
The Barton Springs segment of the Edwards Aquifer (Barton Springs aquifer), located
within parts of Travis and Hays Counties in Central Texas, is an important groundwater resource
for municipal, industrial, domestic, recreational, and ecological needs. Approximately 50,000
people depend on water from the Barton Springs aquifer as their sole source of drinking water,
and the various spring outlets at Barton Springs are the only known habitats of the endangered
Barton Springs salamander. The amount of groundwater in the Barton Springs aquifer that is
available to meet current and future needs will be very limited under severe drought conditions.
The combined effects of drought and significant pumping can result in a decline in water
levels and spring flow in an aquifer. The Barton Springs/Edwards Aquifer Conservation District
(District) evaluated the potential impacts on groundwater availability in the Barton Springs
aquifer following a recurrence of drought-of-record (1950’s) conditions and various rates of
pumping. A numerical groundwater model and hydrogeologic data were the primary tools used
in this evaluation.
A numerical groundwater availability model (GAM) of the Barton Springs aquifer had
been developed by the University of Texas Bureau of Economic Geology and the District. The
GAM was recalibrated so that model results for spring-flow and water-level values better
matched measured values from the 1950’s. The recalibrated model was then used to predict
spring-flow and water-level declines under 1950’s drought conditions and various future
pumping scenarios. Hydrogeological data, such as saturated-thickness maps, water-level maps,
and well-construction and yield data, were evaluated with the model results so that impacts to
water-supply wells under 1950’s drought conditions and various rates of pumping could be
estimated.
Simulations indicate that for a given volume of groundwater pumped from the aquifer
under 1950’s drought conditions, spring flow at Barton Springs would diminish by a nearly equal
amount. The model indicates that with current (2004) permitted pumping rates of 10 cubic feet
per second (cfs) under 1950’s drought conditions, mean monthly spring flow will be about 1 cfs.
Under these conditions, spring flow could cease for periods of less than one month. Under
1950’s drought conditions and the current pumping rate of 10 cfs, as many as 19% of the watersupply wells in the District may be negatively impacted. Simulations of 1950’s drought
conditions with current and future rates of pumping indicate that significantly lower water levels
will occur in most parts of the aquifer. The resulting degradation in water quality could impact
water-supply wells and the endangered species at Barton Springs.