San Antonio is the third largest city in Texas, covering an area of 793
km2 as of 1991, having a population of 935 933 and a metropolitan
population of 1 302 099 as of 1990. It is subject to typical urban sprawl.
Within the downtown core, the city is split into five sub- cities: Alamo
Heights, Balcones Heights, Castle Hills, Olmos Park, and Terrell Hills.
San Antonio was founded in 1718 as a Spanish Military post. It resides
in Bexar county, situated on the San Antonio River, south of the Balcones
Escarpment. The topography is generally flat with few rolling hills in
the north-west corner, and averages an elevation of 215 m above sea level.
The economy is supported by livestock, produce, wool and mohair industries,
wide range manufacturing, and banking, transportation, and retail trades.
Some main attractions in San Antonio include the Alamo, the Tower of Americas,
and quaint areas developed around the San Antonio River. (Ref: 1, 2).
San Antonio is located in south-central Texas, on the edge of the Chihuauhan
desert, where climate is semi-arid. Growing season is 365 days. Average
temperature is 20.5 C, averaging 10 C in January and 29 C in July. Average
annual precipitation is about 732.3 mm, averaging 38.9 mm in January, and
54.7 mm in July. Annual net evaporation rate is about 425 mm. Average sea
level pressure is 1016.1 mb, having 1020.3 mb in January and 1015.1 mb
in July. (Ref: 1, 12, 14, 20).
San Antonio is underlain by the large Edwards Aquifer system, which is
the only source of water for the city. Its transmissivity is about 430
000 feet per day, and its average annual recharge of 635 500 acre-feet
is highly variable (ranging from 44 000 to 2 000 000 acre-feet). Within
the area, the aquifer extends in a 160 mile arch and ranges from 5 to 40
miles in width. Conductivity is also very variable (from 10 to 106 ft/d
in caves and from 10-3 to 10 ft/d in carbonate matrices). Edwards limestone,
being highly karstic, fractured and faulted, makes up the aquifer, and
at the San Antonio region it is confined, having the Glen Rose formation,
predominantly of clayey carbonate mudstone and marl, in the underlying
layer and the Del Rio clay, predominately blue claystone or marl, on top.
Flow generally runs from west to east, and in San Antonio, runs northeastward
to Comal County, but many barrier faults control and complicate the flow
patterns, especially the Balcones fault zone/escarpment that lies just
north of San Antonio. Due to the faulting, anisotropy is high (ratios ranging
from 0.0:1 to 1:1) and water within an area is a mixture of different ages.
There is a divide of freshwater zones, that contain highly recrystalized
calcites with little or no organic material in the aquifer, and a "bad
water line" in the southern edges that contains dolomite, gypsum, pyrite,
celestite and unoxidized organic material in the aquifer, and contains
highly saline (>1000 mg/L TDS) water. Natural spring discharges in San
Antonio occur in San Pedro and San Antonio Springs. The San Antonio River
runs through the city, having headwaters in the northeast corner that are
fed by springs of the confined Edward Aquifer. (Ref: 4, 6, 7, 9).
Groundwater supplies are used extensively for public and agricultural water
supplies, to the point where many natural springs have become intermittent.
Of the total water pumped for usage, 56.6% is used for public water supply,
and 30.1% is used for agriculture, (from 1981- 1990) In the south-central
Texas area (this includes San Antonio, San Angelo, Del Rio, and Austin),
a total of 1 564 590 acre-feet of water was consumed in 1990 with a population
of 2 718 316. It is estimated that in the year 2040 a population of 6 453
944 will consume 1 995 852 acre- feet of water. No major water quality
or pollution problems seem to be known. This may be due to little heavy
industrial practices in the area. (Ref: 10, 14).
Urban sprawl has increased stress on the groundwater supply in the aquifer,
additionally threatening land subsidence and salt water intrusion. (Ref:
4). Increased pumping from the aquifer has decreased the potentiometric
surface and has made the natural springs formed from the Balcones Fault
Zone (ex: Pedro and San Antonio springs) intermittent. (Ref: 5, 10). Many
water quality issues concerning the San Antonio River and connecting creeks,
such as fecal coliform bacterial contamination in the river headwaters,
wastewater effluent in the lower river, agricultural stormwater runoff
in the lower Cibolo Creek, and drying of the mid Cibolo Creek. (Ref: 11).
Natural water springs that are becoming intermittent by extensive water
use are home to many endangered species, including the blind salamander,
the fountain darter, the San Marcos gambusia, and the Texas wild rice.
(Ref: 12, 13).
The low storage limestone aquifer does not deal well with the highly variable
rainfall; it is vulnerable to droughts. (Ref: 19). Degradation from the
"bad water line" along the southern and eastern edges of the aquifer. It
is a zone of high saline waters (TDS 1000 mg/L) where water has contacted
the limestone long enough to increase its salinity to undrinkable levels.
This line threatens to creep up due to overpumping. (Ref: 5, 14, 19).
Further research in the hydrogeology of the aquifer to aid in land and
water management. (For example, TNRCC-Texas Natural Resource Conservation
Commission, EARDC- Edwards Aquifer Research and Data Center, and the USGS
- United States Geological Survey). (Ref: 15, 16, 17 respectively). Proposal
to increase storage behind existing Median Dam as an alternative source
of water in order to relieve stresses on supply. (Ref: 8). Recycling water:
SAWS (San Antonio Water Systems Board) is implementing a 36 000 foot distribution
system that will deliver highly treated effluent to areas of large water
consumption for non-potable uses. This project is due for completion by
January 2001. (Ref: 18). Suggest an increased usage of surface waters to
relieve stress from the aquifer, preserve habitat for endangered species,
prevent land subsidence, and keep the "bad water line" from creeping northward.
(Ref: 4).
The Encyclopedia Americana; Deluxe Library Edition Vol. 24. Pg 190. 1994.
Golier Inc.
The Geological Society of America. 1986. Hydrology of the Edwards
Aquifer.
Ground-Water Supply Issues in Urban and Urbanizing Areas. (Sharp, J.M.
1997). In: Groundwater in the Urban Environment. Vol 1. A.A.Balkema,
Rotterdam, Netherlands.
Hydrology of the Edwards Aquifer. The Geological Society of America.
1986.
Permeability Structure of the Edwards Aquifer, South Texas; Implications
for Aquifer Management. (Hovorka, S. D., R. E. Mace, E. W. Collins.
1998). University of Texas at Austin, USA.
Prediction of Regional Groundwater Flow and Permeability Anisotropy Using
Fractured Outcrop Exposures, Edwards Aquifer, Central Texas, USA.
(Zahm, K.C., R. Marrett, J. M. Sharp. 1998). In: Gambling With Groundwater;
Physical, Chemical, and Biological Aspects of Aquifer-Stream Realations.
1998.
Preliminary Stability Analysis for Normal and Flood Loadings: Medina Main
Dam for Bexar, Medina, Atascocsa Water Control and Improvement District
No. 1. Medina-96-001. Technical Memo. (Bureau of Reclamation,
Denver, CO. Technical Service Center. 1996).
