Urban development on or adjacent to the recharge zone of the Edwards
aquifer is causing concern about the possible pollution of ground water
in the aquifer, which is the principal source of water supply for the San
Antonio area. Water-quality data for many wells and springs and for
selected sites on streams that cross the recharge zone of the aquifer are
being collected to provide background information and to detect any
current pollution of ground water in the area.
Water from the Edwards aquifer is very hard and of the calcium
bicarbonate type. The concentrations of dissolved solids in samples
from wells and springs ranged from about 200 to 470 mg/1 (milligrams per
liter); the chloride and sulfate concentrations ranged from 6.5 to 62
mg/1 and from 0.0 to 65 mg/1, respectively. The nitrate and phosphate
contents of the ground water ranged from 0.0 to 15 mg/1 and from 0.00
to 0. 37 mg/1.
The concentrations of these and other constituents show that the
chemical quality of water in the Edwards aquifer has not been degraded
significantly by domestic, industrial, or agricultural effluents.
However, variations in the number of coliforms, the concentrations of
nitrate and phosphate, and the presence of fecal coliforms and fecal
streptococci in samples from some wells show that fecal pollution is
reaching the aquifer.
Most of these wells, which are located in or just downdip from the
recharge zone, are poorly sealed or inadequately cased. The areal variation
in the locations of these wells indicates that pollution of ground
water in the aquifer is very localized. Prllution results principally
from runoff from the land surface and from effluent from septic tanks
which enters the aquifer through fractures in the recharge zone or which
infiltrates through the thin soil into poorly sealed or inadequately
cased wells in or adjacent to the recharge zone.
Trace amounts of several pesticides have been detected in samples
from two wells in the San Antonio area. Field investigations showed the
source of pesticides in these wells to be. surface drainage that entered
Water-quality data collected at sites on streams that cross the
recharge zone of the Edwards aquifer show the chemical composition of
surface water to be very similar to that of ground water in the area.
Water in most streams is very hard and of the calcium bicarbonate type.
Limited data on the bacteriological quality show that coliforms were
present at each of the site sampled and that fecal coliforms and fecal
streptococci were present at most sites. Although the number of these
bacteria varied greatly in both time and place, their density in samples
from most sites were low for untreated surface water.
Additional publication details
USGS Numbered Series
Chemical and bacteriological quality of water at selected sites in the San Antonio area, Texas, August 1968-January 1975