Lake Brownwood Watershed - Texas State Soil and Water ...

3.4 EXISTING SURFACE WATER HYDROLOGY
The hydrologic characteristics of the Lake Brownwood watershed are closely linked to precipitation patterns in the
watershed, especially the cycles of floods and droughts, which are common in Texas. Major flood and drought events are
those with statistical recurrence intervals longer than 25 years and 10 years, respectively. Streamflow measurements
began in the river basin in 1923, and the data show there has been a significant or major drought in almost every decade
since then. Based on naturalized streamflow estimates, the average monthly discharge of Pecan Bayou at Brownwood
ranges from about 4,500 acre-feet in November to about 31,000 acre-feet in May. The average annual runoff from 1940 to
1998 was 145,331 acre-feet at Brownwood. No major water quality issues have been identified in the watershed, but
contamination from non-point sources of pollution are probably the most likely way that surface water quality may be
impacted. Non-point source pollution is runoff that, as it flows over the land, picks up pollutants that adhere to plants, soils
and man-made objects and eventually infiltrates to the groundwater table or flows into a surface stream. Another source of
non-point pollution is an accidental spill of toxic chemicals near streams or over recharge zones that will send a
concentrated pulse of contaminated water through stream segments or aquifers. Public water supply wells that only use
chlorination water treatment and domestic groundwater wells that may not treat water before consumption are especially
vulnerable to sources of non-point pollution, as are the habitats of threatened and endangered species that live in and near
springs and certain stream segments.
3.5 EXISTING GROUNDWATER HYDROLOGY
The most productive water-bearing unit in the study area is the Trinity Group, which consists mainly of the
Antlers Formation. More wells are completed in the Trinity than other water-bearing units, and the water is of better
quality. The water-bearing portions of the Trinity Group consist of fine- to coarse-grained sand, gravel and shaly
limestone. The maximum thickness of the Trinity in the study area is probably less than 100 feet.
In the western portion of the study area and along the watershed’s northern margin, younger Cretaceous strata of
the Fredericksburg Group, including the Edwards, Comanche Peak and Walnut formations, overlie the Trinity Group. The
Edwards is the most permeable of the Fredericksburg units and is more permeable than the Trinity Group aquifers.
However, it occurs only as thin remnants capping hilltops in portions of the study area, and is not sufficiently saturated to
produce much groundwater. However, the quality of water in the Fredericksburg Group is generally good.
Permian and Pennsylvanian rocks that underlie the watershed produce small amounts of fresh to very saline water
from highly interbedded and laterally discontinuous sandstone and limestone strata. Water is also produced to a limited
degree from older Paleozoic rocks that underlie the Pennsylvanian system, including limestones of the Ellenburger Group
and Hickory Sandstone. Groundwater in the deeper Paleozoics occurs under artesian conditions and the water is slightly
saline to saline. Thin deposits of alluvium along the major streams consist of unconsolidated sand, silt, clay and gravel and
yield small amounts of generally good quality water.
All of the aquifers in the watershed, except the deep Paleozoic aquifers, receive recharge from precipitation in
outcrop areas. The Permian and Pennsylvanian rocks and the alluvium also receive recharge as seepage from surface water
bodies. The deep Paleozoic rocks receive their recharge from areas outside the watershed.
Groundwater in all of the aquifers moves slowly under the force of gravity from areas with relatively high
groundwater elevations to areas with low groundwater elevations, and generally from areas of recharge to areas of
discharge. The direction and rate of groundwater movement in these aquifers are controlled by the hydraulic gradient,
aquifer permeability and structural dip of the rock units comprising the aquifer. Some of the groundwater in the aquifers is
discharged naturally through springs, by channel seeps associated with base flow of effluent streams, by subsurface
underflow, and by evapotranspiration.
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