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

declines, with one reporting a decline of 157 feet and the other two an average of 7.5 feet. The remaining Pennsylvanian
well had a net water level increase of 2 feet. The one well completed in older Paleozoic rock had a net water decrease of
3.3 feet.
Natural water level changes in an aquifer are mainly due to changes in the groundwater recharge/discharge
conditions of the aquifer. Variation in atmospheric pressure and rate of evapotranspiration also may have a lesser effect on
water levels in wells. When natural groundwater recharge is reduced during dry periods, water is discharged naturally from
storage and groundwater levels decline accordingly. As the aquifer is recharged by rainfall, the groundwater lost from
storage is replenished and water levels begin to rise. Groundwater withdrawals from wells disrupt these natural conditions
and artificially cause water level changes.
Hhydrographs for water levels in selected water wells in the watershed show the fluctuation of water level that
occurs over time. Overall, the water level fluctuations are likely the result of variations in rainfall. Localized concentrated
groundwater withdrawals and, to a lesser extent, the rates of evapotranspiration are likely causes of any long-term water
level declines.
Springs
No quantitative information on spring flows in the watershed was found for this study, and little information
appears to be published on the existence of springs in the area. Thompson (1967) inventoried 11 springs in Brown County,
but he did not describe them beyond identifying the geologic unit from which they discharged. Two of the springs
reportedly issue from the Trinity Group, five from Permian formations, and four from Pennsylvanian strata. The locations
of these springs are shown on Figure 3-10. Price et al. (1983), Taylor (1976), and Walker (1967) mention the occurrence
of numerous spring in Callahan, Coleman and Taylor counties but did not identify any that discharge within the Lake
Brownwood watershed. Many of the springs appear to discharge along slopes where permeable, water bearing rocks rest
on less permeable rocks. An excellent example of this is where strata of the Fredericksburg Group (including the Edwards
Formation) occur at the tops of the hills that form the Callahan Divide. Water that infiltrates the surface of the
Fredericksburg percolates downward until it encounters less permeable rocks and flows laterally and by gravity to
discharge points along hillsides.
3.3 GEOLOGICAL CONSIDERATIONS
Geologic units at the surface within the study area are delineated on the geologic map in Figure 3-8. Generally,
streams in the watershed flow across Permian and Pennsylvanian sedimentary rocks that extend to a depth of
approximately 4,000 feet. The rocks are composed of limestone, dolomite, shale and clastics that dip to the northwest at an
average rate of 50 feet per mile. Cretaceous rocks consisting of sandstone, shaly limestone and limestone are exposed at
the surface mainly along the eastern margin of the study area. However, they also occur as erosional remnants, or outliers,
in the localized areas in the western part of the watershed. The Cretaceous rocks unconformably overlie the Pennsylvanian
strata and dip gently to the southeast in the study area. The Cretaceous rocks form the larger hills and mesa-type
landforms in the area. Thin deposits of Quaternary age sand, silt, clay and caliche unconformably overlie the Paleozoic
rocks in the western portions of the watershed, and recent (Holocene) alluvium deposits occur throughout the watershed
in the floodplains of the larger streams. The Qauternary deposits are not depicted in Figure 3-8. An east-west geologic
cross-section that illustrates the subsurface geology within the watershed is presented in Figure 3-9. Table 3-5 identifies
the stratigraphic units, and corresponding aquifers that occur in the study area. The table also summarizes the lithologic
character and water-bearing properties of each significant unit.
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