Geology topics

Research about Comal County, Texas

Source-linked reports with geographic coverage including Comal County, Texas.

4 recordsLinked to original sources

USGS2014

Development of limestone reservoirs in Comal County, Texas

The Edwards limestone, and to a lesser extent the Comanche Peak limestone and the lower part of the Glen Rose limestone below, all of Lower Cretaceous age, form one of the most extensive ground‐water reservoirs in Texas. As a result of normal faulting, these formations are connected by underground channels to form a hydrologic unit which is partly artesian and partly unconfined. Observations in Comal County show that the development of the reservoir by solution is closely related to the geologic history, in which complex faulting and the disconformity between the Edwards and Georgetown limestones are important factors. Various phases of development are shown in different fault blocks, depending upon length of exposure at the surface. The Edwards limestone, which contains the main channels of underground drainage, is almost uniformly permeable from top to bottom, whereas the older, but more recently exposed lower part of the Glen Rose limestone is generally relatively impermeable except where exposed by mature streams.

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USGS2012

Effects of brush management on the hydrologic budget and water quality in and adjacent to Honey Creek State Natural Area, Comal County, Texas, 2001--10

Woody vegetation, including ashe juniper ( Juniperus ashei ), has encroached on some areas in central Texas that were historically oak grassland savannah. Encroachment of woody vegetation is generally attributed to overgrazing and fire suppression. Removing the ashe juniper and allowing native grasses to reestablish in the area as a brush management conservation practice (hereinafter referred to as "brush management") might change the hydrology in the watershed. These hydrologic changes might include changes to surface-water runoff, evapotranspiration, or groundwater recharge. The U.S. Geological Survey (USGS), in cooperation with Federal, State, and local partners, examined the hydrologic effects of brush management in two adjacent watersheds in Comal County, Tex. Hydrologic data were collected in the watersheds for 3-4 years (pre-treatment) depending on the type of data, after which brush management occurred on one watershed (treatment watershed) and the other was left in its original condition (reference watershed). Hydrologic data were collected in the study area for another 6 years (post-treatment). These hydrologic data included rainfall, streamflow, evapotranspiration, and water quality. Groundwater recharge was not directly measured, but potential groundwater recharge was calculated by using a simplified mass balance approach. This fact sheet summarizes highlights of the study from the USGS Scientific Investigations Report on which it is based.

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USGS2006

Hydrologic and water-quality data, Honey Creek State Natural Area, Comal County, Texas, August 2001-September 2003

The U.S. Geological Survey collected rainfall, streamflow, evapotranspiration, and rainfall and stormflow water-quality data from seven sites in two adjacent watersheds in the Honey Creek State Natural Area, Comal County, Texas, during August 2001–September 2003, in cooperation with the U.S. Department of Agriculture, Natural Resources Conservation Service, and the San Antonio Water System. Data collected during this period represent baseline hydrologic and water-quality conditions before proposed removal of ashe juniper ( Juniperus ashei ) from one of the two watersheds. Juniper removal is intended as a best-management practice to increase water quantity (aquifer recharge and streamflow) and to protect water quality. Continuous (5-minute interval) rainfall data are collected at four sites; continuous (5-minute interval) streamflow data are collected at three sites. Fifteen-minute averages of meteorological and solar-energy-related data recorded at two sites are used to compute moving 30-minute evapotranspiration values on the basis of the energy-balance Bowen ratio method. Periodic rainfall water-quality data are collected at one site and stormflow water-quality data at three sites. Daily rainfall, streamflow, and evapotranspiration totals are presented in tables; detailed data are listed in an appendix. Results of analyses of the periodic rainfall and stormflow water-quality samples collected during runoff events are summarized in the appendix; not all data types were collected at all sites nor were all data types collected during the entire 26-month period.

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USGS1994

Geologic framework and hydrogeologic characteristics of the Edwards Aquifer outcrop, Comal County, Texas

All of the hydrogeologic subdivisions within the Edwards aquifer outcrop in Comal County have some porosity and permeability. The most porous and permeable appear to be hydrogeologic subdivision VI, the Kirschberg evaporite member of the Kainer Formation; hydrogeologic subdivision III, the leached and collapsed members, undivided; and hydrogeologic subdivision II, the cyclic and marine members, undivided, of the Person Formation. The two types of porosity in the Edwards aquifer outcrop are fabric selective, which is related to depositional or diagenetic elements and typically exists in specific stratigraphic horizons; and not fabric selective, which can exist in any litho strati graphic horizon. Permeability, the capacity of porous rock to transmit water, depends on the physical properties of the rock such as pore size, shape, distribution, fissuring, and dissolution. Two faults, Comal Springs and Hueco Springs, completely, or almost completely, offset the Edwards aquifer along much of their respective traces across Comal County. Porous and permeable Edwards aquifer limestone is juxtaposed against impermeable upper confining units in these areas. These faults completely, or almost completely, offset the Edwards aquifer and are thought to be barriers or partial barriers to groundwater flow where the beds are juxtaposed. In Comal County, the Edwards aquifer is probably most vulnerable to surface contamination in the rapidly urbanizing areas on the Edwards aquifer outcrop. Possible contamination can result from spills, leakage of hazardous materials, or runoff onto the intensely faulted and fractured, karstic limestone outcrops characteristic of the recharge zone.

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