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Hydrogeologic Report for Evaluation of Groundwater Contamination
Section 3 - Regional Geology and Hydrogeology
The information in this section was taken from Texas Water Development Resources (TDWR) report 273, Ashworth, January, 1983; Hydrogeology of the Lower Geln Rose Aquifer, Hammond, May 1984; and Hydrogeology of the Camp Bullis Area, Wattereus, 1992. These references were used to compile a summary of the regional geology and hydrogeology.
CSSA is located in northwest Bexar County in the south-central Texas Hill Country. The area under study is in northwestern Bexar, southern Kendall, and western Comal counties (Figure 2.1).
The study area is characterized by a rough and rolling terrain in which nearly flat-lying limestone formations have been eroded and dissected by rivers and streams draining to the east and southeast. Normal faulting has occurred near the southeast boundary of the study area, and two sets of regional fractures trending northwest to southeast and northeast to southwest extend across the entire area. Soil cover is relatively thin, and bedrock is exposed in most areas except in the stream valleys.
River and stream dissection of the almost flat-lying limestones is the major surface feature in the study area. Most major rivers and streams originating in the Edwards Plateau to the northwest tend to follow the regional fracture patterns, primarily the northwest-southeast fractures. Resistive limestone beds crop out as topographic highs, but none of these beds form buttes or mesas. Topographic relief across the area ranges from about 1,100 feet to 1,500 feet above sea level.
Sinkholes and caverns are present on the surface and in the subsurface, primarily in areas where the lower Glen Rose Formation is exposed. Small caves and sinkholes also exist in the upper Glen Rose Formation, where evaporite beds are present. The sinkholes and caves result from solution of limestone and gypsum by infiltrating surface water. Commonly, fractures are enlarged by moving groundwater which permits the enhancement of secondary porosity and permeability.
Faulting in the limestone units has juxtaposed strata of different ages, but fault scarps and traces are almost absent because of the similar lithologies. The faults are northeast-southwest trending, but most are not as continuous as the fractures.
Information on the stratigraphy of the study area was taken from TDWR report 273, January 1983. The stratigraphic names and groupings used follow the format set up in report 273. Previous stratigraphic groupings in TDWR report 60, August 1976, differ slightly from report 273 regarding the same area.
The oldest and deepest rocks known in the area are rocks of Paleozoic age (225 to 570 million years ago, mya) Ouchita structural belt. These rocks represent deformation thrusting and mountain building during the late Paleozoic time. The lithologies consist of slate, schist, and hard massive limestone and dolomite. None of these rocks are exposed in the study area.
Following a long period of aerial exposure and erosion of the Paleozoic-age rocks during early to middle Mesozoic (135 to 225 mya), Cretaceous-age (65 to 135 mya) sediments were deposited on the uneven Paleozoic surface. The sediments were deposited on the uneven Paleozoic surface. The sediments were deposited as onlapping sequences on a vast, flat, submerged plain known as the Comanche shelf. The Cretaceous-age sediments are observed in well logs and outcrops to thicken southeastward towards San Antonio and thin to the northwest near Llano in the updip areas. These are sediments of the Trinity Group Travis Peak Formation which were first deposited duriong a time of sea level rise across the area. The Travis Peak Formation attains a maximum thickness of about 940 feet and is divided into five members, in ascending order: the Hosston Sand, the Sligo Limestone, the Hammett Shale, the Cow Creek Limestone, and the Hensell Sand (locally known as the Bexar Shale). These members represent sediments that were depositiied in shallow marine environments except the Hosston Sand, which represents continental deposition. Figure 3.1 is a regional stratigraphic diagram showing the stratigraphy of the area.
The Glen Rose Formation of the Trinity Group was deposited over the Travis Peak Bexar Shaled in the study area. The Glen Rose Formation represents a thick sequence of shallow water marine shelf deposits and is divided arbitrarily into upper and lower members.
The lower Glen Rose member is approximately 300 feet thick in Bexar County and consists primarily of massive bedded limestone wiht few layers of marl and marly limestone. The upper member is approximately 500 feet thick in Bexar County and consists primarily of alternating beds of resistive and nonresistive beds of limestone and marly limestone. A distinct widespread fossil stratigraphic marker known as the "Corbula bed" (a bed of small clamshells 3 to 5 millimeters in diameter) is the dividing boundary between the upper and lower members. In addition, two distinct evaporite beds (primarily gypsum) are present in the upper member. The depth of the second (deeper) evaporite bed is close to that of the Corbula bed, allowing the contact between upper and lower members to be found on subsurface geophysical logs.
Fredricksburg Group sediments overlie the Glen Rose Formation in many areas as erosional remnants cropping out as topographic highs. For this report, the Fredricksburg Group limestone will not be discussed because of the lack of outcrop in the immediate study area.
The predominant structural features in the study area are the regional vertical fractures, the regional dip, and the Balcones fault zone (escarpment). Fractures are areas of structural weakness where rupturing without displacement has occurred, whereas faults are locations of vertical or near-vertical movement. The regional fractures are the result of faulting in the Cretaceous sediments and in the deeper Paleozoic rocks. The two sets of fracture patterns trend northwest-southeast and northeast-southwest across the region. The regional dip of the formations is to the east and southeast at a rate of about 100 feet per mile near the fault zone in Bexar and Comal Counties, but decreases to 10 to 15 per mile in northern Kerr and Gillespie Counties, northwest of the study area.
The Balcones fault zone is a series of roughly parallel high-angle normal faults trending northeast to southwest, as shown in Figure 3.2. Total displacement in northwest Bexar County is approximately 1,200 feet. The faulting is a result of structural weakness in the underlying Paleozoic rocks and subsidence in the Gulf of Mexico basin to the southeast. The downdrop blocks (shown as hatch marks on the faults in Figure 3.2) crop out as younger strata from northwest to southeast across the fault zone. In addition to the major faulting along the Balcones fault zone, numerous minor northeast-trending faults occur also. These faults are laterally discontinuous and their displacement is small.
The occurrence of groundwater in the Trinity Group sediments is related to the stratigraphic intervals which are capable of transmitting and containing water. Groundwater-bearing units in limestone are those in which the enhanced secondary permeability has permitted enough space for groundwater to move freely. Fracturing, faulting, and dissolution all play a role in groundwater occurrence in limestones. Shale, marl, and clay horizons are predominantly confining units which do not yield water. However, if displaced enough by faulting, the shale, marl, and clay units may provide pathways for water to move vertically along fault planes.
Groundwater Occurrence
There are three aquifers in the study area underneath CSSA: the upper, the middle, and the lower Trinity aquifers. Table 3.1 lists the stratigraphic units and their water-bearing properties. Underneath the Trinity aquifers are the oldest known rocks in the study area, i.e., the Paleozoic-age schist, slate, and hard massive limestone and dolomite. These rocks are not know to yield water. The lack of adequate permeability in these rocks permits them to act as a lower hydrologic barrier.
Water-bearing formations in the study area consist of the Travis Peak Formation and the Glen Rose Formation. Collectively, these formations make up the Trinity Group of the Cretaceous-age Comanche series (Table 3.1). The formation and member names are combined into groups consisting of the lower, middle, and upper Trinity aquifers. These formations and members are grouped based on different hydraulic continuities and are discussed in ascending order.
Lower Trinity Aquifer
The lower Trinity aquifer is made up of the Hosston Sand and the Sligo Limestone of the Travis Peak Formation. These members do not crop out in the study area. Updip to the northwest of CSSA, the Hosston consists of red and white conglomerate, sandstone, and claystone, and the main constituent is quartz sand. Downdip in the study area, it becomes increasingly more dolomitic and shaley. Thin conglomeratic zones, near the base, persist throughout the downdip limit of the study area. The Sligo Limestone exists downdip where the Hosston Sand grades upward into a sandy dolomitic limestone. The Hosston and the Sligo thicken in a downdip direction (south and southeast) to as much as 500 feet near the Balcones fault zone. The lower boundary of the lower trinity aquifer is the Paleozoic basement rocks on which the Hosston was deposited.
Middle Trinity Aquifer
The middle Trinity aquifer is made up of the Cow Creek Limestone, the Hensell Sand, and the lower Glen Rose limestone. Only the lower Glen Rose limestone crops out in the study area. The Cow Creek Limestone is a massive, fossiliferous, white to gray, shaley to dolomitic limestone with local thinly bedded layers of sand, shale, and lignite. The Cow Creek attains a maximum thickness of about 90 feet downdip in the study area, although 50 to 60 feet is average over most of its extent. updip northwest of Kendall County, the unit thins and pinches out.
The Hensell Sand member consists of alluvial and near-shore marine sediments. Updip in Kendall County, the Hensell is the depositional equivalent of the Glen Rose Limestone that was deposited offshore. The Hensell Sand is composed of thick continental deposits of red clay, silt, sand, and conglomerate. Downdip in the study area, the unit grades into marine deposits of silty dolomite, marl, calcareous shale, and shaley limestone, known as the Bexar Shale in Bexar County. The thickness of the Hensell varies considerably because of the uneven surface on which it was deposited and the nature of the upper gradational boundary with the Glen Rose. In the study area, the Bexar Shale thins by interfingering into the Glen Rose, and its thickness averages from 80 to 150 feet.
The lower Glen Rose consists of massive, fossiliferous limestone at the base, grading upward into thin beds of limestone, dolomite, marl, and shale. The top of the unit is capped by the "Corbula bed," a thin, resistant, limonite-stained bed of biosparite containing numerous casts and steinkerns of the small bivalve shell Carycorbula harveyi. The lower Glen Rose has a maximum thickness of about 320 feet near the fault zone in Bexar County and thins updip by grading laterally into the underlying Hensell Sand in Kendall County. The lower boundary of the middle Trinity aquifer is the Hammett Shale. The Hammett Shale acts as hydrologic barrier between the middle and lower Trinity aquifers except in areas where the shale is disrupted by faulting.
Upper Trinity Aquifer
The upper Trinity aquifer (upper Glen Rose Limestone) consists of laterally continuous, alternating resistant and nonresistant beds of blue shale, nodular marl, and impure, fossiliferous limestone. The alternating resistance to erosion of the limestone and marl beds creates the characteristic "stairstep" topography. The upper Glen Rose attains a maximum thickness of 450 feet near the fault zone, but pinches out updip. At the northern updip limit of the Glen Rose (northwest of Kendall County), the upper and middle Trinity aquifers are indistinguishable. The lower boundary of the upper Trinity aquifer is the basal marl beds in the upper Glen Rose, which generally prevent vertical migration.
Groundwater Movement
Groundwater movement in the upper, middle, and lower Trinity aquifers is highly variable because of the nature of the lithology. Limestones and calcareously cemented sandstones depend on secondary porosity in the form of solution channels and fractures for the transmission of water. These solution channels are nonuniform in their occurrence and size, which results in unpredictable yields at different locations.
Lower Trinity Aquifer
The lower Trinity aquifer (Hosston Sand and Sligo Limestone) derives its recharge from the overlying Hensell Sand in the updip direction where the overlying Hammett Shale and Cow Creek Limestone is thin or absent (figure 3.1). Some recharge may come from leakage of overlying strata in areas where the Hammett Shale is disrupted by faulting. Groundwater in the lower Trinity aquifer is under artesian conditions because the overlying Hammett shale acts as a confining bed. The average coefficient of transmissibility in the lower Trinity aquifer is about 10,000 gallons per day per foot (gpd/ft), and the highest values are in northern Kerr County. Groundwater movement is generally downdip to the south and southeast except in areas of continuous pumpage, where flow is toward these discharge points. Discharge from the lower Trinity aquifer occurs primarily by pumpage from wells.
Middle Trinity Aquifer
The lower Glen Rose portion of the middle Trinity aquifer derives its recharge from direct precipitation on the outcrop and stream flow losses crossing the outcrop. A prime example of stream flow losses has been observed in the channel of Cibolo Creek between Boerne and Bulverde, where the entire stream flow is diverted underground through sinkholes except during flood stages.
In the study area, Bexar Shale acts as a hydrologic barrier to vertical leakage except where faulted; therefore, most recharge to the Cow Creek Limestone comes from the overlying Hensell Sand in the updip areas (Kendall County) where the Bexar Shale has pinched out (Figure 3.1). No wells in the study area are known to have been completed only in the Cow Creek; therefore, the Cow Creek is presumed to be in natural hydraulic communication with the lower Glen Rose. A number of wells are reported as completed in both the Cow Creek and the lower Glen Rose, allowing artificial hydraulic communication between the lower Glen Rose Limestone and the Cow Creek Limestone. The middle Trinity aquifer is under water table conditions.
The average coefficient of transmissivity in the middle Trinity aquifer (Cow Creek Limestone and Lower Glen Rose Limestone) is 1,700 gpd/ft. Movement of groundwater in the middle Trinity aquifer is in the downdip direction towards the south and southeast except in areas of heavy pumpage, where groundwater flows towards the discharge points. Middle Trinity discharge occurs both artificially by pumpage from wells and naturally by springs and seeps.
Upper Trinity Aquifer
The upper Trinity aquifer consists of the upper Glen Rose Limestone. The upper Glen Rose Limestone is exposed over much of the study area. Recharge to the upper Trinity aquifer is from direct precipitation on the outcrop of the upper Glen Rose Limestone and stream flow losses. Additional recharge in the Camp Bullis area occurs from reservoirs built by the U.S. Soil Conservation Service located on Salado and Lewis Creeks, approximately 1 mile south of CSSA.
No transmissivity values have been determined for the upper Trinity aquifer, but can be expected to be much lower than for the middle and lower Trinity aquifers. Movement of groundwater in the upper Trinity is restricted to lateral flow along bedding planes between marl and limestone, where solution has enhanced the permeability of the limestone. Individual beds which are capable of transmitting very small amounts of groundwater vertically within fractures are bound by the extensive beds of impermeable marl and clayey limestone. The marl layers can act as vertical barriers and do not respond as well as limestone to groundwater solution or fracturing because of the typical swelling nature of marls and clays. Static water levels in adjacent wells completed in different beds within the upper Glen Rose are not concordant, demonstrating the possibility that beds are not hydraulically connected by avenues of vertical permeability. The only reported place where extreme development of solution channels exists is in evaporite layers in or near the outcrop of the upper Glen Rose Limestone. Discharge from the upper Trinity aquifer is predominantly from natural rejection through seeps and springs and pumping.
The upper Trinity aquifer is under water table conditions. Artesian conditions occur where the water-bearing zone is overlain by low-permeability strata and well water levels rise above the top of the aquifer under pressure. Water table conditions occur where the aquifer is unbound above the water table. Fluctuations in water levels are predominantly a result of seasonal rainfalls and pumpage from domestic and public wells. The average rainfall in Bexar County is 32 inches per year (NOAA 1991). In 1990 and 1991, the average precipitation increased to 40.5 inches per year. Most of the rainfall ends up as surface runoff or is rejected by springs and seeps (TDWR 1983).
Groundwater Quality and Usage
Lower Trinity Aquifer
In the study area, the lower Trinity aquifer yields fresh to slightly saline water with a total dissolved solids (TDS) content ranging from 900 to 1,500 milligrams per liter (mg/L). Approximately 40 miles updip out of the study area, the aquifer yields water with a TDS content of 500 mg/L or less. The lower Trinity aquifer is not used as a primary source of water. The overlying Hammett Shale is reported to be a heaving shale and must be cased off in order to complete a well in the lower Trinity aquifer. The high cost of constructing a well in the lower Trinity makes it economically unfeasible to pump for most purposes.
Middle Trinity Aquifer
The middle Trinity aquifer yields fresh to slightly saline water throughout the study area. Water in the lower member of the Glen Rose Limestone is normally very good quality although hard. This water increases in dissolved solids content near the Balcones fault zone owing to an increase in sulfate ions. No wells are near the Balcones fault zone owing to an increase in sulfate ions. No wells are known to have been completed only in the Cow Creek Limestone; therefore, the water qualify is unknown. The middle Trinity aquifer is the most widely used source of water in the area. Most water wells in the area obtain middle Trinity water for public supply, irrigation, domestic, and livestock purposes.
Upper Trinity Aquifer
Upper Trinity water is generally of poor quality, and most wells achieve only low production. The existence of the evaporite beds in the upper Trinity causes excessive sulfate content in the water. Few wells obtain water solely from the upper Trinity aquifer.