Geology and Hydrology

Topography

Topography in Fayette County consists of rolling to hilly uplands, and flat flood plains along the major streams. Flood-plain terraces, river flats, and marshes typify the valley bottoms. Elevation ranges from about 200 feet above sea level where the Colorado River crosses the Fayette-Colorado County line to over 550 feet in the southwest and northeast parts of the county. Most of the county is drained by the Colorado River and its tributaries. Major tributaries of the Colorado River draining Fayette County include Rabbs, Buckners, and Cummins Creeks. The southern part of the county is drained by the east and west branches of the Navidad River and their tributaries, and the westernmost corner of the county is drained by Peach Creek, a tributary of the Guadalupe River.

Groundwater Resources

Aquifers of Fayette County have been divided by the Texas Water Development Board (TWDB) into two types, namely, major and minor aquifers. The TWDB has classified two major aquifers in Fayette County: the Carrizo-Wilcox and Gulf Coast. Queen City and Sparta Sands are classified as minor aquifers. In addition to these aquifers, the alluvium of the Colorado River, as well as other geologic formations, are being tapped by wells within the County for domestic uses.

Most of the formations in Fayette County will yield some water, but only the sands of the Sparta Sand, Yegua Formation, Jackson Group, Catahoula Tuff, and Oakville Sandstone yield fresh to slightly saline water (having less than 3,000 parts per million dissolved solids) in significant quantities. The Carrizo Sand, sands of the Wilcox Group, the Queen City Sand, and the Quaternary alluvium are also capable of yielding water in the county; however, these contain usable quality water over limited areas of the county or occur at relatively great depths in comparison to other fresh water-bearing formations and consequently are not developed in Fayette County. Neither the Queen City Sand nor the Wilcox Group is known to yield water to wells in Fayette County. The Weches Greensand and Cook Mountain Formation generally do not yield usable quality water in sufficient quantities to constitute a supply.

Major Aquifers

Carrizo-Wilcox

The Wilcox Group and the overlying Carrizo Formation of the Claiborne Group form a hydrologically connected system known as the Carrizo-Wilcox Aquifer. The Carrizo-Wilcox crops out in a north-westerly line roughly parallel through Lee, Bastrop and Caldwell Counties and about six to eight miles in width. This aquifer downdips in Fayette County and is the deepest aquifer in the County which yields fresh to slightly saline water. The top of the Carrizo Sand’s altitude ranges from 500 feet below mean sea level in the southwestern part of the County to 5,000 feet below mean sea level in the northeastern part of the County. The Wilcox Group consists of various sediment material such as clay, silt, fine- to medium-grained sand and sandstone, shale, and some seams of lignite.

Total groundwater pumpage from the Carrizo-Wilcox in 1994 was 488,802 acre-feet. Municipal pumpage accounted for 31% of the total and irrigation accounted for 51%.

There is one well in Fayette County known to be tapping this aquifer. This well, located about two miles east of the town of Muldoon, is an abandoned oil and gas test well. It is reported to have a total dissolved solids (TDS) concentration of 1,091 mg/l which exceed s the 1,000 mg/l recommendation for municipal and domestic use. The fresh to slightly saline water of this aquifer extends under the uppermost one-third of Fayette County. This aquifer is undeveloped in Fayette County because of the high costs of producing water from the formation.

Gulf Coast

The Gulf Coast aquifer consists of several geologic formations within Fayette County, including the Oakville Sandstone, Lagarto Clay, and Catahoula. The Gulf Coast aquifer extends inland approximately 100 to 150 miles from the Gulf of Mexico in line approximately parallel to the Texas Gulf Coast. Total pumpage was approximately 1.1 million acre-feet in 1994. Municipal pumpage accounted for 51% of the total, irrigation accounted for 36%, and industrial accounted for 12%.

The aquifer consists of complex interbedded clays, silts, sands, and gravels, which are hydrologically connected to form a large, leaky artesian aquifer system. This system is comprised of two major components in Fayette County, consisting of the following generally recognized water producing formations. The deepest is the Catahoula, which contains groundwater near the outcrop in relatively restricted sand layers. Above the Catahoula is the Jasper Aquifer, primarily contained within the Oakville Sandstone.

Water quality is generally good in the shallower portion of the aquifer. In several areas at or near the coast, including Galveston Island and the central and southern parts of Orange County, heavy municipal or industrial pumpage has caused an updip migration, or saltwater intrusion, of poor quality water into the aquifer. Recent reductions in pumpage in those areas have resulted in a stabilization and, in some cases, even improvement of groundwater quality.

Years of heavy pumpage for municipal and manufacturing use in portions of the aquifer have resulted in areas of significant water-level decline. Some of these declines have resulted in compaction of dewatered clays and significant land-surface subsidence.

Oakville Sandstone and Lagarto Clay Formations

These formations are part of the Gulf Coast aquifer and are composed of two separate units within Fayette County. The Oakville Sandstone overlays the Lagarto Clay and is considered as one unit in Fayette County due to the difficulty in distinguishing each unit uniquely. The outcrop of the two units are east of a northeasterly line from Flatonia to Carmine. The outcrop area for the two units is approximately 13 miles in width in Fayette County. The combined unit consists of sand, gravel, clay and shale. The rate of dip within the County is not known at the present time. This aquifer is currently providing water supply for the cities of Ellinger, Fayetteville, and Flatonia. Some rural domestic users are also taping this formation.

Water quality from these two formations is generally acceptable although hardness is somewhat of a problem. Six wells were sampled from this unit and had an average TDS concentration of 800 mg/l although 14 percent of all samples had TDS concentrations in excess of this average. Chloride concentrations averaged 118 mg/l with 14 percent of the samples having concentrations greater than the average. Sulfate concentrations averaged 49 mg/l for all samples although 29 percent of the samples had sulfate concentrations greater than the average. Iron was not detected in any water quality sample. Calcium carbonate (CACO3) averaged 445 mg/l for all samples but 14 percent had concentrations in excess of this average.

Catahoula Tuff Formation

The Catahoula Tuff Formation is part of the Gulf Coast aquifer and crops out in Fayette and Lee Counties varying in width from one to six miles in Fayette County. The formation follows a northeasterly line from Flatonia to La Grange. The formation consists of clay, sand, silt, and tufaceous sand. The rate of dip which the aquifer has within the County is unknown as is the downdip limit of fresh to slightly saline water. The Catahoula Tuff formation is supplying water to the cities of Carmine, La Grange, Flatonia, and Schulenburg and the rural population between these cities.

Water quality of the water in this formation is generally acceptable for municipal and domestic purposes although hardness is somewhat of a problem. Water quality samples obtained from 50 wells reported an average TDS concentration of 680 mg/l with 28 percent of the samples having concentrations in excess of the average. Chloride was detected in all samples and averaged 145 mg/l although 28 percent of these samples had a concentration greater than this average. Sulfate concentrations averaged 44 mg/l for all samples with 29 percent of all samples having sulfate concentrations in excess of the average. Iron was detected in 12 samples with an average concentration of .12 mg/l. Approximately one third of all samples containing iron had concentrations which exceeded .12 mg/l. Hardness was observed to averaqe approximately 298 mg/l for all samples although 38 percent of all samples had levels which exceeded this average.

Queen City Sand

The Queen City Sand crops out in Bastrop and Lee Counties in a narrow band approximately three to five miles in width and roughly parallel to the Bastrop-Fayette County line. In Fayette County, this formation downdips at a rate of approximately 150 feet per mile from east to west. The formation's altitude ranges from 10 feet above mean sea level near the intersection of Buckner's Creek and State Highway 95 to approximately 4,000 feet below mean sea level near Fayetteville.

Water quality from this formation is adequate for municipal and domestic purposes though TDS values approach the recommended secondary limit. Fresh to slightly saline water is available west of a line from Flatonia to Ledbetter. Presently only one well near the town of West Point taps this aquifer. The reported TDS, chloride, sulfate and iron levels for this well are 952 mg/l, 186 mg/l, 255 mg/l, and 0.16 mg/l respectively. Water hardness for this well was reported at 6 mg/l.

Sparta Sand

The Sparta Sand Formation crops out in Bastrop and Lee counties in a very narrow band approximately one to two miles wide and along a line approximately parallel to the Bastrop-Fayette County line. The formation downdips approximately 175 feet per mile from the southwestern part of the County to the northeastern part of the County. The Sparta Sand's altitude ranges from 272 feet above mean sea level near the Bastrop County State Highway 95 intersection to 3,500 below mean sea level near Fayetteville.

Water quality from this formation is acceptable for municipal and domestic purposes although hardness and TDS concentrations approach Texas Department of Health's (TDH) recommended limits in some locations. Fresh to slightly saline water is available west of a line from sliqhtly west of Carmine to Flatonia. A total of 21 wells which tap this aquifer were sampled resultinq in an average TDS concentration of 923 mg/l. Approximately 30 percent of the samples had TDS concentrations which exceed the average TDS concentration. Sampled chloride concentrations from these 21 wells averaged 259 mg/l with approximately 30 percent of the samples exceeding the average chloride concentration. Sulfate concentrations averaged 235 mg/l with approximately 30 percent of the samples having concentrations which exceeded the average. Iron was present in only two of the samples and averaged 5 mg/l in concentration. Hardness for the Sparta Sand averaged 358 mg/l although 26 percent of all samples had concentrations greater than average.

Yegua Formation

The Yegua Formation crops out in Fayette and Lee County in a band approximately four to eight miles wide and along the Bastrop-Fayette County line. The Yegua Formation is composed of alternating layers of clay and silt with some thin seams of lignite. The formation downdips at a rate of 150 feet per mile. The formation reaches its deepest depth of 2,800 feet below mean sea level along the Fayette-Lavaca County line. Presently, the Yegua Formation is being utilized by rural landowners for domestic and livestock water supply.

The water quality of water from this formation is acceptable for municipal and domestic purposes although TDS and sulfate constituents exceeded the recommended maximum limits, and chloride and hardness constituents approached the maximum limits. A sampling of 46 wells which tap this aquifer produced an average TDS concentration of 1,197 mg/l which exceeds the TDH's recommended TDS limit of 1,000 mg/l. Of these 46 samples, approximately 25 percent had TDS concentration which exceeded the average. Chloride concentration for these sampled wells averaged 283 mg/l with approximately 20 percent of the samples exceeding the average. Sulfate concentration averaged 334 mg/l with approximately 35 percent of all samples having concentrations greater than the average. Iron was detected in only two of the water quality samples and averaged 2 mg/l in concentration. Hardness averaged 414 mg/l but approximately 24 percent had greater concentrations than average.

Jackson Group

The Jackson Group Formation crops out in Fayette and Lee Counties in a band approximately three to eight miles in width and along a northeasterly line from Flatonia to La Grange. The formation is composed of clay and silt with some minor deposits of sandstone. The formation dips within the County at a rate of approximately 150 feet per mile. The formation reaches an estimated 2,200 feet below mean sea level near Fayetteville. Current use of the Jackson Group is by the cities of Ledbetter, Flatonia, and Schulenburg as well as rural property owners.

Water quality from this formation is marginal for municipal and domestic purposes due to constituent levels exceeding recommended maximum limits for TDS, chloride, sulfate, and calcium carbonate in many locations throughout the County. Samples from 31 wells which tap this aquifer had an average TDS concentration of 1,557 mg/l with approximately 40 percent of all samples having TDS concentrations greater than the average. These samples contained chloride concentrations which averaged 404 mg/l with approximately 40 percent of all samples having chloride concentrations in excess of the average. Sulfate concentrations averaged 450 mg/l for all samples but approximately 40 percent of all samples had sulfate concentrations that exceeded the average. The presence of iron was not detected in any water quality sample. Hardness was detected in all samples and averaged 587 mg/l although 40 percent had concentration levels in excess of the average.

Yegua-Jackson Aquifer

According to the 2002 State Water Plan, Water for Texas, on the basis of recent hydrogeologic studies and reviews of groundwater production data, the Texas Water Development Board is designating the Yegua Formation and the Jackson Group as a minor aquifer, the Yegua-Jackson aquifer. The primary rationale for this designation is that water use from the Yegua-Jackson aquifer ranks in the upper half of annual water use for the minor aquifers, with more than 11,000 acre-feet of water produced in 1997. In addition, a review of the TWDB Groundwater Well Database indicates that there are currently more than 1,450 wells producing from the Yegua-Jackson aquifer. The Yegua-Jackson aquifer extends in a narrow band from the Rio Grande and Mexico across the State to the Sabine River and Louisiana. Although the occurrence, quality, and quantity of water from this aquifer are erratic, domestic and livestock supplies are available from shallow wells over most of its extent. Locally water for municipal, industrial, and irrigation purposes is available. Yields of most wells are small, less than 50 gallons per minute, but in some areas, yields of adequately constructed wells may range to more than 500 gallons per minute. The Yegua-Jackson aquifer consists of complex associations of sand, silt, and clay deposited during the Tertiary Period. Net freshwater sands are generally less than 200 feet deep at any location within the aquifer. Water quality varies greatly within the aquifer, and shallow occurrences of poor-quality water are not uncommon. In general, however, small to moderate amounts of usable quality water can be found within shallow sands (less than 300 feet deep) over much of the Yegua-Jackson aquifer.

Jasper Aquifer

The Jasper Aquifer is a hydrologic groundwater formation consisting of parts of the Oakville Sandstone and, in some localized areas, parts of the Catahoula Tuff. Therefore, it is not indicated on most maps as a separate geologic formation. Hydrologically, it is part of the Gulf Coast aquifer. The aquifer crops out in the southern part of Fayette County near the Lavaca County line. The formation overlays the Catahoula Tuff and underlies the Burkville Unit in Fayette County. The aquifer contains local pockets of sand, shale, and clay. The aquifer’s rate of dip in Fayette County is not known at the present time. Currently, rural domestic users are tapping this formation for water supply.

Water quality of the Jasper Aquifer is adequate for municipal and domestic uses although hardness is somewhat elevated. A total of 79 wells were sampled which tap this aquifer and had an average TDS concentration of 601 mg/l. Approximately 43 percent of the samples had a TDS concentration in excess of the average. Chloride concentrations for all samples averaged 109 mg/l although 38 percent of the samples had concentrations greater than the average.

Sulfate concentration averaged 34 mg/l for all samples but approximately 40 percent of the samples contained concentrations greater than the average. Iron was present in only one sample from this aquifer and had a concentration of 0.6 mg/l. Hardness averaged 245 mg/l for all samples although 52 percent had concentrations greater than average.

Alluvium

The alluvium (clay, silt, gravel, etc. deposited by running water) generally follows the flood plain of the Colorado River. The band's width varies from approximately one to eight miles. The alluvial's thickness is not known although some observations have estimated it does not exceed 60 feet. Wells in the alluvium are generally shallow and provide water in small quantities for rural domestic and livestock purposes within Fayette County.
Water quality from alluvial deposits is generally adequate for most uses in Fayette County although quantity is limited. These shallow wells use the alluvial deposits as a sand filter to provide some measure of water treatment. A total of 22 shallow alluvial wells were sampled with a resulting average TDS concentration of 605 mg/l. Approximately 42 percent of all samples contained TDS concentrations which exceeded the average concentration. Chloride concentrations averaged 93 mg/l although one third of all samples had concentrations greater than the average. Sulfate concentration averaged 58 mg/l although one third of all samples had sulfate concentrations greater than the average. Iron was not detected in any sample. Hardness averaged 345 mg/l for all samples with 37.5 percent of all samples having concentrations greater than this average.

Physical Characteristics & Water-Bearing Properties of Geologic Units

Midway Group

Rocks of the Midway Group crop out in a northeast-trending belt, 2 to 3 miles wide, along the Bastrop-Travis County line and dip southeast toward the Gulf Coast. They underlie Fayette County at depths ranging from about 3,800 feet (well 67-14-901) to over 9,100 feet (well 66-18-402).

The Midway consists principally of shale, clay, and a few thin sand lenses. The thickness of the Midway Group in Fayette County is about 900 to 950 feet.

No water wells and only a few oil tests penetrate the Midway in Fayette County. The Midway generally does not yield usable quality water in significant quantities, even in its outcrop area, and is well below the base of fresh to slightly saline water in Fayette County.

Wilcox Group

Rocks of the Wilcox Group crop out in a northeast-trending belt, 9 to 15 miles wide, across northwestern Bastrop and adjoining counties (Figure 4 in Appendix A). The Wilcox unconformably overlies the rocks of the Midway Group and unconformably underlies the Carrizo Sand of the Claiborne Group. The Wilcox is stratigraphically below all other aquifers in Fayette County and is the deepest rock unit containing fresh to slightly saline water.

The Wilcox consists of horizontally discontinuous beds of clay, silt, fine- to medium-grained sand and sandstone, sandy shale, and thin beds of lignite. The thickness of the Wilcox Group in Fayette County ranges from 2,400 to 3,800 feet. The depth to the top of the Wilcox Group in Fayette County ranges from 1,400 to about 6,000 feet.

Although the Wilcox Group occurs in the subsurface at varying depths throughout Fayette County, only that portion underlying the western and north western part of the county is believed to contain water of usable quality. The sands of the Wilcox Group contain fresh to slightly saline water at depths ranging from about 2,400 to over 3,800 feet in the county. The deepest fresh to slightly saline water in the Wilcox is east of Winchester and near the Lee County line. No water wells are known to penetrate the Wilcox Group in Fayette County, and the portion of the aquifer believed to contain fresh to slightly saline water is defined by interpretation of electric logs of oil tests penetrating the Wilcox.

Claiborne Group

Carrizo Sand

The Carrizo Sand crops out in a northeast band parallel to the Bastrop-Fayette County line about 4 to 5 miles wide through Bastrop and Lee Counties (Figure 4 in Appendix A).

The Carrizo Sand lies unconformably on the Wilcox Group and underlies the Reklaw Formation. In the outcrop, the Carrizo is a white to gray, fine- to coarse-grained, massive sand containing abundant cross-beds and very thin laminae of carbonaceous material. Its thickness ranges from 200 to 300 feet. The top of the formation is about 500 feet below sea level in the northwest part of the county and about 5,500 feet below sea level in the southeast part of the county; the dip of the beds is variable, ranging from about 160 to over 250 feet per mile to the southeast.

Although the Carrizo is capable of yielding moderate to large quantities of water to wells, and is extensively developed in many areas of the State, only one well (67-16-404), yielding slightly saline water, is known to produce from the Carrizo in Fayette County.

Reklaw Formation

The Reklaw Formation conformably overlies the Carrizo Sand and crops out in a narrow belt, 1 to 1 1/2 miles wide, across Bastrop, Lee, Gonzales, and adjoining counties (Figure 4 in Appendix A). The formation dips southeast and occurs in the subsurface throughout Fayette County.

The Reklaw consists of glauconitic sandstone interbedded with shale in the lower part of the formation and mainly clay and shale in the upper part. The thickness of the Reklaw ranges from about 225 to 400 feet in Fayette County.

In places in Fayette County the lower sands are very well developed and apparently are in hydrologic connection with the underlying Carrizo Sand. Although no wells are known to obtain water from the Reklaw in Fayette County, the lower sands probably contain fresh to slightly saline water in the northwestern part of the county.
Queen City Sand

The Queen City Sand conformably overlies the Reklaw Formation and is overlain conformably by the Weches Greensand. The Queen City crops out in Bastrop and Lee Counties and dips southeast toward the Gulf Coast at about 150 feet per mile.

The Queen City ranges from about 480 to 750 feet in thickness in Fayette County. Electric logs of oil tests penetrating the formation in Fayette County indicate that the formation consists of two or three 60-foot thick sands, usually near the top of the formation, separated by relatively thick sequences of thin sands interbedded with clay and sandy clay.

No water wells are known to be completed in the Queen City in Fayette County. However, the formation yields small to moderate quantities of water to wells in adjoining counties and provides a supply for the cities of Smithville and Giddings in adjoining Bastrop and Lee Counties, respectively. Small to moderate supplies of water could probably be developed in the northwestern part of Fayette County, but the water is very likely to be more mineralized than that from shallower formations such as the Sparta Sand and Yegua Formation.

Weches Greensand

The Weches Greensand conformably overlies the Queen City Sand and crops out in a northeast-trending belt about 1 mile wide in southeastern Bastrop County.

The Weches consists of about 75 to 150 feet of glauconitic shale with a few interbedded glauconitic sand and marl stringers. The Weches is relatively impermeable and is not known to yield water to wells in Fayette County.

Sparta Sand

The Sparta Sand is exposed in a band 1 to 2 miles wide from the west corner of Fayette County to near Smithville in Bastrop County generally paralleling the Fayette-Bastrop County line (Figure 4 in Appendix A).

The Sparta Sand lies conformably on the Weches Greensand and grades upward into the sandy shale base of the Cook Mountain Formation.

The Sparta consists of fine- to medium-grained sand interbedded with a few lignitic shale beds. The thickness of the Sparta ranges from 0 to 275 feet and averages about 150 feet in Fayette County. The Sparta dips southeast at about 175 feet per mile.

The Sparta yields small to moderate quantities of fresh to moderately saline water to wells near the outcrop in western and northwestern Fayette County.

Cook Mountain Formation

The Cook Mountain Formation overlies the Sparta Sand and crops out in the extreme western and northwestern part of Fayette County. The Cook Mountain consists of clay, shale, and a few thin lenses of sandstone, limestone, glauconite, and gypsum.

The Cook Mountain ranges in thickness from 0 to 500 feet in Fayette County. The Cook Mountain is not known to yield water to wells in the county.

Yegua Formation

The Yegua Formation crops out in a 3½ to 5 mile wide band across western Fayette County. The trend of the outcrop is northeast, the median line of which extends generally from Winchester to about 2½ miles south of Elm Grove in the southwest portion of the county.

The Yegua Formation conformably and semi-gradationally overlies the Cook Mountain Formation and conformably underlies the Jackson Group. Local disconformities between the Yegua and Jackson have been observed but are not of regional extent.

The Yegua Formation consists of alternating beds of fine- to medium grained clay, silt, thin beds of lignite, and small quantities of gypsum. Thickness of the individual sand beds ranges up to 2 or 3 feet where observed but generally is much thinner. Some bentonite occurs in the upper beds.

Total thickness along the outcrop ranges from about 500 to 700 feet. Downdip in Fayette County, the thickness increases, ranging from 600 to over 1,000 feet. Over most of the area in which fresh water occurs, the total sand thickness ranges from 300 to 430 feet and is about 40 to 50 percent of the total formation thickness. The formation dips to the southeast approximately 150 feet per mile, attaining a depth of 2,800 feet below sea level at the southeast edge of the county.

The Yegua yields small to large quantities of water to wells in Fayette County for industrial, irrigation, livestock, and rural domestic purposes. All wells presently pumping from the Yegua in the county are in the outcrop or less than 4 miles downdip.

Jackson Group

The Jackson Group conformably overlies the Yegua Formation of the Claiborne Group and crops out in a band 4 to 6 miles wide trending northeast across central Fayette County. The Jackson consists mainly of clay, silt, and volcanic ash, interbedded with a few relatively thin lenticular beds of tuffaceous sandstone. The thickness of the Jackson in Fayette County ranges from 0 at the updip extent of the formation to a total thickness of from 600 to 1,100 feet. The strata comprising the Jackson Group dip toward the Gulf Coast at about 150 feet per mile, coincident with the general regional structure.

The Jackson Group yields moderate quantities of water to wells, principally for livestock and rural domestic purposes in the outcrop areas. The most productive strata consist of about 50 to 185 feet of tuffaceous sands in the uppermost part of the group. These upper Jackson sands apparently yield water of usable quality some distance downdip from the outcrop and are generally developed in conjunction with the overlying Catahoula Tuff.

Frio Clay

The Frio Clay does not crop out in Fayette County, but overlies the Jackson Group unconformably in the subsurface and is in turn overlain and overlapped by the Catahoula Tuff. The Frio Clay consists principally of clay and shale interbedded with a few thin sand beds. The Frio ranges in thickness from 0 at its updip pinchout to over 520 feet in southeast Fayette County. The Frio Clay is not known to yield water to wells in Fayette County.

Catahoula Tuff

The Catahoula Tuff overlies the upper part of the Jackson Group near its outcrop, but downdip in the southeastern part of Fayette County, the Catahoula overlies the Frio Clay which occupies a position stratigraphically between the Catahoula Tuff and the Jackson Group.

The Catahoula crops out in a belt approximately 1/2 to 4 miles wide across central Fayette County trending northeast through Flatonia, La Grange, and Carmine.

In Fayette County, the Catahoula consists of tuffaceous sand and sandstone interbedded with clay, silt, and tuff. The thickness ranges from 0 to over 500 feet. The Catahoula yields small to large quantities of water to wells in central and southeastern Fayette County for municipal, industrial, and irrigation as well as livestock and rural domestic purposes.

Oakville Sandstone and Lagarto Clay

The Oakville Sandstone overlies the Catahoula Tuff and is in turn overlain by the Lagarto Clay. The approximate outcrop areas of these units are shown on the regional geology map (Figure 4 in Appendix A). Because the contact between the Oakville and Lagarto is difficult to distinguish in Fayette County, these formations are considered as a single unit in this report and are not differentiated on the county geologic map.

In general, the Oakville Sandstone consists of laterally discontinuous sand and gravel lenses interbedded with shaly sand, sandy shale, shale, and clay. Massive cross-bedded sandstone beds at the base grade upward into more thinly bedded sandy shale and clay near the top. The Lagarto Clay, in turn, consists mainly of massive clay interbedded with calcareous sand and shale.

The combined thickness of the Oakville and Lagarto ranges from 0 to over 950 feet.

The Oakville and Lagarto yield small to moderate quantities of water to wells for municipal, industrial, irrigation, livestock, and rural domestic purposes.

Alluvium

Alluvial deposits of Quaternary age in Fayette County occur as a broad band ½ to 6 miles wide coinciding generally with the flood plain of the Colorado River and along some of its major tributaries. Terrace gravel deposits, also of Quaternary age, occupy the tops of some of the hills adjoining the Colorado River flood plain, but these have not been mapped and probably are not important as a source of ground water in Fayette County.

The alluvial deposits consist of sand, gravel, black clay, sandy clay, and shale. Maximum thickness of the alluvial deposits is not known but where observed in stream cuts does not exceed 60 feet. Shallow wells completed in the alluvium yield small quantities of water for livestock and rural domestic purposes.

Surface Water Resources

Surface water sources of Fayette County include the Colorado River, the Cedar Creek Reservoir, flood control reservoirs, and numerous small stock ponds. Among these, the Colorado River and the Cedar Creek Reservoir can be considered for any municipal use. At present, only utilization of surface water for municipal use is at the Fayette Power Plant where potable water is supplied by treating water from the Cedar Creek Reservoir. In addition to this, Colorado River provides water for small domestic uses.
Colorado River

Water quality of the Colorado River varies seasonally and along the length of the river. Since January 1984, water samples were collected and analyzed by the Lower Colorado River Authority (LCRA) Water Quality Monitoring Program for two locations on the Colorado River within Fayette County. These sampling sites are located at upstream and downstream of La Grange. The upstream sampling station is located on the Colorado River at the Highway 71 bridge and the downstream site is at the Highway 77 bridge.

Cedar Creek Reservoir

The LCRA water quality monitoring program collects and analyzes water samples from several locations of the Cedar Creek Reservoir since July 1986. One of these sampling sites is located near FM 159. This sampling site was selected for study to represent water quality of the reservoir because of the suitability of its location for an intake structure of a regional surface water system.

Natural or Artificial Recharge

Recharge is the addition of water to an aquifer. The principal source of ground-water recharge in Fayette County is precipitation that falls on the outcrop of the various aquifers. In addition, seepage from streams and lakes located on the outcrop and possibly interformation leakage are sources of ground-water recharge. Recharge is a limiting factor in the amount of water that can be developed from an aquifer, as it must balance discharge over a long period of time or the water in storage in the aquifer will eventually be depleted. Among the factors that influence the amount of recharge received by an aquifer are: the amount and frequency of precipitation; the areal extent of the outcrop of intake area; topography, type and amount of vegetation, and the condition of soil cover in the outcrop area; and the ability of the aquifer to accept recharge and transmit it to areas of discharge. On aquifer outcrops where vegetation is dense, the removal of underbrush and non-beneficial plants will reduce evaporation and transpiration losses, making more water available for ground-water recharge.

Discharge is the loss of water from an aquifer. The discharge may be either artificial or natural. Artificial discharge takes place from flowing and pumped water wells, drainage ditches, gravel pits, and other excavations that intersect the water table. Natural discharge occurs as effluent seepage, springs, evaporation, transpiration, and interformational leakage.

Ground water moves from the areas of recharge to areas of discharge or from points of higher hydraulic head to points of lower hydraulic head. Movement is in the direction of the hydraulic gradient just as in the case of surface-water flow. Under normal artesian conditions, as in Fayette County, movement of ground water usually is in the direction of the aquifer's regional dip. Under water-table conditions, the slope of the water table and consequently the direction of ground-water movement usually is closely related to the slope of the land surface. However, for both artesian and water-table conditions, local anomalies are developed in areas of pumping and some water moves toward the point of artificial discharge. The rate of ground-water movement in an aquifer is usually very slow, being in the magnitude of a few feet to a few hundred feet per year.

The estimated effective annual recharge in acre-feet shown in the table below is taken from Texas Water Development Board Report 238, Ground-Water Availability in Texas, Estimates and Projections Through 2030.

[1] Note that in a Groundwater Availability Model (GAM) run of the Central Carrizo-Wilcox provided for the District on August 19, 2003 by the TWDB, the report states that the “Carrizo-Wilcox model does not have any direct infiltration in Fayette County. However, TWDB rules concerning groundwater management plan certification define recharge as ‘The addition of water from precipitation or runoff by seepage or infiltration to an aquifer from the land surface, streams, or lakes directly into a formation or indirectly by way of leakage from another formation.” In the TWDB GAM run, leakage into the Central Carrizo-Wilcox is estimated to be 8741 acre feet per year in Fayette County, roughly only half of the estimated recharge provided by the TWDB Report 238. According to the Region K’s Adopted Regional Water Supply Plan of December 2000, “the Carrizo-Wilcox aquifer has been studied by the Bureau of Economic Geology (BEG). The BEG study indicates that the average annual recharge for the aquifer in the Lower Colorado Region is 21,900 ac-ft/yr [state-wide].”

How Annual Amount of Recharge May Be Increased

According to the publication Groundwater Recharge in Texas, the main source of recharge for the Carrizo-Wilcox Aquifer is precipitation. Losing streams and flood water provide an additional source of recharge in the southern section of the Carrizo-Wilcox aquifer (L.B.G. Guyton & Associates and HDR Engineering, Inc., 1998). The Gulf Coast is also recharged primarily by precipitation. Losing streams and irrigation canals provide additional sources of recharge in the southwest portion of the aquifer along the Rio Grande. In Report 332, Ground-Water Resources of the Carrizo-Wilcox Aquifer in the Central Texas Region, it is stated:

Replenishment of water to the Carrizo-Wilcox aquifer, or recharge, is mainly by natural means. The major source and controlling factor for recharge is the frequency and the amount of precipitation.

Recharge to the Carrizo-Wilcox aquifer enters the aquifer primarily through infiltration in the outcrop from rainfall and from streams which cross its outcrop. Since the aquifer is nearly full, a substantial amount of recharge is rejected in the outcrop and only a small amount moves downdip. Additionally, a considerable amount of interformational leakage from overlying younger beds is occurring. With increased pumpage, both annual effective recharge to the outcrop and interformational leakage can be increased. Model-derived data on the north one-half of the study area suggest that slightly less than 3 percent of the mean annual rainfall over the outcrop goes to effective recharge. With increased pumpage under controlled conditions, effective recharge can be increased to about 5 percent.

Therefore, precipitation and recharge may be increased through weather modification over the recharge zones. No plans are currently contemplated by the District to attempt to increase the rainfall over the recharge zones, as there does not appear to be any direct recharge of this aquifer in Fayette County.

Groundwater Availability

According to Texas Water Development Board Report 56, Availability and Quality of Ground Water In Fayette County, Texas, computations of the amount of water that may be available from the Carrizo in Fayette County are based upon coefficients of transmissibility and storage of 40,000 gpd per foot and 0.00016, respectively. It is estimated that a maximum of 20,000 acre-feet of water per year could be induced to move through the aquifer from its recharge area to wells in Fayette County.

In the TWDB report Water for Texas – 2002, it is stated in the summary for Region K – the Lower Colorado River Planning Group, of which Fayette County is a part, that “Groundwater availability throughout the region is defined on the basis of local aquifer characteristics. In general, availability is defined on a sustainable basis (estimated aquifer recharge) instead of on the basis of the total volume of water in aquifer storage.”

Table 3 shows estimated amounts of available groundwater as estimated by the Lower Colorado Regional Water Planning Group (LCRWPG) Regional Water Plan adopted December 2000. In the report, it is stated that “The LCRWPG has established a policy for determining the availability of groundwater within the LCRWPA. The policy indicates that the long-term mining of groundwater within the region is not consistent with the LCRWPG’s sustainability goals. Therefore, in determining the availability of water from aquifers within the region, the average recharge rate for the aquifer is typically used.”

However, in the case of the Gulf Coast aquifer, “the nature of the Gulf Coast aquifer makes it very difficult to determine the average recharge rate. As a result, the water availability from the Gulf Coast aquifer is established based on an estimate of maximum usage in the year 2050 by water user groups (WUGs) that are currently using the aquifer as a source plus the average water use for future conjunctive water use at the Lakeside, Gulf Coast, and Pierce Ranch Irrigation Districts.”

Available groundwater in Fayette County, as shown in this table, is sufficient to meet all current municipal water needs, but due to large depths of water tables and locations of availability, development of some of the available water may not be economically feasible.

Note that the wide disparity in estimated amounts of groundwater availability and recharge rates between the State Water Plan, the Region K Water Plan, and the Groundwater Availability Model (GAM) model run provided by the TWDB serves to emphasize the importance of the District’s establishment of a water monitoring network and the need for accurate, reliable and pertinent data.

Table 3: Groundwater Availability in Fayette County Aquifers (in acre-feet/year)

Aquifer	2000	2010	2020	2030	2040	2050
Gulf Coast	8,697	8,697	8,697	8,697	8,697	8,697
Carrizo-Wilcox	400	400	400	400	400	400
Queen City	1,235	1,235	1,235	1,235	1,235	1,235
Sparta	4,500	4,500	4,500	4,500	4,500	4,500
TOTAL	14,832	14,832	14,832	14,832	14,832	14,832

Projected Groundwater Supply and Demand

Historical Rainfall

The table in Appendix B shows the historical rainfall by month through 1997, for each of the three major cities in Fayette County. The graphs below show the yearly rainfall trends for each of those cities.

Figure 1: Flatonia Yearly Rainfall

Click on thumbnail picture to enlarge.

Historical Water Usage

The Texas Water Development Board Water Uses Unit operates an annual survey of ground and surface water use by municipal and industrial entities within the state of Texas. This survey collects the volume of both ground and surface water used, the source of the water, and other pertinent data from the users. The information obtained is then utilized by the Water Development Board for projects such as water use projections and resource allocation. This is the source of the information contained in the historical water usage tables for Fayette County shown below.

Population Projections

Fayette County has grown very modestly. The geographic distribution and population has remained relatively unchanged. The decline in the oil and gas exploration since the early 1980’s and its distance from major population and employment centers have kept Fayette County’s population relatively stable.

The following population projections for total county and three major cities within Fayette County were taken from the Texas Water Development Board 2002 State Water Plan. The three major cites in Fayette County are La Grange, Flatonia, and Schulenburg. Three other smaller cities of Fayette County are Carmine, Fayetteville, and Round Top.

Fayette County has a diversified economy including livestock, poultry, crop production, power production, manufacturing industries, oil, gas and other mineral exploration, and recreation. Cattle raising and beef production is a major industry of the County. Agricultural products include grains, cotton, fruits, and vegetables.

The table below shows the expected population growth for Fayette County total and for each of the three major cities for each decade through 2050.

Table 7: Population Projections by City for 2000-2050

Water Supply and Demand Projections

The water use categories shown in the projections below are defined by the Texas Water Development Board in Water for Texas and include: municipal, irrigation, livestock, steam electric, manufacturing, and mining.

The municipal water use category includes water used by persons in their private dwellings for drinking, cooking, dishwashing, laundry, bathing, toilet flushing, lawn watering, car washing, swimming pools, and other purposes. It also includes water for sanitation, landscape maintenance, fire protection, and other general municipal purposes.

The other user categories generally represent farm and industry. The agricultural water use categories (irrigation and livestock) include water used for on-farm irrigation of crops and livestock water consumption. Manufacturing water use primarily focuses on the five largest water-using industries in the state: chemicals, petroleum, paper and pulp, metals, and food processing. Mining use represents water used in the extraction of fuel and non-fuel minerals. Steam electric represents water used by the steam generating power plants – in this case, the Fayette Power Project.

The tables below show the expected water demands in acre-feet for Fayette County total and for each of the three major cities, as well as for each major industry, for each decade through 2050.

Table 8: Municipal Demand Projections by City for 2000-2050