| The Dockum Aquifer |
| Taken from the District's Management Plan. |
Although the Dockum aquifer underlies more than 40 counties in West Texas, its low water-yielding ability and generally inferior quality results in
its categorization as a minor aquifer.
The boundaries of the Lone Wolf Groundwater Conservation District are coextensive with the boundaries of Mitchell County, Texas, covering
583,562 acres. The towns of Colorado City, Loraine and Westbrook are the main population centers in Mitchell County, Texas. The City of
Colorado City currently obtains its water supply from water wells located near Loraine with a backup water supply from Lake Colorado City and
Lake Champion. Loraine obtains its water supply from water wells located within the city of Loraine. The City of Westbrook purchases its water
from Mitchell County Utilities with wells located to the east of Colorado City.
Geology
The geologic rock formations of fresh water-bearing significance in Mitchell County consist of strata of Permian age, the Dockum Group of
Triassic age, the Trinity and Fredricksburg Groups of Cretaceous age, the Ogallala Formation of Tertiary age and alluvium of Quaternary age.
All of these strata outcrop in Mitchell County. Of paramount importance are the Santa Rosa Formation of the Dockum Group and the sands of
the Trinity Group which constitute the principal source of groundwater in the area.
Historically, the uppermost Dockum shale rocks were thought to be correlative with the Chinle Formation found in New Mexico and Arizona. The
sandstones below the Chinle were called the Santa Rosa and Trujillo Formations water bearing units and correlated with sandstones found in
northeastern New Mexico. The Santa Rosa typically is composed of an upper sandstone unit, a middle shale member, and lower conglomerate
sandstone. This division of the Triassic geology has commonly been used in West Texas and was the terminology followed in a report on the
groundwater resources in Mitchell County prepared by Victor Shamburger and published by the Texas Water Development Board in June 1967.
Although recent studies contest the historic Triassic correlations and nomenclatures and advance proposals for new divisions to the Triassic
section found in Mitchell County, the Arcadis G&M report chose to base its findings from the TWDB 1967 report as it is apparent the stated
debate will remain ongoing for quite some time.
Permian Strata
Strata of Permian age underlie much of the area but outcrop on the surface in the southeastern part of Mitchell County. The Permian strata
consist mainly of red beds which are dense red silt shale with gray-green inclusions interbedded with tight reddish-brown, fine-grained laminated
sandstones and occasional gypsum or anhydrite beds. The Permian beds dip westward at a slope of about 25 to 30 feet per mile, steepening
considerably in the western part of Mitchell County.
Dockum Group (Santa Rosa and Chinle Foundations)
Strata of the Dockum Group occur on the surface or subsurface in much of Mitchell County. The Dockum Group is generally subdivided into the
Santa Rosa Sandstone, the Tecovas Formation, the Trujillo Sandstone and the Cooper Canyon Formation by Lehman. The Cooper Canyon
Formation is generally absent in the area except in the extreme western part of Mitchell County. The Cooper Canyon Formation is predominately
red clay and shale with thin, ventricular, sandstone interbeds and it overlies the Trujillo Sandstone in the areas where the Cooper Canyon
occurs. The Cooper Canyon Formation is generally unimportant as a source of water except for livestock because it yields only small quantities
of water which are usually highly mineralized.
The Trujillo Sandstone is a cross-bedded unit composed of sandstones and conglomerates. The base of the unit (top of the Tecovas Formation)
is marked by erosional unconformity. The Trujillo may be as much as 100 feet or more in thickness.
The Tecovas shale underlies the Trujillo and is composed of mostly dark gray mudstones and shales. The thickness of the unit may be as much
as 45 to 50 feet in some areas.
The Santa Rosa Sandstone occurs beneath the Tecovas and it underlies unconformity on older Permian rocks. It consists of a basal
conglomerate overlain by alternating beds of red and gray micaceous shale, clay and sand. The thickness of the formation ranges from a few
feet to as much as 45 to 50 feet or more in other areas based on the work done by Lehman and Lucas. The thickness of the entire Dockum
Group ranges from a few feet to over 300 feet in the area northeast of Colorado City.
Cretaceous Rocks (Trinity and Fredricksburg Groups)
The Cretaceous rocks which occur in the area are of Lower Cretaceous age and belong to the Trinity and Fredricksburg Groups. These rocks
outcrop in southwestern and central Nolan County and underlie Tertiary Ogallala deposits in northwestern Nolan County. Cretaceous rocks are
completely absent in Mitchell County, except for the extreme eastern part of the county.
Sands of the Trinity Group consist of moderate to loosely consolidated, white to purplish, fine to medium-grained quartz sand with occasional
lenses of quartz gravel at the base of the unit. The thickness of the Trinity sands ranges from 60 to approximately 100 feet. The Trinity sand
overlies the Dockum Group (Santa Rosa Formation) in Western Nolan County but it lies directly on Permian strata farther to the east.
The Fredricksburg Group consists of up to 220 feet of calcareous sediments which overlie the Trinity Group in Nolan County. These rocks are of
little importance as a source of groundwater in the area.
Tertiary Ogallala Formation
Ogallala sediments of Tertiary age occur in the northwestern part of Nolan County (around Roscoe), the northeastern part of Mitchell County and
in west central and northwestern Mitchell County. Near Roscoe, the Ogallala sediments consist of up to 50 feet of caliche, sand and gravel
interbedded with light-colored clay. In this area, the Ogallala sediments are generally above the regional water table and are not a source of
groundwater. However, they appear to constitute an effective avenue for recharge to the underlying Santa Rosa Formation and Trinity sand.
In the western part of Mitchell County, the Ogallala consists of up to 100 feet of unconsolidated buff-brown sand with a zone of coarse gravel at
the base of the formation. In this area, the Ogallala sediments yield small quantities of usable water of variable quality to domestic and livestock
wells.
Hydrology
The water-bearing formation of primary interest in Mitchell County is the Santa Rosa Formation which consists of basal gravel and sand of
Triassic age overlain by alternating beds of red and gray micaceous shale, clay and sand (which comprises the Tecovas Formation and the
Truijillo Sandstone based on Lehman’s nomenclature). These strata occur on the surface over most of the county. The Permian rocks only yield
small quantities of water to wells and are generally regarded as the base of the fresh water occurrence in the area. In the western part of the
county, the Ogallala sediments yield small quantities of usable water of variable quality to domestic and livestock wells. The Permian beds dip
westward at an approximate slope of 25 to 30 feet per mile for most of the county, but the dip steepens considerably in the western part of the
county.
The literature indicates that the basal gravel and sand of the Santa Rosa Sandstone is highly productive and provides most of the water to wells
in the area. In the area north and northeast of Colorado City, the upper part of the Dockum Group (probably the Trujillo Sandstone) is saturated
and makes a significant contribution to well yields in the area. However, these upper sands apparently have a different water level than the lower
Santa Rosa and generally contain water of inferior quality to that found in the basal sand and gravel.
Although the Santa Rosa/Trujillo Aquifer is very productive over most of the area, the literature indicates that the groundwater quality in the
aquifer west of the Colorado River is poor and is not suitable for public consumption. In view of this, the remainder of this report focuses primarily
on the Santa Rosa/Trujillo Aquifer and the upper productive sands of the Dockum Group in the area east of the river. The thickness of the
Dockum Group as a whole in this area may be as much as 300 feet, but the saturated thickness is only approximately 50% or less of the total
thickness. Reported yields for water supply wells in this area are up to 1,000 gallons per minute (gpm).
Santa Rosa/Trujillo Aquifer Water Table
Groundwater in the Santa Rosa/Trujillo Aquifer and the overlying rocks of the Dockum Group that are saturated (Trujillo Sandstone) occurs
under either slightly artesian conditions or water table conditions. Pumping tests conducted on several wells completed in the Santa Rosa/Trujillo
Aquifer and/or the Trujillo Sandstone in the area indicate that, under static condition, the water in the aquifer may be artesian, but with pumping
and lowering of the water table below confining strata, water table conditions are produced.
Recharge to the aquifer results from infiltration and percolation of precipitation on the outcrop areas (including the overlying Ogallala and
alluvium formations where they occur). The area west of Loraine (where the surface is fairly sandy) is highly conducive to recharge. Significant
recharge also occurs along the creeks in the area where alluvium occurs on the surface along the stream channel. The amount of recharge to
the Santa Rosa and the Trujillo Sandstone in this area has not been determined. A rough estimate of recharge in this area is approximately 0.5
inches per year which amounts to approximately 26.7 acre-feet per section of land.
The altitude as shown in TWDB maps of the water table in the Santa Rosa/Trujillo Aquifer and or the Trujillo Sandstone for the period of 1960-
1961 shows that the direction of groundwater movement in the aquifer was to the west toward the Colorado River where significant discharge to
the river occurred. West of the river, the direction of groundwater movement was to the east toward the river.
The static water levels in most (or all) of the Santa Rosa/Trujillo water wells in the area were as high as or higher in the mid-1990s than they were
back in the early 1960s. This is reflected by the hydrographs of State observation wells which have historical records spanning the period from
the early-1960s to the mid-1990s. Several of the hydrographs show that the water table/piezometric surface in the Santa Rosa/Trujillo
Aquifer/Trujillo Sandstone responds quite rapidly and significantly to heavy pumping or cessation in pumping of water wells.
The fact that the water table in this area is at or above the levels in the early 1960s indicates a substantial cessation of groundwater withdrawal
from the aquifer for irrigation purposes during that time. The elevation of the water table appears to be approximately 20 feet higher in the mid
1990s than in 1960-61. However due to the sustained drought conditions during the late 1990s, groundwater usage in Mitchell County increased
dramatically with irrigation and municipal use. As part of this plan, the District will monitor the groundwater levels regularly to determine the
continued effects of increased pumping.
Groundwater Reserves
The gross saturated thickness of the Santa Rosa/Trujillo sediments in the eastern part of Mitchell County range from less than 60 feet in the
southern part of the area to over 200 feet in the north. In the areas situated north, northeast and east of Colorado City, the thickness of Santa
Rosa/Trujillo sediments ranged from 140 feet to over 200 feet in 1960-61. Accounting for the additional 20 feet in the water table by the mid-
1990s, the gross saturation of the aquifer in this area in the mid 1990s ranged from approximately 160 feet to over 220 feet.
An estimate of the amount of groundwater reserves in storage in the aquifer can be made knowing the saturated thickness of Santa Rosa/Trujillo
sediments and the effective porosity of the sediments. The effective porosity of the aquifer represents the void space from which water can be
drained by gravity expressed as a percentage of the total volume of sediments. No values of the effective porosity for the Santa Rosa/Trujillo
Aquifer have been reported in literature. However, based on Arcadis Geraghty and Miller’s experience in working with this and other aquifers in
West Texas, a conservative value of 10 percent is assumed for the effective porosity of the aquifer. This value was used to estimate the amount
of reserves in the aquifer.
Based on the range of gross saturated thickness of the aquifer discussed above for the areas north, northeast and east of Colorado City (160
feet to over 220 feet), the assumed effective porosity of the sediments of 10% and a recovery factor of 70%, the volume of recoverable
groundwater presently inplace in the aquifer is estimated to range from approximately 7,168 acre-feet per section to over 9,856 acre-feet per
section depending on the location of the property. This represents groundwater reserves present in the aquifer that can be produced by
pumping, and it does not include any recharge to the aquifer or exterior drainage from adjoining properties that may be captured once a well field
is developed and production begins.
These estimates for groundwater reserves in the aquifer include the apparent poorer quality water that may exist in the upper part of the aquifer
which may not be suitable for municipal purposes and may have to be sealed off during construction of water supply wells. The saturated
thickness of this upper productive zone is not known with any degree of certainty and would need to be addressed in any subsequent exploratory
work to verify the aquifer reserves, quality and productivity.
Groundwater Quality in the Santa Rosa/Trujillo Aquifer
State observation wells completed in the Dockum Group aquifer for which chemical analysis data were available in 1967 and more recent water
quality data obtained from the TNRIS are available for a limited number of these observation wells. Data from these observation wells indicate the
quality of the groundwater in the Santa Rosa/Trujillo Aquifer is considerably more mineralized in the western part of the county than in the
eastern part of the county. Generally speaking, west of the Colorado River the groundwater quality in the aquifer is poor and is unsuitable for
municipal purposes. However, east of the river, the water quality in the aquifer is less mineralized and is generally suitable for municipal purposes
(with some exceptions). More recent water quality data, where available, confirm this conclusion. For example, State observation well 28-40-608
(located about 10 miles northwest of Colorado City) contained chloride, sulfate and total dissolved solids (TDS) of 560 miligrams per liter (mg/L),
337 mg/L and 1,891 mg/L, respectively, in 1963. In 1986, the chloride, sulfate and TDS concentration in this well were 519 mg/L, 386 mg/L and
1,893 mg/L, respectively. By contrast, State observation well 29-35-702 (located about eight miles east of Colorado City in Loraine) contained
chloride, sulfate and TDS of 34 mg/L, 73 mg/L and 418 mg/L, respectively, for these same constituents in 1995. This also indicates that the
groundwater quality in this well had not changed appreciably over the indicated time period. In fact, the quality in well 29-35-702 actually
improved over the period.
Another important observation concerning the quality of groundwater in the Santa Rosa/Trujillo aquifer is the fact that the quality in the upper
sands (Trujillo Sandstone) appears to be inferior to the quality in the deeper basal sands and gravels (Santa Rosa Sandstone). This appears to
be true even for wells located east of the Colorado River.
Based on the available chemical quality data, it appears that wells completed in the lower (basal) sands or gravels (the Santa Rosa/Trujillo
Aquifer) contain groundwater which would meet the TCEQ standards for municipal water supplies in terms of the chloride, sulfate and TDS
content. These standards are 300 mg/L, 300 mg/L and 1,000 mg/L respectively, for these constituents.
The concentrations of nitrate in the groundwater are another important factor in determining the suitability of a water supply for municipal
purposes. The MCL for nitrates in public water supplies (as established by the EPA) is 10 mg/L of nitrogen (or 45 mg/L as nitrates). Above this
level, adverse health effects can result. The groundwater quality in the Santa Rosa/Trujillo Aquifer in the area east of Colorado City appears to
be generally acceptable for municipal purposes from the standpoint of the nitrate content of the water. However, several wells in the area do
exhibit elevated nitrate concentrations above the MCL of 45 mg/L. For example, State Well 29-27-902 had nitrates of 81 mg/L in 1978 which
increased to 109.9 mg/L in 1986. Well 29-34-515 had nitrate of 66 mg/L in 1963, well 29-34-801 had nitrate levels of 98 in 1946 and well 29-35-
108 had nitrate levels of 320 in 1963. No recent nitrates data are available for these wells. The source could be septic systems or areas where
nitrate-rich fertilizers are stored. Additional exploration would be necessary to identify and delineate the nature and extent of this problem.
Hydraulic Properties of the Santa Rosa/Trujillo Aquifer and Aquifer Productivity
The results of pumping tests conducted by the Texas Water Development Board in the 1960s on several water wells in the area completed in the
Santa Rosa/Trujillo Aquifer were used to estimate the transmissivity and storage coefficient of the aquifer. The transmissivity of the aquifer is
defined as the rate at which water flows through a vertical strip of the full saturated thickness of the aquifer one foot wide and under a unit
hydraulic gradient. It is a measure of the ability of the aquifer to transmit water. High values indicate greater transmitting capabilities of the
aquifer. The storage coefficient is defined as the volume of water released from storage or taken into storage per unit of surface area of the
aquifer per unit change in head in the aquifer. For water table aquifers, the storage coefficient is the same as the specific yield (or effective
porosity). As discussed earlier, in this area the Santa Rosa/Trujillo Aquifer appears to exhibit slightly artesian conditions under static conditions
due to the stratified nature of the aquifer. However, when the aquifer is pumped and the water level lowered below confining strata, water table
conditions may be produced. The specific yield (effective porosity) of an aquifer is the volume of water which can be drained by gravity from a
unit volume of the aquifer expressed as a fraction or percentage of the unit volume.
The transmissivity values obtained from the pumping tests conducted by the Texas Water Development Board ranged from 5,868 gallons per
day (gpd/ft) to 12,300 gpd/ft and averaged 8,845 gpd/ft. Because the tested wells were located over a wide area (east of Colorado City), this
range of transmissivity values appears to be representative of the Santa Rosa/Trujillo Aquifer in this area.
The storage coefficient values from the pumping tests ranged from 0.00008 to 0.00044 which are typical of aquifers under artesian conditions.
With sustained pumping of the aquifer and lowering of the water table below confining strata, water table conditions are expected to be produced.
Storage coefficients (or specific yields) in the range of 0.01 to 0.35 are typical of aquifers under water table conditions.
Reported yields for Santa Rosa/Trujillo water supply wells in the north, northeast and east of Colorado City are up to 1,000 gpm. However, well
yields and the productivity of the aquifer will vary across the area and depend on factors such as the lithology of the formation and the gross
saturated thickness of the aquifer. The design of the wells also has a significant impact on the yield of the well. Therefore, it would be imperative
to conduct exploration and testing to better assess these factors and to determine the productivity of the aquifer and well yields in specific areas
of interest.
its categorization as a minor aquifer.
The boundaries of the Lone Wolf Groundwater Conservation District are coextensive with the boundaries of Mitchell County, Texas, covering
583,562 acres. The towns of Colorado City, Loraine and Westbrook are the main population centers in Mitchell County, Texas. The City of
Colorado City currently obtains its water supply from water wells located near Loraine with a backup water supply from Lake Colorado City and
Lake Champion. Loraine obtains its water supply from water wells located within the city of Loraine. The City of Westbrook purchases its water
from Mitchell County Utilities with wells located to the east of Colorado City.
Geology
The geologic rock formations of fresh water-bearing significance in Mitchell County consist of strata of Permian age, the Dockum Group of
Triassic age, the Trinity and Fredricksburg Groups of Cretaceous age, the Ogallala Formation of Tertiary age and alluvium of Quaternary age.
All of these strata outcrop in Mitchell County. Of paramount importance are the Santa Rosa Formation of the Dockum Group and the sands of
the Trinity Group which constitute the principal source of groundwater in the area.
Historically, the uppermost Dockum shale rocks were thought to be correlative with the Chinle Formation found in New Mexico and Arizona. The
sandstones below the Chinle were called the Santa Rosa and Trujillo Formations water bearing units and correlated with sandstones found in
northeastern New Mexico. The Santa Rosa typically is composed of an upper sandstone unit, a middle shale member, and lower conglomerate
sandstone. This division of the Triassic geology has commonly been used in West Texas and was the terminology followed in a report on the
groundwater resources in Mitchell County prepared by Victor Shamburger and published by the Texas Water Development Board in June 1967.
Although recent studies contest the historic Triassic correlations and nomenclatures and advance proposals for new divisions to the Triassic
section found in Mitchell County, the Arcadis G&M report chose to base its findings from the TWDB 1967 report as it is apparent the stated
debate will remain ongoing for quite some time.
Permian Strata
Strata of Permian age underlie much of the area but outcrop on the surface in the southeastern part of Mitchell County. The Permian strata
consist mainly of red beds which are dense red silt shale with gray-green inclusions interbedded with tight reddish-brown, fine-grained laminated
sandstones and occasional gypsum or anhydrite beds. The Permian beds dip westward at a slope of about 25 to 30 feet per mile, steepening
considerably in the western part of Mitchell County.
Dockum Group (Santa Rosa and Chinle Foundations)
Strata of the Dockum Group occur on the surface or subsurface in much of Mitchell County. The Dockum Group is generally subdivided into the
Santa Rosa Sandstone, the Tecovas Formation, the Trujillo Sandstone and the Cooper Canyon Formation by Lehman. The Cooper Canyon
Formation is generally absent in the area except in the extreme western part of Mitchell County. The Cooper Canyon Formation is predominately
red clay and shale with thin, ventricular, sandstone interbeds and it overlies the Trujillo Sandstone in the areas where the Cooper Canyon
occurs. The Cooper Canyon Formation is generally unimportant as a source of water except for livestock because it yields only small quantities
of water which are usually highly mineralized.
The Trujillo Sandstone is a cross-bedded unit composed of sandstones and conglomerates. The base of the unit (top of the Tecovas Formation)
is marked by erosional unconformity. The Trujillo may be as much as 100 feet or more in thickness.
The Tecovas shale underlies the Trujillo and is composed of mostly dark gray mudstones and shales. The thickness of the unit may be as much
as 45 to 50 feet in some areas.
The Santa Rosa Sandstone occurs beneath the Tecovas and it underlies unconformity on older Permian rocks. It consists of a basal
conglomerate overlain by alternating beds of red and gray micaceous shale, clay and sand. The thickness of the formation ranges from a few
feet to as much as 45 to 50 feet or more in other areas based on the work done by Lehman and Lucas. The thickness of the entire Dockum
Group ranges from a few feet to over 300 feet in the area northeast of Colorado City.
Cretaceous Rocks (Trinity and Fredricksburg Groups)
The Cretaceous rocks which occur in the area are of Lower Cretaceous age and belong to the Trinity and Fredricksburg Groups. These rocks
outcrop in southwestern and central Nolan County and underlie Tertiary Ogallala deposits in northwestern Nolan County. Cretaceous rocks are
completely absent in Mitchell County, except for the extreme eastern part of the county.
Sands of the Trinity Group consist of moderate to loosely consolidated, white to purplish, fine to medium-grained quartz sand with occasional
lenses of quartz gravel at the base of the unit. The thickness of the Trinity sands ranges from 60 to approximately 100 feet. The Trinity sand
overlies the Dockum Group (Santa Rosa Formation) in Western Nolan County but it lies directly on Permian strata farther to the east.
The Fredricksburg Group consists of up to 220 feet of calcareous sediments which overlie the Trinity Group in Nolan County. These rocks are of
little importance as a source of groundwater in the area.
Tertiary Ogallala Formation
Ogallala sediments of Tertiary age occur in the northwestern part of Nolan County (around Roscoe), the northeastern part of Mitchell County and
in west central and northwestern Mitchell County. Near Roscoe, the Ogallala sediments consist of up to 50 feet of caliche, sand and gravel
interbedded with light-colored clay. In this area, the Ogallala sediments are generally above the regional water table and are not a source of
groundwater. However, they appear to constitute an effective avenue for recharge to the underlying Santa Rosa Formation and Trinity sand.
In the western part of Mitchell County, the Ogallala consists of up to 100 feet of unconsolidated buff-brown sand with a zone of coarse gravel at
the base of the formation. In this area, the Ogallala sediments yield small quantities of usable water of variable quality to domestic and livestock
wells.
Hydrology
The water-bearing formation of primary interest in Mitchell County is the Santa Rosa Formation which consists of basal gravel and sand of
Triassic age overlain by alternating beds of red and gray micaceous shale, clay and sand (which comprises the Tecovas Formation and the
Truijillo Sandstone based on Lehman’s nomenclature). These strata occur on the surface over most of the county. The Permian rocks only yield
small quantities of water to wells and are generally regarded as the base of the fresh water occurrence in the area. In the western part of the
county, the Ogallala sediments yield small quantities of usable water of variable quality to domestic and livestock wells. The Permian beds dip
westward at an approximate slope of 25 to 30 feet per mile for most of the county, but the dip steepens considerably in the western part of the
county.
The literature indicates that the basal gravel and sand of the Santa Rosa Sandstone is highly productive and provides most of the water to wells
in the area. In the area north and northeast of Colorado City, the upper part of the Dockum Group (probably the Trujillo Sandstone) is saturated
and makes a significant contribution to well yields in the area. However, these upper sands apparently have a different water level than the lower
Santa Rosa and generally contain water of inferior quality to that found in the basal sand and gravel.
Although the Santa Rosa/Trujillo Aquifer is very productive over most of the area, the literature indicates that the groundwater quality in the
aquifer west of the Colorado River is poor and is not suitable for public consumption. In view of this, the remainder of this report focuses primarily
on the Santa Rosa/Trujillo Aquifer and the upper productive sands of the Dockum Group in the area east of the river. The thickness of the
Dockum Group as a whole in this area may be as much as 300 feet, but the saturated thickness is only approximately 50% or less of the total
thickness. Reported yields for water supply wells in this area are up to 1,000 gallons per minute (gpm).
Santa Rosa/Trujillo Aquifer Water Table
Groundwater in the Santa Rosa/Trujillo Aquifer and the overlying rocks of the Dockum Group that are saturated (Trujillo Sandstone) occurs
under either slightly artesian conditions or water table conditions. Pumping tests conducted on several wells completed in the Santa Rosa/Trujillo
Aquifer and/or the Trujillo Sandstone in the area indicate that, under static condition, the water in the aquifer may be artesian, but with pumping
and lowering of the water table below confining strata, water table conditions are produced.
Recharge to the aquifer results from infiltration and percolation of precipitation on the outcrop areas (including the overlying Ogallala and
alluvium formations where they occur). The area west of Loraine (where the surface is fairly sandy) is highly conducive to recharge. Significant
recharge also occurs along the creeks in the area where alluvium occurs on the surface along the stream channel. The amount of recharge to
the Santa Rosa and the Trujillo Sandstone in this area has not been determined. A rough estimate of recharge in this area is approximately 0.5
inches per year which amounts to approximately 26.7 acre-feet per section of land.
The altitude as shown in TWDB maps of the water table in the Santa Rosa/Trujillo Aquifer and or the Trujillo Sandstone for the period of 1960-
1961 shows that the direction of groundwater movement in the aquifer was to the west toward the Colorado River where significant discharge to
the river occurred. West of the river, the direction of groundwater movement was to the east toward the river.
The static water levels in most (or all) of the Santa Rosa/Trujillo water wells in the area were as high as or higher in the mid-1990s than they were
back in the early 1960s. This is reflected by the hydrographs of State observation wells which have historical records spanning the period from
the early-1960s to the mid-1990s. Several of the hydrographs show that the water table/piezometric surface in the Santa Rosa/Trujillo
Aquifer/Trujillo Sandstone responds quite rapidly and significantly to heavy pumping or cessation in pumping of water wells.
The fact that the water table in this area is at or above the levels in the early 1960s indicates a substantial cessation of groundwater withdrawal
from the aquifer for irrigation purposes during that time. The elevation of the water table appears to be approximately 20 feet higher in the mid
1990s than in 1960-61. However due to the sustained drought conditions during the late 1990s, groundwater usage in Mitchell County increased
dramatically with irrigation and municipal use. As part of this plan, the District will monitor the groundwater levels regularly to determine the
continued effects of increased pumping.
Groundwater Reserves
The gross saturated thickness of the Santa Rosa/Trujillo sediments in the eastern part of Mitchell County range from less than 60 feet in the
southern part of the area to over 200 feet in the north. In the areas situated north, northeast and east of Colorado City, the thickness of Santa
Rosa/Trujillo sediments ranged from 140 feet to over 200 feet in 1960-61. Accounting for the additional 20 feet in the water table by the mid-
1990s, the gross saturation of the aquifer in this area in the mid 1990s ranged from approximately 160 feet to over 220 feet.
An estimate of the amount of groundwater reserves in storage in the aquifer can be made knowing the saturated thickness of Santa Rosa/Trujillo
sediments and the effective porosity of the sediments. The effective porosity of the aquifer represents the void space from which water can be
drained by gravity expressed as a percentage of the total volume of sediments. No values of the effective porosity for the Santa Rosa/Trujillo
Aquifer have been reported in literature. However, based on Arcadis Geraghty and Miller’s experience in working with this and other aquifers in
West Texas, a conservative value of 10 percent is assumed for the effective porosity of the aquifer. This value was used to estimate the amount
of reserves in the aquifer.
Based on the range of gross saturated thickness of the aquifer discussed above for the areas north, northeast and east of Colorado City (160
feet to over 220 feet), the assumed effective porosity of the sediments of 10% and a recovery factor of 70%, the volume of recoverable
groundwater presently inplace in the aquifer is estimated to range from approximately 7,168 acre-feet per section to over 9,856 acre-feet per
section depending on the location of the property. This represents groundwater reserves present in the aquifer that can be produced by
pumping, and it does not include any recharge to the aquifer or exterior drainage from adjoining properties that may be captured once a well field
is developed and production begins.
These estimates for groundwater reserves in the aquifer include the apparent poorer quality water that may exist in the upper part of the aquifer
which may not be suitable for municipal purposes and may have to be sealed off during construction of water supply wells. The saturated
thickness of this upper productive zone is not known with any degree of certainty and would need to be addressed in any subsequent exploratory
work to verify the aquifer reserves, quality and productivity.
Groundwater Quality in the Santa Rosa/Trujillo Aquifer
State observation wells completed in the Dockum Group aquifer for which chemical analysis data were available in 1967 and more recent water
quality data obtained from the TNRIS are available for a limited number of these observation wells. Data from these observation wells indicate the
quality of the groundwater in the Santa Rosa/Trujillo Aquifer is considerably more mineralized in the western part of the county than in the
eastern part of the county. Generally speaking, west of the Colorado River the groundwater quality in the aquifer is poor and is unsuitable for
municipal purposes. However, east of the river, the water quality in the aquifer is less mineralized and is generally suitable for municipal purposes
(with some exceptions). More recent water quality data, where available, confirm this conclusion. For example, State observation well 28-40-608
(located about 10 miles northwest of Colorado City) contained chloride, sulfate and total dissolved solids (TDS) of 560 miligrams per liter (mg/L),
337 mg/L and 1,891 mg/L, respectively, in 1963. In 1986, the chloride, sulfate and TDS concentration in this well were 519 mg/L, 386 mg/L and
1,893 mg/L, respectively. By contrast, State observation well 29-35-702 (located about eight miles east of Colorado City in Loraine) contained
chloride, sulfate and TDS of 34 mg/L, 73 mg/L and 418 mg/L, respectively, for these same constituents in 1995. This also indicates that the
groundwater quality in this well had not changed appreciably over the indicated time period. In fact, the quality in well 29-35-702 actually
improved over the period.
Another important observation concerning the quality of groundwater in the Santa Rosa/Trujillo aquifer is the fact that the quality in the upper
sands (Trujillo Sandstone) appears to be inferior to the quality in the deeper basal sands and gravels (Santa Rosa Sandstone). This appears to
be true even for wells located east of the Colorado River.
Based on the available chemical quality data, it appears that wells completed in the lower (basal) sands or gravels (the Santa Rosa/Trujillo
Aquifer) contain groundwater which would meet the TCEQ standards for municipal water supplies in terms of the chloride, sulfate and TDS
content. These standards are 300 mg/L, 300 mg/L and 1,000 mg/L respectively, for these constituents.
The concentrations of nitrate in the groundwater are another important factor in determining the suitability of a water supply for municipal
purposes. The MCL for nitrates in public water supplies (as established by the EPA) is 10 mg/L of nitrogen (or 45 mg/L as nitrates). Above this
level, adverse health effects can result. The groundwater quality in the Santa Rosa/Trujillo Aquifer in the area east of Colorado City appears to
be generally acceptable for municipal purposes from the standpoint of the nitrate content of the water. However, several wells in the area do
exhibit elevated nitrate concentrations above the MCL of 45 mg/L. For example, State Well 29-27-902 had nitrates of 81 mg/L in 1978 which
increased to 109.9 mg/L in 1986. Well 29-34-515 had nitrate of 66 mg/L in 1963, well 29-34-801 had nitrate levels of 98 in 1946 and well 29-35-
108 had nitrate levels of 320 in 1963. No recent nitrates data are available for these wells. The source could be septic systems or areas where
nitrate-rich fertilizers are stored. Additional exploration would be necessary to identify and delineate the nature and extent of this problem.
Hydraulic Properties of the Santa Rosa/Trujillo Aquifer and Aquifer Productivity
The results of pumping tests conducted by the Texas Water Development Board in the 1960s on several water wells in the area completed in the
Santa Rosa/Trujillo Aquifer were used to estimate the transmissivity and storage coefficient of the aquifer. The transmissivity of the aquifer is
defined as the rate at which water flows through a vertical strip of the full saturated thickness of the aquifer one foot wide and under a unit
hydraulic gradient. It is a measure of the ability of the aquifer to transmit water. High values indicate greater transmitting capabilities of the
aquifer. The storage coefficient is defined as the volume of water released from storage or taken into storage per unit of surface area of the
aquifer per unit change in head in the aquifer. For water table aquifers, the storage coefficient is the same as the specific yield (or effective
porosity). As discussed earlier, in this area the Santa Rosa/Trujillo Aquifer appears to exhibit slightly artesian conditions under static conditions
due to the stratified nature of the aquifer. However, when the aquifer is pumped and the water level lowered below confining strata, water table
conditions may be produced. The specific yield (effective porosity) of an aquifer is the volume of water which can be drained by gravity from a
unit volume of the aquifer expressed as a fraction or percentage of the unit volume.
The transmissivity values obtained from the pumping tests conducted by the Texas Water Development Board ranged from 5,868 gallons per
day (gpd/ft) to 12,300 gpd/ft and averaged 8,845 gpd/ft. Because the tested wells were located over a wide area (east of Colorado City), this
range of transmissivity values appears to be representative of the Santa Rosa/Trujillo Aquifer in this area.
The storage coefficient values from the pumping tests ranged from 0.00008 to 0.00044 which are typical of aquifers under artesian conditions.
With sustained pumping of the aquifer and lowering of the water table below confining strata, water table conditions are expected to be produced.
Storage coefficients (or specific yields) in the range of 0.01 to 0.35 are typical of aquifers under water table conditions.
Reported yields for Santa Rosa/Trujillo water supply wells in the north, northeast and east of Colorado City are up to 1,000 gpm. However, well
yields and the productivity of the aquifer will vary across the area and depend on factors such as the lithology of the formation and the gross
saturated thickness of the aquifer. The design of the wells also has a significant impact on the yield of the well. Therefore, it would be imperative
to conduct exploration and testing to better assess these factors and to determine the productivity of the aquifer and well yields in specific areas
of interest.