| DOCKUM AQUIFER |
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The data provided for this section of the management plan, unless otherwise noted, is obtained from a study conducted by Arcadis Geraghty and Miller for Mitchell County in
October 1998. The study was conducted primarily to determine an alternate resource for the public water supply since the surface water resources were quickly evaporating
due to drought. The study consisted of researching and reviewing available information (including published literature, reports, files, data, etc) which contain information
pertinent to evaluating the groundwater resources available in the county.
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 within 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 is 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 Formation 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 a 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.
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, lenticular, 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 is 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.
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 north-east 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 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 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 Aquifer Water Table
Groundwater in the Santa Rosa 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 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 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 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 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 1990’s 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 area 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 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 area 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 in-place 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 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 75 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 for gravels (the Santa Rosa Aquifer) contains 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 as 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 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 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 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.
Estimated Annual Recharge to the Dockum
Recharge occurs rapidly where the Dockum outcrops or is near the surface in Scurry, Mitchell, Sterling and Coke counties. Elsewhere, the Dockum is buried at depths that
generally eliminate recharge. The TWDB estimates that 19,880 acre-feet of annual effective recharge occur in these four counties. The annual effective recharge in the District
is estimated to be 8,744 acre-feet.
Developed areas of the Santa Rosa Formation of the Dockum aquifer derive recharge both locally from the land surface and from undeveloped areas, including the Cretaceous
sediments in Nolan County which contribute water to the areas of development. The amount of recharge any particular area might receive depends on a number of factors such
as soil and rock type, topography, extent of cultivation and intensity and distribution of rainfall.
The best indicator of the adequacy of recharge is the performance of water levels in wells. Water levels, together with the amount of water pumped, can be used to estimate the
amount of water available on a continuing basis. To provide a historical reference of water levels the Lone Wolf GCD began an ongoing quarterly water level monitoring
program in the Spring of 2002.
Taken from the Lone Wolf Groundwater Conservation District Management Plan.
The data provided for this section of the management plan, unless otherwise noted, is obtained from a study conducted by Arcadis Geraghty and Miller for Mitchell County in
October 1998. The study was conducted primarily to determine an alternate resource for the public water supply since the surface water resources were quickly evaporating
due to drought. The study consisted of researching and reviewing available information (including published literature, reports, files, data, etc) which contain information
pertinent to evaluating the groundwater resources available in the county.
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 within 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 is 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 Formation 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 a 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.
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, lenticular, 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 is 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.
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 north-east 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 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 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 Aquifer Water Table
Groundwater in the Santa Rosa 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 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 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 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 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 1990’s 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 area 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 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 area 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 in-place 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 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 75 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 for gravels (the Santa Rosa Aquifer) contains 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 as 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 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 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 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.
Estimated Annual Recharge to the Dockum
Recharge occurs rapidly where the Dockum outcrops or is near the surface in Scurry, Mitchell, Sterling and Coke counties. Elsewhere, the Dockum is buried at depths that
generally eliminate recharge. The TWDB estimates that 19,880 acre-feet of annual effective recharge occur in these four counties. The annual effective recharge in the District
is estimated to be 8,744 acre-feet.
Developed areas of the Santa Rosa Formation of the Dockum aquifer derive recharge both locally from the land surface and from undeveloped areas, including the Cretaceous
sediments in Nolan County which contribute water to the areas of development. The amount of recharge any particular area might receive depends on a number of factors such
as soil and rock type, topography, extent of cultivation and intensity and distribution of rainfall.
The best indicator of the adequacy of recharge is the performance of water levels in wells. Water levels, together with the amount of water pumped, can be used to estimate the
amount of water available on a continuing basis. To provide a historical reference of water levels the Lone Wolf GCD began an ongoing quarterly water level monitoring
program in the Spring of 2002.
Taken from the Lone Wolf Groundwater Conservation District Management Plan.
