The relationship between the productivity of wells and the location of lineaments has been investigated for 47 wells in that part of the Edwards Aquifer that discharges to Barton Springs. Controlled aerial mosaics were used to map two sets of lineaments (short and long) by the use of different techniques. Transmissivities were then estimated for all wells with specific capacity data. The distance from each of the wells to the nearest short and long lineaments was then determined. Wells located greater than 300 feet from short lineaments had transmissivities that ranged from less than 25 to 2,800 gal/day/ft and averaged 434 gal/day/ft. Transmissivities for wells within 300 feet of short lineaments averaged 56,300 gal/day/ft and ranged from 200 to 350,000 gal/day/ft.
There is a strong correlation between increased water-well productivity and decreased distances to short lineaments. However, no correlation is evident between well productivity and distances to long lineaments. Locations of short lineaments can therefore be used to increase the probability of locating high-yield wells in the Edwards Aquifer associated with Barton Springs.
Figure 1 : Boundary of the Edwards Aquifer--Barton Springs Segment (Slade and others, 1985).
Figure 2 : Block diagram showing various structural and stratigraphic influences on porosity development (modified from Parizek, 1975).
Figure 3 : Location of the wells used in the study and short and long lineaments mapped on the Austin West, Texas 7.5" U.S.G.S. quadrangle map.
Figure 4 : Location of the wells used in the study and short and long lineaments mapped on the Oak Hill, Texas 7.5" U.S.G.S. quadrangle map.
Figure 5 : Locations of wells used in the study and short and long lineaments mapped on Signal Hill, Texas 7.5" U.S.G.S. quadrangle map.
Figure 6 : Locations of the wells used in the study and short and long lineaments mapped on the Buda, Texas 7.5" U.S.G.S. quadrangle map.
Figure 7 : Locations of the wells used in the study and long and short lineaments mapped on the Mountain City, Texas 7.5" U.S.G.S. quadrangle map.
Figure 8 : Location of the wells used in the study and short and long lineaments mapped on the Driftwood, Texas 7.5" U.S.G.S. quarangle map.
Table 1 : Summary of wells with specific capacities, estimated transmissivities, and relative distance to nearest lineaments.
Figure 9 : Relation between distance of wells to short linements and transmissivities.
Figure 10 : Relation between distance of wells to long lineaments and transmissivities.
Linear features on the surface of the earth have attracted the attention of geologists for over one hundred years. This interest has grown most rapidly since the introduction of aerial photographs into geological studies. Geologists have recently proven that lineaments perceived in remotely sensed images are reliable indicators of geologic structure (Caran and others, 1982). Lineaments have been used in many applications: petroleum and mineral exploration (Blanchet, 1957); nuclear energy facility siting (Seay, 1979); geothermal assessments (Woodruff and others, 1982); and water resource investigations (Lattman and Parizek, 1964). Lattman and Parizek (1964) established a relationship between the occurrence of groundwater and fracture traces for carbonate aquifers, and in particular that lineaments are underlain by zones of localized weathering and increased permeability and porosity. LaRiccia and Rauch (1977) tested this theory on a limestone aquifer in Frederick Valley, Maryland. They reported that short lineaments are usually associated with higher transmissivities, thus greater water well productivities.
Lineaments have been called lineations, linears, fracture traces, and many other names. Woodruff and others (1982) did a thorough review of published works on lineaments and have proposed a concise terminology, and a systematic method of perceiving and interpreting lineaments that improves data reproducibility. Their terminology and methodology have been used within this report.
The purpose of this report is to determine and present the relation between productivity of water wells and proximity to lineaments in that part of the Edwards aquifer that discharges to Barton Springs. If wells with high yields can be related to the location of lineaments, then the locations of lineaments could be useful in locating zones of high productivity in the aquifer, and areas of high recharge potential in the unsaturated zone of the aquifer. Furthermore, flow in the Edwards aquifer is primarily through the cavities and caves associated with faults, fractures, and joints. (Slade, 1984). The intent of this report therefore is to test the hypothesis that lineaments relate to fracture zones--areas of greater hydraulic conductivity.
DESCRIPTION OF STUDY AREA AND HYDROGEOLOGIC SETTING
The study area for this report is that part of the Edwards Aquifer that discharges to Barton Springs. It extends from a discharge boundary formed at the Colorado River in the north to a southern boundary about 25 miles south of the river--a surface water divide that separates the Blanco River Basin from that of Onion Creek. The western boundary of the aquifer is the western extent of the Edwards Limestone (mainly controlled by the Mt. Bonnell Fault), and the eastern boundary, known as the "bad-water" line, is the eastern limit of water within the aquifer that contains less than 1,000 mg/l dissolved solids (figure 1). These boundaries form a hydrologically independent segment of the Edwards aquifer, which covers about 155 square miles. The westernmost 90 square miles of the aquifer area are within the recharge zone, an area that covers the outcrop of the Edwards and Georgetown Limestones. About 85 percent of the recharge occurs in the main channels of six major streams that cross the recharge zone. These creeks have a total combined drainage of 264 square miles upstream from the recharge zone, an area called the contributing zone. The main discharge point of this segment of the Edwards Aquifer is Barton Springs.
The aquifer is composed of the Edwards and Georgetown Limestones, a series of limestones and dolomites that range from 400 to 460 ft thick where not outcropped. Where exposed at the surface in the recharge zone, the aquifer varies from about 100 to 460 feet thick. The upper confining layer is the Del Rio Clay, which ranges from 60 to 75 ft thick, and the lower confining unit is the Walnut Formation, which ranges from 15 to 60 ft thick. The Balcones Fault Zone extends over much of the aquifer, creating numerous fault blocks "stair stepping" downward to the east, with vertical displacements of as much as 200 ft. However, no evidence has been presented to indicate that the aquifer is discontinuous; thus, groundwater flow probably is not greatly impeded by faults (Slade and others, 1985).
DEVELOPMENT OF POROSITY AND PERMEABILITY
Significant porosity in the Edwards aquifer was created along particular bedding planes through dissolution by meteoric water during an interval of subaerial exposure at the close of the Edwards Limestone period of deposition (Abbott, 1976). Enhanced porosity also occurs laterally and vertically along faults and fractures as a result of enlargement by dissolution. Laterally continuous cavities are thought to be caused by dissolution of the limestone along faults. Wermund and others (1978) compared the incidence of caves with the incidence of short fractures (lineaments) on the southern Edwards Plateau and reported that cave frequency increases as fracture incidence increases. Parizek (1975) show how various factors influence cavity distribution in carbonate rocks (figure 2). Furthermore, interpretation of geophysical and drillers' logs for many wells in the study area indicates that most wells that penetrated cavities within the aquifer are near faults (Slade and others, 1985). Wells that have penetrated cavities commonly have high yields.
DEFINITION OF LINEAMENTS AND METHOD OF INVESTIGATION
Woodruff and others (1982) define lineaments as a figure (either simple or composite) that 1) is perceived in an image of a solid planetary body, 2) is linear and continuous, 3) has definable end points and lateral boundaries, 4) has a relatively high length-to-width ratio and hence a discernible azimuth, and 5) is shown or presumed to be correlative related to stratigraphy or geologic structure.
A "false lineament" is defined as a perceived lineament that meets all but criterion number 5 as described above. "False lineaments" could be: 1) cultural manifestations that do not coincide with linear topography, such as fencelines, roads, pipelines, railroads, and animal trails; 2) artifacts of imaging process or perceptual aberrations (illusions); or 3) geomorphic features that are not controlled by stratigraphy or geologic structure. "False lineaments" are sometimes mapped as true lineaments. However, in the process of transferring the lineaments marked on aerial photographs, most if not all false lineaments are thought to be eliminated from the study.
Lineaments are polygenetic and inherently ambiguous features. They are ambiguous because they cannot always be field-verified, nor are they precisely reproducible. Lineaments are polygenetic in that they owe their expression to a number of different causes. Lineaments could be 1) straight stream and valley segments, 2) aligned surface sags and depressions, 3) soil tonal changes revealing variations in soil moisture, 4) alignments in vegetation, 5) vegetation type and height changes, and 6) abrupt topographic changes. All of these phenomena might be a result of structural phenomena such as faults, joint sets, or folds.
The method employed for this study began by obtaining controlled mosaics of 1:20,000 black-and-white aerial photographs taken in 1937 by Tobin Research, Inc. Six mosaics were needed to cover the study area, which corresponds to the Austin West, Buda, Driftwood, Mountain City, Oak Hill, and Signal Hill, 7.5 minute quadrangle maps of the U.S. Geological Survey.
Two sets of lineaments were identified--short and long sets. Both sets were viewed by three interpreters; however, the mapping and viewing methods were different. For short lineaments, the six mosaics were viewed individually for two 20-minute sessions by each interpreter, thus a total of 120 minutes of viewing time for each 7.5 minute map. In order for a linear feature to be mapped as a lineament, a minimum length of 112 inches on the image (about 1000 feet on the ground) was established. No maximum length was set; therefore, the maximum length could be the length of a diagonal line across the 7.5 minute map. The end points of each lineament were identified with arrows, then transferred to a set of maps (figures 3 to 8).
For the long lineaments the same set of mosaics were used, but the six photo mosaics were taped together on the floor of a large room and viewed concurrently by all three investigators. In viewing long lineaments, minimum lengths of one foot on the image (about 4.5 miles on the ground) were identified only when all three interpreters agreed on their presence. The lineament was then marked by tape and later transferred to the same set of maps. Again, the main difference in short and long lineaments lies in the method by which they were perceived.
METHOD OF ESTIMATING WELL YIELDS FROM PUMP DATA
Well data were obtained from the U.S. Geological Survey (USGS). Information for all wells with recorded pump tests was gathered by the USGS from the Texas Water Development Board for a study that simulated the flow of Barton Springs and associated Edwards Aquifer (Slade and others, 1985). No known aquifer-test data exist for the study area (Slade and others, 1985). Therefore, transmissivities were estimated from specific capacities. For this study, water well yields were represented by transmissivities of the aquifer at the well location. Transmissivities were used rather than specific capacities, because transmissivities represent the hydraulic characteristic of the aquifer, whereas specific capacities can be a function of how efficient wells are completed and developed.
Pumpage and drawdown data were used to compute specific capacities of the wells. Transmissivities were estimated from specific capacities by a method derived by Rex R. Meyer (Bentall, 1963). Meyer created a graph that relates well diameter, specific capacity (SC), transmissivity (T), and storage (S). The graph was prepared by 1) computing, for various values of T and S, the theoretical drawdown in wells having different diameters, 2) computing the SC of those wells (on the assumption that they were 100% efficient), and 3) plotting the SC against S to form a family of curves that represent the different values of T.
Meyer explains that the graph has certain limitations, however, these limitations do not apply to the Edwards Aquifer. A principal factor affecting the transmissivity at a well is the entrance loss of water to the bore. The graph is based on the assumption that the wells are 100 % efficient or, in other words, the water level is the same inside and immediately outside the casing or screen when the wells are pumped. This assumption that wells are 100 % efficient applies to wells within the Edwards Aquifer, most of which are uncased within the aquifer. Thus, good communication between the bore and aquifer should exist (Slade, pers. comm., 1986).
Another factor affecting the transmissivity at a well is the diameter of the well. The well diameters of 6, 12, and 24 inches are shown on the graph and are considered to be the effective diameters of the well. Meyer explains that the actual and effective diameters may be different for wells in unconsolidated aquifers. However, if an aquifer is composed of consolidated rocks, as is the Edwards Aquifer, the effective diameter should not appreciably vary from the actual diameter of the well. It is also evident from the graph that differences in diameters do not greatly affect the transmissivities.
A time interval of one day was used for computing the specific capacities on the graph. It was noted that an error is introduced if the specific capacity determined in the field is based on shorter or longer periods of pumping. Meyer (Bentall, 1963) notes that the amount of this error is small for high values of T and low values of S, but increases substantially for low values of T and high values of S. This error will be small for wells in the Edwards, where small values of S and large values of T normally occur.
The graph shows that large changes in S correspond to relatively small changes in T and SC; therefore, any errors in S would not significantly affect the values of T. Reasonable values for S can be estimated based on previous studies done on the Barton Springs segment of the Edwards Aquifer. Slade and others (1985) estimated storage coefficients using a two-dimensional, finite difference ground-water model of the study area; specific yield was calibrated for a transient state simulation using time-dependent gauged or measured data for recharge, discharge, and water levels. The mean specific yield for the unconfined part of the aquifer was determined to be 0.014, and the coefficient of storage of the confined portion was calculated to be .00005. Specific yield is defined as the volume of water that an unconfined aquifer releases from storage per unit surface area of aquifer per unit decline in the water table. Specific yield increases with porosity and, because of the karstic nature of the aquifer, specific yield also increases with hydraulic conductivity (Slade and others, 1985). Slade reported that hydraulic conductivities generally increased with proximity to Barton Springs, and thus a similar pattern for distribution of specific yield probably exists.
INTERPRETATION AND CONCLUSIONS
Locations of the wells with specific capacities were plotted on the composite set of lineament maps, figures 3 to 8. A total of forty-seven wells were available for this analysis. The specific capacity, and distance from each well to the nearest lineament is summarized in Table 1. Distances were recorded to the nearest 50 feet. The transmissivities for the 47 wells range from less than 25 to 350,000 gal/day/ft. The distances from these wells to short lineaments vary from 0 to more than 1000 feet. However, all 12 wells that had T's greater than 10,000 gal/day/ft were within 300 feet of a short lineament. Also 17 of 20 wells with T's greater than 1,000 gal/day/ft were within 300 feet from a short lineament. Of the 27 wells with T's less than 1,000 gal/day/ft, 21 were greater than 300 feet of a short lineament. The 20 wells with T's greater than 1,000 gal/day/ft were located an average of 190 feet from short lineaments, whereas the 27 wells with T's less than 1,000 gal/day/ ft were located at an average distance of 520 feet from short lineaments. A correlation therefore exists between higher transmissivities and decreased distances to short lineaments.
Figures 9 and figure 10 show the distance from each well to the nearest lineament plotted against the estimated T for each well. A correlation can be made from the short lineament set; however, there does not appear to be any type of correlation between well transmissivities and proximity to longer lineaments. A correlation between areas of increased aquifer productivity and decreased distances to short lineaments can also be made. Some lineaments can therefore be used to increase the probability of locating high yield zones of the Edwards aquifer. This association probably indicates that lineaments are associated with fracture zones and areas of solution porosity and high permeability.
There are four limitations to conclusions from this report. First of all, most of the well locations have not been field-verified. Therefore, the distances from the wells to the nearest lineaments are subject to error. Also, the locations of lineaments marked on the mosaics are subject to error in the transferring process, which could affect the distances between lineaments and wells. Next, the specific capacities were obtained from drillers' logs. It is not known whether or not steady-state conditions were reached before water-level declines were measured, or if constant discharges were maintained throughout the tests. Finally, conclusions are based on only 47 wells. It is not known if this is a representative sample of wells with varying specific capacities and locations with respect to lineaments.
The lineaments were derived by the mapping work done by Fred Snyder, Albert E. Ogden, and C. M. Woodruff. The lineament maps used in the study are available from the City of Austin's Department of Environmental Protection. Special thanks to Fred Snyder for his time and effort that went into the compilation of the lineaments and also for the idea and encouragement to do this study.
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in Abbott, Patrick L. and Woodruff, C.M., Jr., eds., 1986, The Balcones Escarpment, Central Texas: Geological Society of America, p. 131-144
in Abbott, Patrick L. and Woodruff, C.M., Jr., eds., 1986, The Balcones Escarpment, Central Texas: Geological Society of America, p. 131-144