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The Balcones Escarpment :

Pipeline Oil Spills and the Edwards Aquifers, Central Texas, p. 163-183

by Peter R. Rose

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Shallow-marine shelf carbonates of the Lower Cretaceous Edwards Group form two important aquifers in central Texas. Both are fractured, cavernous, and highly transmissive. On the Edwards Plateau, the Edwards Group forms a simple, widespread, unconfined aquifer that is the water source for most streams, upland ranches, and many towns. In the Balcones fault zone, the Edwards forms an unconfined aquifer that is recharged by area streams where they cross the Edwards outcrop, and an adjacent confined aquifer farther downdip where the Edwards is overlain by impervious younger formations. The fault-zone aquifer is the main municipal water supply for San Antonio and many other smaller towns, and also provides much of the agricultural and recreational water for the region.

The Edwards is exceptionally vulnerable to pollution from pipeline oil spills, because its high permeability allows spilled crude oil to sink into the bedrock before cleanup crews have time to recover it. Thereafter, moving ground water and irregular pore distribution make recovery operations ineffective. It is estimated that oil spills larger than 999 barrels have the potential for reaching and contaminating the Edwards aquifers. On the average, 40 to 50 percent of spilled oil is recovered in cleanup operations.

The region is crossed by nearly 4,000 miles of pipelines, including small-bore gathering systems, product pipelines, and large-bore trunk pipelines (which pose the greatest hazard because they tend to incur larger spills). In the counties of the Edwards Plateau, Hill Country, and Balcones fault zone, 33 oil spills larger than 999 barrels have been reported since 1971. The largest was 25,200 barrels. Most of the 33 spills were from 10 trunk pipelines, having 10- to 24-inch diameters.

Two proposed large-bore (30- and 42-inch-diameter), high-pressure pipelines pose new danger of aquifer pollution. Although the average spill from 30-inch pipelines is about 6,000 barrels, reasonable spill scenarios suggest that spills may commonly be much larger, and conventional mechanical safeguards proposed for such "mega-pipelines" may be inadequate to counter the higher pressures and volumes.

Suggested measures to help protect Edwards aquifers from future oil-spill pollution include:

1. Selective retrofitting of older existing pipelines,

2. Closer spacing of block-valves,

3. Electronically linked hydrocarbon sensors,

4. Centralized shutdown systems,

5. Establishment of continuously staffed "Spill-response stations," strategically located near especially vulnerable areas along the pipeline route, and

6. Approval of proposed pipeline routes by the Texas Water Commission, prior to construction.

Additional crude-oil pipeline construction over the Edwards aquifers seems imprudent, and an alternative northern route appears much more sound from an environmental viewpoint.


Figure 1: Regional deposition and structural features influencing deposition of the Edwards Group, in relation to Central Texas geography and surface geology (Rose, 1972).

Figure 2 (445 K): General geologic map of Central Texas.

Figure 3 (440 K): General thickness of Edwards group and associated carbonate formations.

Figure 4 (437 K): General geologic structure on base of Edwards group.

Figure 5 (474 K): Main geologic and hydrologic provinces affecting the Edwards aquifers, Central Texas.

Figure 6: Schematic cross section showing relations of the Edwards Aquifer, Central Texas.

Table 1: Statewide data, Texas--all pipelines.

Figure 7: Cumulative percent distribution of oil pipeline spill (1971-1985).

Figure 8: Frequency of pipeline oil spills (1971-1985) by size of spill.

Figure 9 (368 K): Major trunk crude-oil pipelines, with proposed and alternate routes for All-American pipeline.

Table 2: Summary of liquids-pipelines in counties of Edwards Plateau, Hill Country, and Balcones fault-zone provinces, Texas.

Table 3: Data from Edwards Plateau, Hill Country, and Balcones fault-zone provinces, Texas: all pipelines.

Table 4: Data from six main large-bore crude trunk lines crossing area (excludes counties with possible gathering systems and product-lines: Crockett, Irion, Reagan, Upton, and Tom Green Counties, Texas).

Table 5: Large spills from crude oil pipelines (01-71 thru 05-86) in Edwards Plateau, Hill Country, and Balcones Fault-zone provinces, Texas.

Table 6: Comparison of median and mean spill volumes from smaller (10" and 12") versus larger (18" and 24") trunk pipelines, 22 counties of subject area.

Figure 10: Mean spill size and pipe diameter.

Table 7: Scenarios for antipated oil spills from a 30-inch pipeline, transmitting at 300,000 barrels per day.


Most of the drinking, agricultural, and recreational water supplies of Central Texas come ultimately from highly porous limestone and dolomite aquifers in the Lower Cretaceous Edwards Group. Public awareness of the vulnerability of these aquifers to contamination has been growing in the region for more than 20 years. It reached new heights in late 1985 and early 1986 as a result of the proposal by the All-American Pipeline Company to build a large-bore, high-pressure, heated crude-oil pipeline across about 300 miles of combined Edwards outcrop and watershed of streams recharging Edwards aquifers. In trying to make an objective assessment of potential risks posed by the All-American Pipeline, it has also been necessary to analyze hazards represented by existing crude-oil trunk and products pipelines in the region.

This paper has three purposes:

1. To assess the risk of large oil spills that have the potential to pollute Edwards aquifers;

2. To examine the effects and consequences of aquifer contamination by petroleum liquids from a geological perspective;

3. To recommend measures for reducing potential for pollution of Edwards ground water by oil pipelines.



A thick succession of resistant, flat-lying, porous, Lower Cretaceous limestone and dolomite, known traditionally as "Edwards," covers much of west-central Texas and comprises one of the dominant physiographic elements of the state, the Edwards Plateau (Rose, 1972). Along the northwestern edge of the Gulf Coastal Plain, in the Balcones fault zone, the Edwards is also exposed in fault blocks, where it has been severely altered by ground water. South and east of the Balcones fault zone, the Edwards dips gulfward beneath the coastal plain, reaching depths of 15,000 to 20,000 ft.

Edwards limestone and dolomite were deposited mostly as very shallow-marine carbonate shelf-sediments, on a vast, flat submarine plain called the Comanche shelf (Figure 1). Deeper water lay to the southeast in the ancestral Gulf of Mexico basin. A long, narrow belt of skeletal carbonate sediments, the Stuart City reef (Winter, 1962), marked the gulfward edge of the Comanche shelf. Seaward of the Stuart City reef, water depth apparently increased abruptly, so that open-marine carbonate sediment accumulated in water hundreds of feet deep (Van Siclen, 1958; Winter, 1962). On the Comanche shelf, however, water was generally quite shallow, although there were broad depressions and swells in the interior of the shelf that exerted great influence on thickness and lithology of the Edwards and its counterpart formations. The two dominant depressions were the Maverick basin (Winter, 1962) on the southwest and the north Texas/Tyler basin (Fisher and Rodda, 1967) on the north and northeast. Separating these two depressions was a broad, elongate swell, the Central Texas platform, bearing southeasterly from the vicinity of San Angelo across the Llano Uplift to the Stuart City reef. Most of the carbonate sediments now included in the Edwards Group accumulated in shallow-marine shelf environments on the Central Texas platform.

Figure 2 (base map) is a generalized map showing the distribution of major geologic units that crop out in Central Texas. Figure 3 shows regional thickness patterns of the Edwards Group and its counterpart formations. This map especially illustrates the wedge form of the Edwards, which thickens in a southwesterly direction from about 100 ft near Waco to about 500 ft throughout most of the Edwards Plateau and Balcones fault zone, to more than 1,000 ft in the Maverick basin where Edwards equivalents lose much of their shallow-shelf character.

Following Edwards deposition, the Central Texas platform was submerged and buried by Upper Cretaceous shales and chalks. Gulfward subsidence had already begun, so depth of burial was much greater to the southeast--up to perhaps 5,000 ft. To the northwest, on what is now the Edwards Plateau, and over the perennially positive Llano Uplift, structural contours on the base of the Edwards reflect a broad, domal uplift (Figure 4).

Edwards carbonates on the Edwards Plateau may never have been buried more than perhaps 1,000 to 2,000 ft. Gradual emergence began during early Eocene, and by the time of Balcones faulting (Miocene), it is probable that the top of the resistant Edwards carbonate mass in the Edwards Plateau region was subaerially exposed, and karstification processes were acting on the carbonate terrane. Especially in the Balcones fault zone, such subaerial solutional processes were already enhancing early Edwards porosity, precursors to the fault-bound sluiceway that would develop later into the Edwards underground aquifer of the Balcones fault zone trend (Abbott, 1975; Woodruff and Abbott, 1979). Post-Miocene headward erosion formed the "Hill Country" region, northwest of the Balcones fault zone and southeast of the Edwards Plateau proper, by removal of Edwards divides, leaving a highly dissected terrain of Glen Rose hills and valleys. During the past 10 million years, continued uplift and erosion to the northwest and subsidence beneath the Gulf Coastal Plain to the southeast have produced the present geological configuration of the Edwards Plateau, "Hill Country," Balcones fault zone, and Gulf Coastal Plain (Figure 5).


Edwards Plateau Aquifer

The Edwards Plateau aquifer is a simple, widespread, flat-lying, unconfined aquifer in the lower part of the Edwards Group. The aquifer is present--in fact, is the sole or dominant aquifer--in all, or large parts, of 19 Texas counties (Fig. 2 and Fig. 5; Glasscock, Sterling, Coke, Pecos, and Terrell counties are excluded based on lithologic and hydrologic criteria). The lower part of the Edwards succession is an aquifer because claystones, mudstones, and sandstones of the underlying Glen Rose, Hensell, and Antlers formations of the Trinity Group are, for the most part, aquitards, or at least much less permeable than the honeycombed and cavernous limestones and dolomites of the lower Edwards Group. As a result of this differential permeability, rain water and run-off water percolate downward through the cavernous carbonate sequence and accumulate above the base of the Edwards. Thickness of the saturated zone varies, of course, but water columns in excess of 200 ft thick are common. This ground water then moves laterally, emerging as prolific springs which form the headwaters of streams draining radially from the Edwards Plateau: the Concho, San Saba, North and South Llano, Pedernales, Blanco, Guadalupe, Medina, Sabinal, Frio, East and West Nueces, and Devils rivers (Figs. 2 and 5).

Edwards permeability is very high because of karstic porosity development, such as sinkholes, caverns, horizontal fissures, and honeycombed zones. In addition, the Edwards Plateau is crisscrossed by a network of tectonic fractures and joints (Wermund et al., 1978) that provide effective avenues for both vertical and lateral migration of ground water through the carbonate mass. As a result, water moves downward to the top of the water table very rapidly--within hours or, at most, several days. Many area residents can describe small caves in cliff walls from which issue strong but temporary water-flows a day or so after summer cloudbursts occur miles away on top of the plateau.

Even though sinkholes and solution-enlarged fractures are present throughout the Edwards Plateau area, matrix porosity and permeability increase to the southeast. There are multiple causes for this lateral variation in aquifer quality:

1. Evaporite solution-collapse breccias are concentrated over the interior of the Central Texas platform, where evaporitic-restricted environmental conditions were maximized. Such collapse-breccia zones are particularly prone to porosity enhancement and, thus, to cavernous permeability development.

2. For similar environmental reasons, dolomite is much more abundant in the interior of the Central Texas platform, and dolomite tends to be more porous and permeable than limestone, other factors being equal.

3. Annual rainfall increases in an easterly direction across the Edwards Plateau. There is ample evidence that ground water has acted as a "positive feedback process" that has steadily enhanced porosity and permeability in the Edwards aquifer (Abbott, 1975; Woodruff and Abbott, 1979: Maclay and Small, 1984). In other words, the more water available to move in and through the aquifer, the more enhancement of secondary porosity will occur.

Overall, however, porosity and permeability increase downward in the Edwards section, again reflecting the long-term influence of ground-water saturation, movement, and enlargement of pores and vugs. Honeycomb porosity zones are especially common in the lower part of the Edwards aquifer. Such vuggy networks seem to develop preferentially in burrowed limestones and dolomites, as well as in rudist limestones in which individual rudistids are preferentially dissolved, leaving a vuggy limestone matrix. About 200 ft above the base of the Edwards, a widespread evaporate solution-collapse horizon, the Kirschberg evaporate, also forms a horizontal zone of high transmissibility.

In the northern part of the map area, the Edwards is underlain by siltstones and sandstones of the Antlers and Hensell sandstones of the Trinity Group, with which the Edwards may be in hydrologic continuity, even though such sandstone aquifers generally yield much less prolific flows. However, impervious claystones and mudstones of the Glen Rose Formation intervene wedge-like in the Trinity, going southward and eastward, between the Hensell (or Travis Peak) Sandstone and the overlying Edwards. As a result, these sandstones are no longer in continuity with the Edwards and, in fact, these sandstone aquifers become artesian, because the overlying tight Glen Rose serves as a confining aquitard. Also, aquifer-quality sandstones in the Hensell/Antlers/Travis Peak are commonly erratic in their distribution. For these reasons, the classification by the Texas Department of Water Resources (Walker, 1979), which treats the Edwards and Trinity as one aquifer in the Edwards Plateau region, is clearly a misleading and inappropriate simplification.

Balcones Fault Zone Aquifer

The basic geohydrology of the Edwards aquifer in the Balcones fault zone is, by now, well understood and has been capably described by many authors, especially Arnow (1959), Abbott (1975), Maclay and Small (1984), and Senger and Kreitler (1984). The following brief review merely summarizes the conclusions of many previous workers. Because of the presence of other carbonate formations and facies, such as the thin Georgetown Limestone which overlies the Edwards, and the Devils River, Salmon Peak, McKnight, and West Nueces formations, which are lateral facies equivalents of the Edwards, hydrogeologists have adopted the useful term "Edwards and Associated Limestones" to refer to the hydrogeologic carbonate aquifer unit in the Balcones fault zone province.

In the Balcones fault zone of Central Texas, Edwards and associated limestones are present in a series of linear fault blocks that trend eastward through Kinney, Uvalde, and Medina counties and thence northeastward through Bexar, Comal, Hays, Travis, and Williamson counties. Structure contours on the base of the Edwards (Figure 4) indicate that vertical displacement across the several en echelon faults of the Balcones system totals about 1,000-1,500 ft in the Austin/San Antonio corridor and perhaps half that amount west of San Antonio, in Medina and Uvalde Counties.

Streams draining southward and eastward from their sources in the Edwards Plateau, such as the East and West Nueces, Frio, Sabinal, Medina, Guadalupe, and Blanco rivers cross a belt of Glen Rose outcrop, between the Edwards Plateau to the west and the Balcones fault zone to the east, called here the Hill Country drainage area (Fig. 5 and Fig. 6). This belt represents the area in which headward erosion since the Miocene has stripped resistant Edwards carbonates away, leaving the mudstones and carbonates of the underlying Glen Rose Formation at the surface. Farther east, these streams then flow across outcropping Edwards carbonates in downthrown Balcones fault blocks, and much of their water moves downward, recharging the Edwards aquifer of the fault zone. Therefore, where the Edwards crops out in the Balcones fault zone, the aquifer in the lower part is, for the most part, unconfined, analogous to the plateau aquifer.

Slightly farther eastward and southward, however, the Edwards dips underground, covered by relatively impermeable younger formations such as the Del Rio and Eagle Ford clays. These argillaceous formations serve as top-seals. Where they are present, the subsurface Edwards forms a confined, or artesian, aquifer. At distances of 1 to 20 miles downdip from the Edwards outcrops of the Balcones fault zone lies the "bad water line," a remarkable hydrologic boundary within the Edwards, marking the abrupt transition from fresh water (250-450 mg/l total dissolved solids) to sulphurous salt water (more than 1,000 mg/l total dissolved solids). Fresh ground water moves generally eastward across Kinney, Uvalde, and Medina counties, then northeastward across Bexar, Comal and southern Hays counties within the confined "Edwards underground aquifer," and emerges at San Marcos Springs, the lowest discharge point along the fault zone. About halfway between San Marcos and Austin, in northern Hays County, may be located a vague "ground-water divide," defined by a potentiometric high (Senger and Kreitler, 1984). Water appears to flow southwestward from that divide to San Marcos Springs and northward to Barton Springs in Austin (Slade et al., 1985). Apparently, the narrow belt of the Edwards underground aquifer has long served as a natural subsurface sluiceway, possibly even since the Miocene (Abbott, 1975; Woodruff and Abbott, 1979). Because of this long history and the "positive feedback" effects of ground-water solution on aquifer transmissibility, reservoir rocks within the Edwards underground aquifer are much more porous and permeable, with a markedly more cavernous character, than Edwards rocks downdip of the "bad water line."

Prolific permanent springs are present along the fault-bounded south and east margins of the Edwards outcrop. Discharge from Comal Springs, near New Braunfels, is mostly artesian, derived from the confined Edwards underground aquifer. At San Marcos Springs, as well as at Barton Springs in Austin, discharge is from both the unconfined Edwards of the recharge zone, and from the artesian Edwards of the Edwards underground aquifer.

As pointed out by Abbott (1975), the upper part of the Edwards Group in the Balcones fault zone appears to be more cavernous and permeable than the lower part, possibly because of the presence in the middle of the Edwards succession of a slightly argillaceous, tight limestone layer, the Regional Dense Member, and/or because of Cenozoic enhancement of porosity created by mid-Cretaceous subaerial exposure and karstification in the upper part of the Edwards (Rose, 1972).


There are two Edwards aquifers. They are the most sensitive aquifers in Texas, and they are crucial to the water supplies and character of the Central Texas region.

In the Edwards Plateau, upland ranches throughout this sparsely populated region derive nearly all of their agricultural water from the unconfined Edwards aquifer, commonly via windmill-powered water wells. Towns such as Ozona, Sonora, and Rocksprings also get their municipal water from Edwards water wells; streams fed by Edwards springs provide water supplies to towns such as Junction and Kerrville. Limited irrigation, primarily in cultivated river valleys, depends on water supplied by such streams. The Llano, Pedernales, Guadalupe, Medina, and Devils rivers, which issue from spring-fed streams in the Edwards Plateau and drain eastward and southward across the Hill Country drainage area (Figure 5), are major contributors to area lakes used for storage, irrigation, power, and recreation, such as LBJ, Travis, Austin, Canyon, Medina, and Amisted lakes. Also, such streams are the primary source of fresh water for recharge of the second Edwards aquifer in the Balcones fault zone.

The Edwards underground aquifer provides most of the municipal water supply for the City of San Antonio. In addition, smaller towns such as Uvalde, Hondo, New Braunfels, and San Marcos derive their water from Edwards springs or water wells. Smaller semi-rural residential developments, ranches, and considerable irrigated farming all depend on water from the Edwards underground aquifer. A number of spectacular springs, such as Barton Springs in Austin, represent cherished recreational and scenic locales to the Central Texas community.

The importance and vulnerability to pollution of the Edwards aquifer in the Balcones fault zone led to the creation of the Edwards Underground Water District by the Texas Legislature in 1959. Proceedings are now underway to create a second underground district, in the Austin sector of the Balcones fault zone, north of the previously discussed ground-water divide in Hays County. Special zoning and construction restrictions are in effect for most counties in the fault zone area, reflecting the high public awareness of the fragility of the hydrogeologic system.


Like most events that are caused by a combination of independent variables such as rainfall or mineral deposits, pipeline oil spills appear to fit what is called a "lognormal" distribution (Fox, et al., 1976). The basic attribute of any lognormal distribution is that there will be, in the total population, a very large number of small occurrences, a moderate number of intermediate occurrences, and only a few very large occurrences. A perfectly lognormal distribution will plot as a straight line on a special kind of graph paper, called "lognormal probability paper."

In Texas, all oil spills of five barrels or more are required to be reported by pipeline operators to the Texas Railroad Commission; some operators also report spills of smaller volume. Volume, location, date, recovery, and cause of spills must be reported. Pipelines are separated into three classes: crude-oil trunk lines, gathering lines, and product lines. Total line-miles in each class vary somewhat from year to year, but the average total for the state from 1971 through 1985 was 75,266 miles: crude-oil trunk lines were 25,738 miles; gathering lines were 26,968 miles; and product lines were 22,560 miles. Computerized spill reports do not indicate the class of pipeline from which the spill occurred.

Texas Spill Statistics

From 1971 through 1985, 15,260 pipeline oil spills were reported to the Texas Railroad Commission (Table 1). Curve A, in Figure 7, shows the probability distribution of these spills: 40 percent were larger than 49 barrels in volume, about 28 percent were more than 99 barrels, and nearly 3 percent were 1,000 barrels or more. For the 15-year period, the spill-rate (all spills) was one spill per 78.4 line-miles of pipeline per year (Table 1 and Figure 8, Curve A). For spills of 50 barrels or more, the spill-rate was one spill per 189.1 line-miles per year. Spills of 1000 barrels or more occurred at the rate of one per 2,752.2 line-miles per year. Mean spill size was about 215 barrels and median spill size was about 36 barrels, again reflecting the lognormality of spill distributions. The largest spill reported during the 15-year period was 30,185 barrels, from a Chevron pipeline in Hudspeth County.

Pipeline spills in Texas are occurring with decreasing frequency; for example, 1,403 spills were reported in 1971 versus 751 in 1984. Although all classes of spills decreased, the greatest decrease (nearly 50 % reduction) occurred among spills smaller than 50 barrels, whereas spills larger than 500 barrels showed a reduction of about 35 to 40 percent. This would appear to mean that pipeline operators are preventing smaller spills more successfully than they are preventing larger spills.

The most common reported causes of Texas pipeline oil spills are: corrosion, equipment failure, human error, and "other causes." In the latter two categories, most incidents relate to heavy equipment cutting the pipeline. Although the Texas Railroad Commission data indicate corrosion to be the single largest cause of pipeline oil spills, the U. S. Department of Transportation, Materials Transportation Bureau (1985) indicates that "outside force damage" is the leading cause of pipeline factors, followed by external corrosion and "leaks in system components other than pipe." It appears that considerable latitude exists in accounting for causes of spills. For example, a large oil spill in 1979 by Exxon in Kimble County (25,224 barrels) is shown on Texas Railroad Commission computer reports as caused by corrosion, despite other evidence that the spill was caused by a longitudinal split in the 18-inch pipe.

Typical Sequence of Events in an Oil Spill Incident

Most pipeline oil spills are detected by landowners or other passersby, by heavy equipment operators whose activity caused the pipeline rupture, by company personnel patrolling the pipeline, or by company personnel monitoring pumping stations or pipeline terminals. Some newer pipelines may be fitted with special electronic hydrocarbon-detection sensors to identify leaks. Many experienced pipeliners indicate that spills of smaller than about 0.1 percent of daily shipping volume are difficult to detect volumetrically. Most small spills of less than about 50 barrels and most leaks on older pipelines are detected by manual means. Many small, chronic leaks (so-called "ghost leaks") may be very difficult to locate, once detected.

Large leaks may cause abrupt drops in pressure, which can be detected in pumping stations or terminals. Larger, more modern pipeline systems may have monitoring systems that can indicate the sector of the break, allowing rapid shutdown of the pipeline, closing of block-valves, and preferential evacuation of downstream segments to reduce outflow. Such monitoring systems may also alert clean-up and repair crews promptly and direct them to the general location of the accident.

In the event of a large leak, the entire pipeline system must be shut down, which ordinarily requires a human decision. Pipeline transmission cannot be stopped instantly; block-valves are closed gradually over a period of perhaps 5 to 15 minutes. In addition to the oil that spilled out before shutoff of transmission, a certain amount of oil between the break and the nearest upstream block-valve may leak out of the pipeline. Spacing of such valves varies, but for the newest proposed pipeline in Texas, average spacing between block-valves was about 18 miles. Accordingly, the time between rupture and complete shutdown, the distance between the break and nearest upstream block-valve, the size of the rupture, the terrain-slope, the diameter of the pipe, the current shipping pressure, and the API gravity of the product all affect the total volume spilled.

Once a large oil spill is detected and located, operators generally respond as quickly as possible by shutting down transmission and dispatching repair and cleanup crews to the scene. Some commonly employed equipment and procedures include the following:

1. Bulldozers to build temporary earthen retaining berms,

2. Back-hoes to make ditches and gathering systems,

3. Deployment of plastic-lined catchment systems,

4. Vacuum trucks to collect spilled oil from ditches, catchment basins, and rivers or lakes,

5. Cultivators to till and retill oil-saturated ground so as to let the spilled oil evaporate, volatize, and be consumed by soil bacteria,

6. Booms and other blocking devices to collect crude oil that has spilled into streams or lakes,

Analysis of reported spills and recoveries suggests that, on average, operators recover about 40 to 50 % of oil spilled in pipeline accidents involving 1,000 barrels or more, about 60 % in spills of 100 to 1,000 barrels, and about 40 % of smaller spills. This does not take into account problems in correctly measuring and reporting true sizes of spills, nor does it account for the 10 to 15 % "typical" rate of simple evaporation, which renders suspect many reported recoveries that approach 100 % of the amount claimed to have been spilled. Generally, calculated spill-volumes are probably accurate within about 10 to 15 % plus or minus the actual volume spilled, and there may be room for improvement in reporting accuracy of pipeline spills and recoveries to the Texas Railroad Commission.

Depending on the location and accessibility of the accident site, it may take several hours or longer for cleanup and repair operations to begin. Many weeks of cleanup work may be required to achieve maximum possible correction. Oil spills may lead to serious fires and other human hazards. More common negative effects include killed trees; crop land or pasture land rendered barren for some years; contaminated lakes and streams; infertility, illness, or mortality of livestock and game; fish and waterfowl mortality; and ground water contaminated by hydrocarbons and heavy metals.

In fairness, however, it should be pointed out that most oil spills, especially smaller ones, do not generally cause permanent environmental destruction. Nature does eventually clear herself up, but years and much damage may ensue in the process.

Construction and Retrofitting Countermeasures

Although pipeline companies are clearly motivated to prevent oil spills, their natural corporate interest is in avoidance of crude-oil loss and in the mechanical integrity of the pipeline facility. The issue of environmental damage would appear to be of secondary priority, impacted more by concerns of their corporate public image. For that reason, new pipeline projects may resist the imposition of closer-spaced block- and check-valves; more numerous, continuously maintained, repair/cleanup stations; frequent collection sumps; clay liners; concrete-lined marginal ditches; larger-diameter outer sleeves; and other design measures aimed at minimizing the effects of oil spills once they occur.

A second aspect of limitation of environmental damage during the design/construction phase has to do with pipeline routes. Obviously, there is substantial financial incentive to choose routes offering lowest construction costs, of which distance is probably the most significant. However, some routes offer intrinsic advantages over others vis-a-vis the environmental consequences of pipeline oil spills. In Texas, there is no statutory or regulatory provision applying to location of a proposed pipeline--the pipeline route is entirely in the hands of the pipeline company, which also is endowed with the right of eminent domain. The Texas Railroad Commission grants permits to operate pipelines, not to construct them, and such permits have generally been issued in the past on a pro forma basis. In addition, the Texas Railroad Commission has been assigned the responsibility to safeguard surface and subsurface water supplies from pipeline pollution, even though the Texas Water Commission clearly has the philosophical mandate and technical personnel for such regulation.

Many Texas pipelines are more than 60 years old. Given the cumulative effects of corrosion, we might expect increasing spill frequencies in these older lines. However, Texas spill-statistics would seem to indicate pipeline safety to be improving over time. The question of retrofitting of older pipelines, especially those showing an increased spill-rate, however, is an issue that must eventually be faced. This is especially true in areas of population concentration, environmental sensitivity, and fragile aquifers. Indeed, given the pace of suburban development, expanding population, and the recognition of actual historical spill-rates, the question of retrofitting may be applied even to pipelines that do not have histories of increasing spill-frequencies.


Spilled oil seeps into the ground, obeying forces of gravity and capillarity. The amount of oil that seeps downward into the bedrock depends on many variables, including:

1. The amount of oil spilled,

2. The rapidity of the spill,

3. The viscosity of the spilled oil,

4. The temperature of the ground surface,

5. The amount of vegetative and soil cover,

6. The slope of the terrain,

7. The porosity and permeability of the bedrock,

8. The moisture or water saturation of the bedrock,

9. The response time and effectiveness of cleanup crews.

Obviously, bedrock characterized by a network of very small pores, such as a fine-grained sandstone, will not accept or transmit spilled oil as rapidly as a cavernous, honeycombed limestone. Very large or extensive fractures or bedding fissures also promote rapid transmission of surface fluids downward into the bedrock. Pervasive matrix porosity can absorb a great deal of crude oil. Thin soils and sparse vegetative cover may lead to increased absorption into bedrock. Remote or inaccessible terrain may delay the arrival of cleanup crews until much of the spilled crude oil has flowed downward into the bedrock.

For the reasons outlined above, it is very difficult to determine or predict how large an oil spill must be before it will reach and contaminate a carbonate aquifer. Construction of a series of "reasonable models" led the writer to conclude that any oil spill of 1,000 barrels or more represented a reasonable danger of reaching the water table in the Edwards Plateau area.

All of the above factors make carbonate bedrock more susceptible to oil-spill pollution than sandstone. When oil seeps down through unsaturated bedrock and reaches the ground-water table in an unconfined aquifer, it then spreads out on top of the water surface. The rate of spread is highly sensitive to aquifer permeability, but tends to be rapid and extensive. For example, one barrel of standard 1OW30 motor oil will form a "rainbow" covering about 200 acres of standing water. By analogy, in a highly cavernous carbonate aquifer having about 15 % porosity, one barrel of oil could cover more than two square miles of the water table. Of course, capillary pressure acting on fine matrix porosity would be expected to absorb much of this oil and retard lateral spreading. Free oil entering an unconfined aquifer would be expected to spread and travel in the direction of water flow, emerging eventually at springs and showing up wherever wells sampled the top of the water column. One of the more insidious effects of crude-oil contamination of an aquifer is the dissolution of some hydrocarbon components, such as benzene and heavy metals, which can be toxic in minor concentrations in fresh water. Moreover, the lighter hydrocarbon fractions tend to be more toxic, and they also tend to spread more readily on water.

It is much more difficult to introduce spilled oil into a confined aquifer simply because, by definition, such an artesian aquifer sealed from above is "full" and under pressure, so that the buoyant oil droplets cannot enter the water column. However, soluble crude-oil components can be introduced into confined aquifers.

Several methods have been suggested to recover spilled oil from aquifers, including closely spaced, intensively pumped relief wells surrounding the spill site and the introduction of petroleum-consuming bacteria. Carbonate aquifers in which ground water is moving at rates of a few feet per day through irregular, cavernous fissures do not lend themselves to either approach. Natural biodegradation processes tend to act slowly in fresh-water aquifers. Most knowledgeable hydrogeologists agree that, for all practical purposes, once spilled oil has been introduced into a cavernous carbonate aquifer, only time and nature can take care of the cleanup job.


Spill Data in the Edwards Plateau, Hill Country, and Balcones Fault Zone Provinces

Figure 9 shows the routes of many trunk, petroleum-product, and large gathering-system pipelines in the area of the Edwards aquifers and Hill Country drainage area. In the 27 counties of the subject area, there are 3,867 total line-miles of liquids pipelines (Table 2), consisting of 2,653 line-miles of trunk pipelines having diameters of 6" or larger, 224 line-miles of product pipelines of 4 to 8 inch diameter (mostly in the San Antonio/Austin urban corridor), and approximately 990 line-miles of gathering-system and small-bore pipelines (in some counties where oil production is prolific, gathering-system and small-bore pipeline mileage has been estimated).

This pipeline network experienced a total of 1,373 oil spills in the period from 1971 to 1985 (Table 3). Spills larger than 49 barrels of oil numbered 449, and there were 32 spills of 1,000 barrels or larger. The mean spill was 181.3 barrels, and the median spill was 31.5 barrels (Figure 7, Curve B). The system experienced one spill per 42.2 line-miles per year, considering all spills. For spills Iarger than 49 barrels, the spill-rate was one spill per 129.2 line-miles per year, and for 1,000-barrel (and larger) spills, the rate was one spill per 1,812.7 line-miles per year (Figure 8, Curve B). On both Figures 7 and 8, note the close correspondence between Curve B and Curve A (which represents spill frequency for the entire state), indicating that, overall, pipeline spills in the Edwards Plateau, Hill Country, and Balcones fault zone region followed roughly the same patterns as they did for the State as a whole. However, these data may be misleading for most of the'region because there is a disproportionately high concentration of gathering lines and smaller-bore pipelines in the western counties, due to the abundance of oil fields in that area.

Accordingly, Table 4 was prepared as an attempt to focus exclusively on trunk pipeline oil spills in the subject area, since they probably represent a greater potential hazard. Only spills from the six main crude-oil trunk pipelines that cross the region were analyzed. Measurement of pipeline miles is probably more accurate, and spills can be assigned to the source pipeline without question. Twenty-two counties are included in this analysis, containing 1,225 miles of trunk pipelines. There were 33 spills in the 1971-1985 period: 18 were larger than 49 barrels, and 12 were larger than 999 barrels. The mean spill was 2,741 barrels, and the median was 210 barrels. Curve C, in Figure 7, shows dramatically the departure of the crude-oil trunk pipeline spills from populations that contain all types of pipelines; there is a much larger proportion of 1,000-barrel spills from these generally larger-bore pipelines.

Comparison with Statewide Spill Data

Figure 8 compares graphically the relationships of spill frequency by size/class among the following:

1. Statewide data, including all pipelines (Curve A).

2. Data for the subject area, including all pipelines (Curve B).

3. Data for most of the subject area, for larger-bore trunk pipelines only (Curve C).

The pattern again is clear: crude-oil trunk pipelines in the Edwards Plateau, Hill Country, and Balcones fault zone region incur small leaks much less frequently than do all pipelines. However, they also incur large spills more frequently than do all pipeline populations. The "crossover" occurs in the 500-1,000 barrel-spill range. For the 15 years ending in 1985, crude-oil trunk pipelines in the Edwards Plateau, Hill Country, and Balcones fault zone region incurred large spills (1,000 barrels of oil and larger) at the rate of about one spill per 1,500 miles of pipeline per year.


Based on the above data, we may anticipate that for the entire 27-county area, there will be about two spills per year of 1,000 barrels or larger. Excluding the five highly drilled western counties, the six main trunk pipelines crossing the pollution-vulnerable central and eastern Edwards Plateau, Hill Country, and Balcones fault zone region should experience one 1,000-barrel spill (or larger) about every 15 months.

It is also important to emphasize that when large-bore pipelines break, larger amounts of oil are spilled. Figure 7 shows that the mean spill size for the trunk pipelines was more than 10 times larger (2,741 barrels of oil) than mean spills for the other populations. Figure 8 shows that large spills occur more frequently in trunk pipelines than in other types of pipelines. Table 6 shows that the median and mean spill sizes of the smaller (10" and 12") trunk pipelines are significantly smaller than for the larger (18" and 24") trunk pipelines in the subject area.

It is also noteworthy that causes of pipeline spills for the large-bore trunk pipelines show a different distribution than for all Texas pipeline spills. Table 5 gives details of all 1,000-barrel and larger oil spills in the affected area. Of the 33 total spills, 33 percent were reportedly caused by corrosion and 36 percent by external sources, such as damage by heavy equipment.

Prior and Future Studies

The question of possible oil-spill pollution of Edwards aquifers was first addressed by Fox et al. (1976), who restricted their analysis to the area of the Edwards underground reservoir, the adjacent recharge zone in the Balcones fault zone, and the Hill Country drainage area. Fox et al. utilized a severely limited sample--nine small spills reported from 1970 through 1975 (the largest of which was 200 barrels of oil), and 236.8 total pipeline miles. Unaccountably, they did not include in their data-set two large spills that occurred over the Edwards underground reservoir during the 1970-1976 period: the Exxon pipeline in Medina County suffered a 1,250-barrel spill on June 3, 1971, and a 1,300-barrel spill on July 8, 1973. Fox and his associates correctly noted that the spill-rate for the area of the Balcones fault zone and Hill Country drainage area was much less than the statewide spill-rate, an experience we see duplicated by comparing 15-year spill-rates for the 22-county area versus the statewide spill-rate for the same period. However, because they omitted the two large Medina County spills, they were not able to anticipate that the observed differences applied only to smaller spills having much lower potential for pollution, and that the rate for large spills in the area was similar to, or even larger than, statewide spill-rates.

Utilizing the statewide rates as an alternative, Fox and his co-workers did suggest that there was a high probability of 1,000-barrel and larger spills for time periods of several years or longer in the area of their study. In fact, there have been at least three in the 10 years since their report was published:

1. The 1978 Texas/New Mexico spill of 3,220 barrels of oil in Hays County.

2. The 1977 American Petrofina spill of 3,102 barrels of oil in Medina County.

3. The 1977 Exxon spill of 2,500 barrels of oil in Medina County.

Now that a reliable data base exists with respect to source and location of oil spills in the Edwards Plateau, Hill Country, and Balcones fault zone region, research is underway to document and assess the specific environmental consequences of the 33 large oil spills that have taken place since 1971 (Table 5).


Formal proposals, as well as publicized plans, involved two additional large-bore, high-pressure crude oil pipelines and the Edwards aquifers in 1985 and 1986. The All-American pipeline was planned to cross about 200 miles of the Edwards Plateau aquifer, about 70 miles of the Hill Country drainage area, and, between Austin and San Marcos, about 15 miles of the recharge and confined zones of the Edwards fault zone aquifer. Considerable public concern was expressed (as well as a flurry of lawsuits), focused primarily on protection of the Edwards aquifers from oil-spill contamination. Construction on the second pipeline, the Pacific-Texas (or "Pac-Tex") pipeline, has not commenced.

The All-American pipeline is a 30"-diameter, heated crude-oil pipeline which, in August, 1986, was already essentially completed from the California coast near Santa Barbara, across Arizona, New Mexico, and trans-Pecos Texas to McCamey, in Upton County. The final 450-mile segment, across Texas (Figure 9) to refineries south of Houston, would complete the first major U.S. west-to-east oil pipeline, connecting California-producing facilities to the rest of the U.S. refining/transportation network. All-American Pipeline Company is an affiliated company of Celeron Corporation, a subsidiary of Goodyear Tire and Rubber Company (All-American is presently being absorbed by Celeron). All-American spokesmen indicated that the pipeline was planned to carry low-gravity (i.e., viscous) high-sulphur, California heavy crude oil in the 15-20' API gravity range, hence the need for heating and insulation to facilitate transmission. However, they also acknowledged that the pipeline might carry Alaskan North Slope crude oil, which is approximately 300 API gravity. Considering the proportional magnitude of North Slope versus California crude-oil reserves (about 7 billion barrels versus perhaps 2 billion barrels), it seems highly likely that Alaskan North Slope crude oil may well constitute a large proportion of future All-American pipeline shipments. Average transmission rates are projected to be about 300,000 barrels per day, with a pressure of approximately 1,000 lbs./in.2, and maximum rates would be about 450,000 barrels per day. Preliminary plans called for three pumping/heating stations within the subject area, at McCamey, near Sonora in Sutton County, and near Harper in Gillespie County. Average spacing between block-valves is assumed to be 18 miles, based upon the Santa Barbara/McCamey segment (California State Lands Commission, 1984).

Construction across Texas was halted in April 1986, pending completion of a Supplemental Environmental Impact Statement, in which two alternative routes were also to be considered (Figure 9): one generally north of the Edwards Plateau, the Llano Uplift, and the northernmost extent of the Edwards underground aquifer; and a second southern route across the western Edwards Plateau, crossing the Balcones fault zone just west of the westernmost extent of the Edwards underground aquifer and then turning east. Final decision on the pipeline route is scheduled for May, 1987.

The second proposed pipeline, the Pac-Tex, is a 42"-diameter heated transmission line, also designed to ship heavy crude, as well as lighter-gravity oil. Construction on this line has not yet commenced, but the proposed route is similar to the All-American pipeline. Company announcements since the All-American controversy indicate the Pac-Tex pipeline is now planned to terminate near McCamey, just as All-American first indicated, before deciding shortly thereafter to extend their pipeline on to the Houston area.

Projected Spill Volumes and Frequencies

As previously pointed out, oil spills from large pipelines tend to be larger than spills from small pipelines. Mastandrea (1982) presented a graph (Fig. 10) showing the relationship between pipeline diameter and mean spill size. Mean spill sizes for 10- to 12-inch and 18- to 24-inch Central Texas pipelines are plotted on Mastandrea's diagram to indicate their general consistency with his results. Based on these data, it appears that mean spill size expected from a 30-inch pipeline is about 6,000 barrels of crude oil and from a 42-inch pipeline about 9,000 barrels.

An independent method by which spill volumes can be estimated is to construct a number of "reasonable scenarios," utilizing a responsible range of values for factors that control spill volumes:

1. Time between rupture and shutdown: 5, 10, 20 minutes (assume half of elapsed time is open flow; half is steadily reducing flow).

2. Shipping rates: 300,000 and 450,000 barrels per day (equals 208 and 312 barrels per minute).

3. Percent of contained oil escaping: 10, 50, 90 percent.

4. Distance between rupture and nearest upstream block-valve: 1, 9, 18 miles.

This approach enables one to gain some perspective as to possible spill-volumes under differing conditions, as well as the relative impact of different variables. It turns out that shutdown time and shipping rate are relatively insignificant, in comparison with percent of contained oil that escapes and distance from upstream block-valves, as factors influencing volumes of oil spilled in pipeline accidents.

Table 7 applies to discrete pipeline breaks rather than chronic small leaks. It indicates that small, quickly controlled breaks would spill about 500 barrels of crude oil; intermediate spills might be expected to spill about 20,000 barrels; and very large ruptures, located many miles from block-valves, could spill more than 75,000 barrels.

The cross-sectional area of a 42-inch pipeline is nearly two times (1.92) that of a 30-inch pipeline and more than five times that of an 18-inch pipeline. Therefore, to gain a rough idea as to projected losses from a 42-inch pipeline, one can simply double the minimum, "most likely," and maximum scenario-losses exercise in Table 7. This would indicate that the loss from an "intermediate" 42-inch pipeline spill would probably be a little over 43,000 barrels of crude oil. For perspective, it may be pointed out that, since 1971, the largest recorded pipeline oil spill in Texas, 30,185 barrels, was from a 20-inch pipeline and the second largest- spill, 25,224 barrels, was from an 18-inch pipeline.

Based on the above examples, scenarios, and analogies, it appears that oil spills of a thousand barrels or larger may be expected as the rule, rather than the exception, from a 30-inch pipeline, and that it is not unrealistic to anticipate some spills of 20,000 barrels or more. Catastrophic accidents might spill in the neighborhood of 75,000 barrels or more if not controlled promptly.

If each of the proposed pipelines has the same spill-rate as the six large-bore pipelines reviewed in Table 4, we should expect roughly one spill per year per 556.8 pipeline miles, or about once every two years in its route across the Edwards Plateau, Hill County, and Balcones fault-zone region. Since the median spill from 18- to 24-inch pipelines is 1,100 barrels (Table 6), the expected median spill from each of these proposed, very large pipelines would be larger, probably in excess of 2,000 barrels. Accordingly, a spill of more than 2,000 barrels or larger would be expected to occur about once every four years from each pipeline. It should be reemphasized that 1,000 barrels is considered by the writer as the lower threshold of spill size that may have the potential to pollute the Edwards aquifers.

When such spill sizes and frequencies are viewed over perhaps a 50-year pipeline-life, it appears that the occurrence of one or more large oil spills polluting the Edwards aquifers is a probable, rather than improbable, event, whose environmental consequences could be severe and long-lasting.


A number of design measures have already been suggested herein, which may serve to limit the quantity of oil spilled in a pipeline rupture. In particular, closer spaced block-valves seem to be an effective improvement. A second improvement concerns the emplacement of a continuous, electronically linked series of hydrocarbon sensors in the pipeline trench, which should greatly assist in locating chronic, or "ghost," leaks. In addition, the construction of barriers and berms to guide spilled crude oil away from especially permeable or sensitive areas along the pipeline route should be considered.

Although clay-lined trenches and large, concrete collection sumps are desirable safety features that would help in containing small, "ghost" leaks, they do not constitute effective countermeasures against large volumes of heated oil emerging at high pressures. Also, attempts to seal off fractured areas of Edwards outcrop are not only impractical, but would also hinder the natural ground-water recharge process.

Because of the very large volumes of crude oil capable of being spilled from "mega-pipelines," it would seem essential to have continuously maintained and equipped "spill-response stations," staffed by trained personnel capable of being on the scene of pipeline breaks within about two hours of spill occurrence. In the case of the All-American pipeline, prudence would suggest the location of such a "spill station" in the Austin/San Marcos sector, because of its vulnerability and high population density, with another "spill station" located in the Sonora area.

However, all of the above remedies assume that the All-American pipeline will ultimately be built along the originally proposed central route, directly across the Edwards Plateau, Hill Country, and Balcones fault zone. The two alternative routes may offer more appropriate solutions to the problem (Figure 9). In particular, the northern route appears to be much more attractive, for the following reasons:

1. It avoids most of the Edwards Plateau aquifer, especially the most porous and permeable areas.

2. It avoids all of the especially sensitive Edwards aquifer in the Balcones fault zone in the Austin/San Marcos urban corridor.

3. Along the northern route, from the vicinity of San Angelo to central Mills County, there are no significant or widespread aquifers, and population density is low. In addition, dominantly clayey surface formations there would significantly restrict percolation of spilled crude oil into the subsurface.

4. From central Mills County across Coryell County, the pipeline would cross the Trinity Sand aquifer. This aquifer is much less susceptible to oil pollution than the Edwards aquifers, by virture of its lower permeability and greater amenability to spill-recovery methods. Also, fewer people rely on the Trinity as a water source.


1. There are two "Edwards aquifers" in Texas, one in the Edwards Plateau and the second in the Balcones fault zone area. Both are fractured, cavernous aquifers developed in carbonate rocks that were deposited in very shallow-marine shelf environments. More than a million people in Central Texas depend on these aquifers for drinking, agricultural, and recreational water.

2. Because of their exceptional permeability and the general lack of soil or vegetative cover, the Edwards aquifers are unusually vulnerable to pollution from oil spills. The basic problem is that spilled crude oil flows downward into the bedrock before cleanup crews have time to recover it. Thereafter, such crude oil can move downward to the top of the water table, where it cannot be recovered by mechanical or chemical means.

3. It is estimated that any oil spill of 1,000 barrels or larger probably has a reasonable possibility of reaching the water table in either unconfined Edwards aquifer. Spills of 5,000 barrels or more can probably be expected to contaminate Edwards ground water to some degree.

4. Analysis of more than 15,000 spills from Texas oil pipelines, since 1971, provides a reliable basis for assessing frequency and volume of pipeline oil spills. About 3 % are spills of 1,000 barrels or larger. The most common cause of large spills is outside-force damage, not corrosion. Human errors and failure of mechanical components are also common causes.

5. In large pipeline accidents, Texas operators generally recover about 40 to 50 % of the crude oil that was spilled.

6. Expectable environmental damage from crude oil spills includes killed trees; barren agricultural land; contaminated lakes and streams; infertility, illness, or mortality of livestock and wildlife; and contamination of ground water by soluble and insoluble hydrocarbons and heavy metals. Many of these components are highly toxic. Duration of contamination of fresh-water aquifers is expected to be on the scale of years rather than days.

7. Routes for any newly proposed large-bore trunk pipeline should require approval by the Texas Water Commission, in order to protect sensitive aquifers and surface streams.

8. In the future, selective retrofitting of six existing trunk pipelines in the Edwards Plateau, Hill Country, and Balcones fault zone region is likely to become more desirable in light of the following:

a. Advancing age,

b. Increasing population, and

c. Recognition of especially vulnerable zones of the Edwards aquifers.

9. Within the Edwards Plateau, Hill Country, and Balcones fault zone region (excluding the five western oil-producing counties of Upton, Reagan, Crockett, Irion, and Tom Green, where spills will be more frequent), we may anticipate an oil spill from one of these six trunk pipelines about every five to six months; a spill of 1,000 barrels or larger can be expected to occur about every 15 months.

10. Previous studies (Fox et al., 1976) correctly perceived the lower frequency of all oil spills affecting the Edwards aquifer in the Balcones fault zone and Hill Country drainage area relative to statewide spill-rates, but did not detect that the frequency of large spills was higher than the statewide average, not lower.

11. Based on historical pipeline performance, crude oil spills from a 30-inch pipeline constructed across the Edwards Plateau, Hill Country, and Balcones fault zone can be expected to occur about every two years. About half will probably be greater than 2,000 barrels, and the average spill will be about 6,000 barrels, which is considered to be a polluting spill. Several large oil spills in excess of 20,000 barrels may be expected, and the size of a major oil spill could reach 75,000 barrels or more.

12. Suggested design modifications to reduce spill volumes include: closer spaced block-valves; centralized shutdown systems for older lines; electronically linked hydrocarbon sensors; and, the construction of barriers to guide spilled crude oil away from sensitive areas. Other measures may be useful in controlling small chronic leaks, but will be ineffective against the heat and pressure of major spills.

13. One or more continuously staffed and equipped "spill-response stations," located near especially vulnerable areas of the pipeline route, can significantly increase the amounts of oil recovered from spills.

14. The suggested alternate northern route appears to represent significantly less hazard to ground water than does the central route across the Edwards Plateau, Hill Country, and Balcones fault zone, proposed originally by the All-American Pipeline Company.


The writer acknowledges the support of the Travis County, Texas, Attorney's Office and the Environmental Division of the Texas Attorney General's Office. Special appreciation goes to Ms. Barbara Wiley, who drafted the illustrations, and to Ms. Karen L. Rose and Ms. Robin I. Heap for their care in typing and editing the manuscript.


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in Abbott, Patrick L. and Woodruff, C. M., Jr., ed,., 1986, The Balcones Escarpment, Central Texas: Geological Society of America, p. 163-183