Groundwater Technical Report for the Central Texas Regional Mobility Authority 183 North Mobility Project

Home , Central Texas, Edwards Group, Edwards Plateau, Georgetown Formation, Ranch to Market Road 620

CSJ # 0151-05-100 and 3136-01-185

Prepared for CP&Y, Inc.

Prepared by Cambrian Environmental with

SWCA Environmental Consultants

July 2, 2015 SWCA Project Number 25572-AUS

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Groundwater Technical Report for the Central Texas Regional Mobility Authority 183 North Mobility Project

CAMBRIAN ENVIRONMENTAL 4422 Pack Saddle Pass No. 204 Austin, Texas 78745 with SWCA ENVIRONMENTAL CONSULTANTS 4407 Monterey Oaks Blvd No. 110 Austin, Texas 78749

Submitted to: CP&Y, Inc. The Chase Bank Building Tower of the Hills 13809 Research Blvd, No. 300 Austin, Texas 78750 Attn: Andy Atlas July 2, 2015 Geological analysis and interpretations conducted by and under the direct supervision of Kemble White Ph.D., P.G., Texas Professional Geoscientist license number 3863. As a licensed professional geoscientist I attest that the contents of this report are complete and accurate to the best of my knowledge.

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EXECUTIVE SUMMARY

The proposed 183 North Mobility Project (Project) is a cooperative effort by the Central Texas Regional Mobility Authority (CTRMA) and the Texas Department of Transportation (TxDOT) to improve US 183 from State Highway (SH) 45/Ranch-to-Market Road (RM) 620 to Loop 1 (MoPac) (CSJ 0151-05-100 and 3136-01-185).Proposed Project activities involve a total of approximately 724.6 acres; 714.2 acres of existing right-of-way (ROW), 8.0 acres of proposed ROW, and 2.4 acres of existing easements, as shown in Figure 1 (Project Area).

Construction activities are expected to include the reconfiguration of existing road surfaces and the addition of new lanes. This work will involve reconfiguration of some existing structures; surface grading (generally to a depth of three to four feet below existing grade); excavation for piers to support bridges, overpasses, or flyovers (generally to depths of between 10 and 45 feet); construction of new road surfaces and ancillary structures; the expansion or improvement of existing water quality controls; and the addition of new water quality controls, as needed.

Groundwater resources in the Project Area primarily include the Trinity and Edwards Aquifers. The Trinity Aquifer is an important source of groundwater for public use both in the Project Area and the region, but the Edwards Aquifer is the primary focus of conservation concerns due to its ecological significance and vulnerability to contamination. The Edwards Aquifer is not a sufficiently productive water source within the Project Area to satisfy current demand for human consumption, but it is an important source of groundwater for ecological purposes. The Edwards Aquifer supplies numerous low- flow springs that provide habitat for rare wildlife species including the Jollyville Plateau Salamander (Eurycea tonkawae , JPS) that was listed as threatened under the Federal Endangered Species Act on September 19, 2013 (USFWS 2013). The northern two-thirds of the Project Area generally follows the drainage divide between the Colorado and Brazos River basins. This divide occurs on a broad upland ridge known as the Jollyville Plateau (see Figure 1). The Jollyville Plateau is the primary recharge area for numerous known locations for the JPS in both drainage basins. Spring and aquifer hydrology differ somewhat between these basins due to variations in stratigraphy and terrain. Stream piracy is occurring along the plateau as the more steeply incised Colorado River captures both surface water and groundwater from the upper headwaters of the Brazos River Basin in Brushy Creek. This study was conducted in order to determine the potential for the Project to affect groundwater and associated resources.

The results of groundwater modeling conducted for this study indicate that the Project can be divided into three sections based on the likely direction of groundwater flow and the potential for storm water to reach habitat for the JPS. The first section extends from the northern Project limit to approximately the intersection with McNeil Road. Within this area, the potentiometric surface indicates that karst stream piracy is occurring. Although the surface drainage divide occurs west of this section of the project, well data and spring elevations suggest that groundwater is moving to the southwest, feeding springs and seeps within the Bull Creek Basin (see Figure 2). Known JPS locations down-gradient from this section include those within Critical Habitat Unit 22 (see Figure 1). The second section extends from approximately McNeil Road to approximately the intersection with Loop 360. Along this section of the project it appears that both the surface and subsurface drainage divides occur near or beneath the Project Area. Within this area, groundwater may pass from the Project Area either southwest into Bull Creek or northeast into the Lake Creek or Walnut Creek basins (see Figure 2). Known JPS locations down-gradient from this section include those within Critical Habitat Units 24, 25, 26, 27, and 32 (see Figure 1). The third section extends from approximately the Loop 360 intersection to the southern terminus of the Project. In this area the Project Area is clearly down-gradient of any known JPS locations or habitats. Groundwater in this area may enter the confined zone of the northern Edwards Aquifer.

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TABLE OF CONTENTS

Executive Summary ...... ES-4 Introduction ...... 1 Geology of the Project Area ...... 2 Stratigraphy ...... 7 Trinity Group ...... 7 Fredericksburg Group ...... 7 The Washita, Eagle Ford, and Austin Groups ...... 7 Relationship Between Geomorphology and Hydrogeology ...... 8 The Northern Segment of the Edwards Aquifer ...... 9 Hydrological Modeling ...... 14 Results ...... 18 Literature Cited ...... 21

Appendix A – Potentiometric Surface Points

FIGURES

Figure 1. Project location...... 3 Figure 2. Project location with reference to watersheds...... 4 Figure 3. Generalized stratigraphy of the aquifer-bearing rocks exposed in the Project Area (From Black and Veatch and Stephens and Associates 2010)...... 5 Figure 4. Regional stratigraphy of northern Travis and southern Williamson Counties...... 5 Figure 5. Geologic map of the Project Area using Texas Commission on Environmental Quality boundaries for the Edwards Aquifer...... 6 Figure 6. Hydrogeology of the water-table zone of the Northern Segment of the Edwards Aquifer. (From Snyder 1985)...... 9 Figure 7. Hydrogeology of the artesian zone of the Edwards Aquifer (Snyder 1985)...... 10 Figure 8. Water levels in the Edwards Limestone in the spring of 1978 indicating groundwater flow to the northeast (Modified from Figure 21, Brune and Duffin 1983)...... 12 Figure 9. Hydrogeology of Jollyville Plateau salamander habitat in Upper Bull Creek...... 14 Figure 10. Spring-fed section of Davis Spring Branch, south of the old Ranch to Market Road 620 prior to the construction of SH 45. This was an expression of the potentiometric surface approximately 2.7 miles east of the Project Area...... 15

Figure 11. Spring-fed section of Bull Creek Tributary 6 within Critical Habitat Unit 16 below the Edwards outcrop approximately 2 miles southwest of the Project Area...... 16 Figure 12. Davis Spring occurs in an impounded area on Davis Spring Branch approximately 2 miles northeast of the Project Area...... 17 Figure 13. Riviera Spring discharges from the Edwards Aquifer into the headwaters of Brushy Creek approximately 0.5 mile northwest of the Project Area ...... 17 Figure 14. Potentiometric surface model for a portion of the northern Edwards Aquifer...... 19 Figure 15. Conceptual model of groundwater flow in the northern hydrological segment of the Project Area between RM 620 and Mc Neil Road...... 20

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INTRODUCTION

Proposed Project activities involve a total of approximately 724.6 acres; 714.2 acres of existing right-of- way (ROW), 8.0 acres of proposed ROW, and 2.4 acres of existing easements, as shown in Figure 1 (Project Area).

While a Water Pollution Abatement Plan (WPAP) has not yet been created for this project, the use of best management practices (BMPs) in accordance with the TCEQ Edwards Aquifer Protection Program (EAPP) and associated Edwards Rules are fully anticipated for the entire Project Area. The EAPP is based on state regulations (Texas Administrative Code Chapter 213) stipulating water quality protection for storm water entering the Edwards Aquifer. According to Section 213.1, paragraph 2 of the Texas Administrative Code, Title 30, Part 1, the Edwards Rules and EAPP have been determined to be a non- degradation regulation; therefore, the construction of temporary and permanent BMPs in accordance with an approved WPAP serve to remove sediments and roadway pollutants arising from normal roadway usage and from accidental spills. All stormwater discharges arising from activities within the Project Area are subject to removal of 80% of Total Suspended Solids (TSS) prior to being allowed back into local surface or subsurface aquatic systems. Reasonable and prudent impact avoidance and minimization measures described above (WPAP and BMP requirements) to be implemented as a component of this Project and would greatly reduce the likelihood of negative impacts on any ecosystem directly adjacent to the Project Area.

The Project Area occurs above two regionally important groundwater resources including the Trinity Aquifer and Northern Segment of the Edwards Aquifer. The Trinity Aquifer is an important source of groundwater for public use both in the Project Area and in the region. The middle and lower segments of the Trinity Aquifer are entirely confined in the Project Area and occur more than 300 feet below the surface within the Glen Rose Limestone and associated formations of the Trinity Group. The Trinity Aquifer supplies moderate amounts of water to individual users across much of Travis and Williamson Counties and are recharged mainly through rainfall and stream flow across their outcrop areas in western Travis and Williamson Counties. Locally, Lake Travis also moderates water levels in the Trinity Aquifer (Brune and Diffin 1983). Concerns about the Trinity Aquifer focus mainly on water quantity and water quality issues, which are exacerbated when aquifer storage is low. The Trinity Aquifer is less transmissive than the Edwards Aquifer and recharges more slowly since it is further removed from surface interactions.

This groundwater technical report summarizes the hydrogeological setting of the Project Area (see Figure 1 and 2). This report focuses on the Northern Segment of the Edwards Aquifer because it is exposed to the surface in the Project Area and it is well known for dynamic interactions between surface water and groundwater. The Edwards Aquifer is also the primary focus of conservation concerns due to its ecological significance and vulnerability to contamination as a karst aquifer. “Karst” refers to the modification of bedrock by chemical dissolution resulting in a landscape characterized by caves,

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sinkholes, and springs. The Northern Segment of the Edwards Aquifer is a discrete aquifer entirely isolated from the other better-known segments of the Edwards Aquifer, such as the Barton Springs and San Antonio Segments of the Edwards Aquifer, and the Edwards Plateau Aquifer.

GEOLOGY OF THE PROJECT AREA

The geology of the Project Area consists of a thin veneer of calcareous soil overlying Lower Cretaceous carbonate rocks of the Trinity and Fredericksburg groups and Upper Cretaceous rocks of the Washita, Eagle Ford, and Austin groups (Brune and Diffin 1986, Collins 2005, Garner and Young et al. 1976, Housh 2007, Rodda 1970). The upper Glen Rose Formation is the only Trinity-group representative exposed in the Project Area. The upper Glen Rose Formation is exposed at the surface along channel bottoms in the Bull Creek and Lake Creek basins. Fredericksburg Group strata exposed in the Project Area include the Walnut Formation, the Comanche Peak Formation, and the Edwards Formation. Figure 3 presents the stratigraphic columns for units exposed in the Project Area. Figure 4 presents the stratigraphic columns for units exposed in the area of northern Travis and southern Williamson counties. Figure 5 is a geologic map of the Project Area.

The Project Area north of the MoPac and US 183 interchange is located entirely within the recharge zone of the Northern Segment of the Edwards Aquifer (Senger et al. 1990). The water-bearing limestone that composes the Edwards Aquifer is Cretaceous in age and consists of the Comanche Peak, Edwards, and Georgetown strata; however, the Edwards Limestone contains most of the aquifer. The Northern Segment of the Edwards Aquifer has an outcrop area covering approximately 400 square miles (Slade 1985). The Project Area traverses an upland area that forms the southern terminus of the Northern Segment of the Edwards Aquifer known as the Jollyville Plateau.

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Figure 1. Project Area Location Map

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Figure 2. Project Location with Reference to Watersheds.

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Figure 3. Generalized Stratigraphy of the Aquifer-bearing Rocks Exposed in the Project Area (from Black and Veatch and Stephens and Associates 2010).

Figure 4. Regional Stratigraphy of Northern Travis and Southern Williamson Counties.

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Figure 5. Geologic Map of the Project Area Using Texas Commission on Environmental Quality Boundaries for the Edwards Aquifer.

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STRATIGRAPHY

Trinity Group

The upper Glen Rose Limestone is composed of up to 700 feet of alternating layers of marl with limestone and dolomitic limestone. When eroded these alternating layers form the stair-step topography that is characteristic of the hill country west of Austin. Black and Veatch and Stephens and Associates (2010) found the Glen Rose to have a low hydraulic conductivity in the Bull Creek Basin.

Fredericksburg Group

The Walnut Formation is composed of approximately 100 feet of alternating layers of marl and nodular limestone in the Project Area (Black and Veatch and Stephens and Associates 2010). Moore (1964) subdivided the Walnut into five members including (from oldest to youngest) the Bull Creek, Bee Cave, Cedar Park, Keys Valley Marl, and Upper Marl members. Only the Cedar Park, Bee Cave, and Bull Creek members were found to be present in the Bull Creek Basin (Black and Veatch and Stephens and Associates 2010). In the Buttercup Creek area located approximately 2.5 miles northwest of the Project Area, the Walnut is known to be highly cavernous (Russell 1993). Absent of karst modification, the Walnut is less permeable than the Edwards and typically yields little to no water to wells.

The Comanche Peak Formation generally consists of 40 to 50 feet of white, irregularly bedded, nodular limestone inter-bedded with marl. To the east of the Project Area (and likely within it) the Comanche Peak is underlain by the Walnut Formation and overlain by the Edwards Formation. However, in multiple locations within the Bull Creek Basin it is known to inter-finger with the Edwards Formation. For this reason some observers (Snyder 1985) have chosen to include it within the Edwards Formation.

The Edwards Limestone consists of chert-rich and rudistid-bearing limestone, dolomitic limestone, grainstone, and mudstone. It is up to 200 feet thick to the south of the Project Area along the Bull Creek Basin where the Comanche Peak is interbedded but thins rapidly to the north to approximately 130 feet thick in the Brushy Creek Basin (Moore 1964). Solution-collapse features are common in the Edwards.

The Georgetown Limestone is not exposed in the Project Area but consists of approximately 40 to 60 feet of relatively soft nodular limestone with abundant oyster fossils. It overlies the Edwards and is the last of the Lower Cretaceous age rocks in the sequence.

The Washita, Eagle Ford, and Austin Groups

The upper confining units of the Edwards Aquifer are only exposed within the southern end of the Project Area where MoPac crosses the Mount Bonnell fault at the base of the escarpment. The upper units consist of low-permeability clays, shales and chalks of Upper Cretaceous age.

The Del Rio clay overlies the Georgetown Formation and consists of approximately 55 to 75 feet of dark olive to bluish-gray to yellow clay which weathers to heavy clay soil. It can be recognized in outcrop due to a distinct change in vegetation to species such as mesquite that prefer clay soil. This formation is exposed in the Project Area only near the intersection of MoPac and FM 2222.

The Buda Limestone consists of approximately 35 feet of thin-to-massively bedded tan crystalline limestone. This formation is not exposed in the Project Area.

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The Eagle Ford Shale consists of approximately 40 feet of petroliferous shale. Exposures of the Eagle Ford Shale are generally poor because it weathers rapidly. This formation is exposed in the Project Area only east of MoPac, north of the intersection of MoPac and FM 2222.

The Austin Chalk consists of approximately 430 feet of white chalk and limestone which has been subdivided in multiple ways according to texture and fossil assemblage. This formation crops out nearly continuously along the east side of the MoPac portion of the Project Area where it is juxtaposed against the Edwards along the Mount Bonnell Fault.

RELATIONSHIP BETWEEN GEOMORPHOLOGY AND HYDROGEOLOGY

The Project Area traverses the Balcones Escarpment, a fault-controlled topographic break between rocky upland ranchlands to the north and west and blackland prairie farmland to the east and south. The Balcones Fault Zone (BFZ) defines the escarpment and consists of a series of northeast-trending, predominantly normal, nearly vertical, en echelon (stair-step sequence) faults. During the middle Tertiary, structural down-warping occurred to the southeast associated with the formation of the ancestral Gulf of Mexico. The Earth’s crust was stretched in response and the BFZ formed along a zone of weakness that today marks the boundary between the Edwards Plateau and the Gulf Coastal Plain throughout Central Texas. Movement along the Balcones Fault system generally ceased more than 10 million years ago.

Recharge into the Edwards Aquifer primarily occurs in areas where the Edwards Group is exposed at the surface. Most groundwater recharge is from direct infiltration via precipitation incident on upland areas and streamflow loss. Recharge to the aquifer, as well as the habitat for a variety of threatened or endangered species, occurs predominantly along secondary porosity features such as caves, solution cavities, and sinkholes. These features commonly develop along faults, fractures, and karst features associated with the BFZ. Development of recharge pathways and groundwater transmission routes within the Edwards Aquifer are highly influenced by numerous unmapped faults and bedrock fractures produced during the faulting. The portion of the Project Area along MoPac expressway follows the base of the escarpment where the Mount Bonnell Fault occurs. The Mount Bonnell Fault is the master fault in the BFZ system.

Along the escarpment, the BFZ separates the water table aquifer to the west from the confined aquifer to the east. The Project Area crosses this boundary as it approaches the MoPac corridor which generally follows the base of the escarpment. This boundary divides the recharge zone from the transition zone. The recharge zone contains an unconfined water-table aquifer where the Edwards and Georgetown limestones are exposed at the surface and absorb rainfall and surface runoff through pore spaces and larger karst- modified, fracture-controlled recharge features such as caves and sinkholes. Because of this, dynamic surface water and groundwater in this area are in close communication. The Texas Commission on Environmental Quality implements the Edwards Aquifer Protection Program which regulates development activities throughout much of this area and requires the protection of recharge features. The recharge zone is also characterized by many low-flow seeps and springs that discharge primarily from the base of the Edwards Aquifer where it overlies the less-permeable Comanche Peak Formation (Collins 2005; Woodruff 1985). Figure 6 presents an interpretation of spring hydrology in the recharge zone.

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Figure 6. Hydrogeology of the Water-table Zone of the Northern Segment of the Edwards Aquifer (Snyder 1985).

The Northern Segment of the Edwards Aquifer starts near the Colorado River and thins northward through Williamson County until it ultimately pinches out in Bell County near Salado. Woodruff (1985) described the differences between the Barton Springs and Northern Segments of the Edwards Aquifer in terms of their positions relative to the Mount Bonnell Fault and the Balcones Escarpment. The Barton Springs segment is exposed on the downthrown, coast-ward side of that fault at the base of the escarpment and is recharged by streams draining the Edwards Plateau. This is a relatively young, fault- controlled karst landscape with integrated flow paths leading primarily to Barton Springs. The Northern Segment, by contrast, is an erosional remnant of a formerly continuous sheet of Edwards Limestone that once stretched at least as far west as the Stockton Plateau of West Texas.

The Northern Segment of the Edwards Aquifer is an erosional outlier of today’s Edwards Plateau, consisting of a nearly flat upland along the drainage divide between the Colorado and Brazos river basins where the Edwards Limestone forms a resistant caprock even while being sapped from within by karst processes (Woodruff 1985). The Colorado River has maintained a higher rate of down-cutting over geologic time than has the Brazos River. As a result, the southern edge of the Northern Segment of the aquifer has been more steeply incised by erosion, and steeper slopes are present along the Colorado River tributaries, such as Bull Creek and Cypress Creek. By contrast, the tributaries of Brushy Creek (a tributary of the Brazos River) form low-gradient drainages that cross the gently rolling surface of the plateau. The Northern Segment of the Edwards Aquifer, especially along its southern terminus known as the Jollyville Plateau, has been dissected into a series of relatively discrete water-table aquifer segments each recharging and discharging with relative independence from one another. Woodruff (1985) also identified protection of discrete recharge features and of drainage ways as being vital to water quality protection in the Northern Segment.

The Northern Segment of the Edwards Aquifer

The Edwards Limestone is a lower Cretaceous carbonate deposit that once stretched in a continuous sheet from the upper Gulf Coastal Plain to the trans-Pecos area of West Texas (Woodruff 1985). Tectonic events have flexed and broken the Edwards into discrete segments and stream incision and erosion have

9 GROUNDWATER TECHNICAL REPORT for the Central Texas Regional Mobility Authority 183 North Mobility Project removed intervening sections of its outcrop. Differences in relief, faulting intensity, and aquifer thickness have resulted in multiple discrete aquifer segments with different hydrologic properties. Of the four major Edwards Aquifer segments, the Northern Segment has been the focus of the least amount of formal study.

The Northern Segment is a hydrologically distinct groundwater system with a unique combination of the karstic characteristics common to its corollaries: the Barton Springs and San Antonio Segments of the Edwards Aquifer and Edwards Plateau Aquifer. It shares the fault-influenced rapid flow paths of the Barton Springs and San Antonio Segments (which also occur within the BFZ), but the recharge and discharge mechanisms of the Northern Segment are more similar to the Edwards Plateau Aquifer. The Barton Springs and San Antonio Segments are recharged primarily by streams that gather their base flow from the contributing zone and then lose their base flow to the aquifer as they cross the recharge zone. The Northern Segment, however, is recharged primarily by rainfall on the outcrop within the recharge zone. Bull Creek, the San Gabriel River, Brushy Creek, and Salado Creek are all gaining streams within the Edwards Aquifer recharge zone where their base flow is fed by aquifer springflow.

To the east of the Edwards Aquifer recharge zone, the Edwards formation is at its full thickness and is overlain by the aquifer confining units of the Del Rio Clay, the Buda Limestone, the Eagle Ford Formation, and the Austin Chalk. The artesian zone also contains many springs which commonly exhibit a higher rate of discharge. At these sites, water from the confined zone of the Edwards Aquifer rises vertically along faults to discharge at the surface through the upper confining units. Figure 7 presents an interpretation of spring hydrology for both the recharge and confined zones. The confined zone is proximal to the Project Area only along the MoPac section.

Figure 7. Hydrogeology of the Artesian Zone of the Edwards Aquifer (Snyder 1985).

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The direction of groundwater flow within the Northern Segment of the Edwards Aquifer is typically to the north and east (Brune and Diffin 1983, Senger et al. 1990). Figure 8 presents the results of a potentiometric surface model from 1978 which includes the Project Area. The lack of well data for the Bull Creek Basin prevented accurate modeling in that area. As a result of the general northeastern potentiometric gradient, springs and seeps in the Northern Segment of the aquifer tend to be formed preferentially along the south banks of the larger streams (such as Brushy Creek and the north and south branches of the San Gabriel River) where the base of the Edwards Limestone is exposed. This pattern can be seen in the distribution of known springs on local geologic maps, especially that of Collins (2005); which shows a total of 11 springs discharging from the Comanche/Edwards contact along the south bank of the north San Gabriel, with only two along the north bank. Similarly, seven springs are documented along the south bank of the south San Gabriel, with none along the north bank. Local influences such as faults and a locally high hydraulic gradient can modify that general pattern and some springs are known to be formed on the north banks of major streams.

Springs in both the water-table and artesian zones contribute to the base flow of streams as they cross the Gulf Coastal Plain to the east. This is one of the major distinctions between the Northern Segment and the Barton Springs and San Antonio Segments of the Edwards Aquifer; where surface streams predominantly lose base flow to the aquifer.

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Figure 8. Water Levels in the Edwards Limestone in the Spring of 1978, Indicating Groundwater Flow to the Northeast (Modified from Figure 21, Brune and Duffin 1983).

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Additional springs occur at lower stratigraphic locations and with seemingly different hydrologic characteristics within the Bull Creek Basin. In this type of system, the distances between recharge and discharge areas are typically short. Because the Edwards Formation is highly eroded in parts of the recharge zone, the storativity of the Edwards Aquifer is reduced and many springs are highly ephemeral as a result; ceasing to flow in dry weather. Some springs in the area have been known to continue to flow during major drought years and may be fed by the underlying Trinity Aquifer, which is under artesian pressure in some areas (Snyder 1985). Although the Northern Segment is the least studied of the Edwards Aquifer segments, recent work by the City of Austin and others does provide a good baseline for interpreting the spring hydrology of the Bull Creek Basin and the western and southern flank of the Jollyville Plateau. The baseline hydrogeological study for the City of Austin’s Water Treatment Plant 4 project (Black and Veatch and Daniel B. Stephens and Associates 2010) was the first in-depth analysis of the hydrogeology of the Bull Creek Basin where many JPS sites occur. Based on the results, JPS sites can be divided into two hydrogeological categories. The first category is the Edwards\Walnut Flow System, or the Edwards Aquifer caprock. Eighty-two percent of the springs in the Upper Bull Creek Basin discharge from small, fragmented groundwater lenses, typically not more than 10 feet thick, within the Edwards Limestone and the Walnut Formation (Black and Veatch and Daniel B. Stephens and Associates 2010). These springs typically occur in the upper reaches of the watershed at higher elevations and in steeper terrain. These springs generally provide access to the karstic subsurface refugia (wetted caves) that have been well-documented in both the Edwards Limestone and Walnut formations in Travis and Williamson Counties. The Spicewood Springs, Stillhouse Hollow, Tanglewood, and Barrow monitoring locations (see Figure 1) fall into this category; as do the great majority of known locations for all salamander species in the Northern Segment.

The second category is the Glen Rose flow system. As spring flow from the Edwards/Walnut groundwater system moves lower in the basin, the valley floors bottom-out into a lower gradient terrain where the main channel of Bull Creek is incised into the upper surface of the Glen Rose Limestone. At the transition between the high and low gradient channels, baseflow is often lost altogether to alluvial deposits, which form minor aquifers recharged both by springflow from the Edwards\Walnut flow system and by surface water entering the Bull Creek Basin. Water in these deposits returns to the surface downstream through "springs" in the Glen Rose Limestone terrain. These discharge points tend to be broader areas of creek baseflow emergence rather than the open karstic portals more typical of the Edwards headwater springs. The City of Austin (Davis et al. 2001) notes that "no distinct spring" occurs at the Tributary 3, 5, and 6 monitoring sites. Black and Veatch and Daniel B. Stephens and Associates (2010) describe the section of Bull Creek between Lanier and Pit springs in these terms. Depending upon the size of the alluvial deposit, the springflow from these sources may be extremely ephemeral to nearly permanent. It should be noted that although temporary refugia may occur in the alluvial gravels at these sites, the more effective karstic subsurface refugia are not generally known from the upper Glen Rose Limestone in Travis and Williamson Counties. Monitoring sites at Tributaries 3, 5, 6, 7 (Franklin) and Long Hog Hollow (Wheless) fall into this hydrogeologic category (see Figure 1).

Figure 9 illustrates the hydrogeologic relationship between types of JPS habitat sites. Jollyville Plateau salamanders are found in headwater springs discharging from the Edwards/Walnut caprock. In the higher terrain, spring runs typically disappear where flow is lost to alluvial deposits that thicken downstream. Those alluvial deposits form minor aquifers which are also recharged by surface water entering the channel. Salamanders also occur further downstream in channels on the valley floor where the Edwards Aquifer spring water resurges to the surface after mixing with small amounts of discharge from the Trinity Aquifer.

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Figure 9. Hydrogeology of Jollyville Plateau Salamander Habitat in Upper Bull Creek (see Figure 2 for Location of Bull Creek).

The Tributary 3, 5, and 6 sites (see Figure 1) would be expected to respond differently to climatic variation as they are more distant from the source of groundwater and lack cavernous subsurface refugia. During long periods of beneficial rainfall (1991–1998), these sites apparently maintained high quality habitat, supporting large numbers of salamanders, many of which may have dispersed from the upstream Edwards/Walnut headwater springs. Containing much more habitat area than is available in individual spring runs, the channels of Bull Creek can sustain much higher numbers of salamanders as long as conditions remain suitable.

HYDROLOGICAL MODELING

Potential project-related groundwater impacts could result in impacts to karst aquatic fauna if the fauna occur down-gradient in terms of subsurface flow paths. In order to establish a clearer understanding of site-specific hydrogeologic conditions, including the direction of groundwater flow along the Project Area, geoscientists constructed a conceptual hydrogeologic model of the Project Area to establish the likely hydrogeologic zone of influence for the springs.

Existing well data was collected from the Texas Water Development Board Water Information Integration and Dissemination System (WIID) groundwater database and other sources in order to map the local potentiometric surface to the extent practicable. Using the depth to water and surface elevation, the elevation of the water table was calculated for each data point. Using ArcGIS, the data points were plotted based on their recorded latitude and longitude locations. The groundwater surface model was then interpolated from these plotted data points using the Inverse Distance Weighting (IDW) Tool in ArcGIS’s Spatial Analyst extension. To eliminate spurious data, a low-pass filter was applied to the model. Data were then reclassified using the natural breaks (Jenks Method) to display seven categories of data. For display purposes, the model was clipped to the edges of the mapped Edwards Limestone geologic unit.

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The potentiometric surface indicates the general direction of groundwater flow (at right angles to the potentiometric surface contours). Non-Edwards wells were excluded from the database. The locations of known springs and other expressions of the potentiometric surface as determined though the literature review were also added to the database. Figures 10 through 13 present a sample of the natural reflections of the groundwater surface used in the model. Appendix A contains a database of the potentiometric surface points used in the model.

The structural setting of the potential springshed area was studied for indications of local fault-induced recharge and flow patterns. Faults within the recharge zone define a series of fault blocks which are down-dropped toward the coast and often slightly rotated relative to one another. Faults provide opportunities for vertical infiltration of large amounts of surface water where fracture-controlled karst conduits occur beneath areas where surface water collects. The direction of groundwater flow can be influenced by the orientation of the host strata, especially that of the lower confining unit (Comanche Peak).

Figure 10. Spring-fed Section of Davis Spring Branch, South of the Old Ranch to Market Road 620, Prior to the Construction of SH 45; Expressing of the Potentiometric Surface Approximately 2.7 Miles East of the Project Area.

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Figure 11. Spring-fed Section of Bull Creek Tributary 6 Within Critical Habitat Unit 16 Below the Edwards Outcrop Approximately 2 Miles Southwest of the Project Area.

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Figure 12. Davis Spring Occurs in an Impounded Area on Davis Figure 13. Riviera Spring Discharging from the Edwards Aquifer Spring Branch Approximately 2 Miles Northeast of the Project Area. into the Headwaters of Brushy Creek; Approximately 0.5 mile Northwest of the Project Area.

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RESULTS

The results of hydrogeologic modeling (see Figure 14) indicate that groundwater in the local portion of the Northern Edwards Aquifer moves generally to the northeast and to the southwest away from a groundwater divide that runs sub-parallel to the US 183 corridor. These trajectories generally parallel the surface paths of Brushy Creek, Lake Creek, and Bull Creek with some exceptions. Figure 14 depicts the model results with groundwater flow lines which follow at right angles to the potentiometric surface contours.

Flow direction and accumulation for underground water was calculated using ArcGIS Hydrology Tool Set. A basic model was set up beginning with the Fill Tool which fills any artificially calculated sinks in the underground water raster surface. Next, the Flow Direction tool was used to determine the direction each cell in the raster flows. Finally, the Flow Accumulation tool was employed. This tool utilizes the input of flow direction and summarizes the accumulated flow of cell. To make display and visualization more clear, the Log10 of the accumulated flow was calculated.

The results of groundwater modeling conducted for this study indicate that the Project can be divided into three sections based on the direction of groundwater flow and potential for storm water to reach habitat for the JPS. The first section extends from the northern project limit to approximately the intersection with McNeil Road. Within this area, the potentiometric surface indicates that karst stream piracy is occurring. Although the surface drainage divide occurs west of this section of the Project Area, well data and spring elevations suggest that groundwater is moving to the southwest, feeding springs and seeps within the Bull Creek Basin (see Figure 15). Similar karst groundwater piracy is known to occur elsewhere in the vicinity of the Project Area (Russell 1993). The Buttercup Creek karst area is a discrete, hydrologically integrated karst landscape containing more than 75 significant caves and sinkholes. Many of the caves contain small stream passages inhabited by the JPS. Cave mapping conducted by the University Speleological Society (Russell 1993) and subsequent dye traces conducted by Mike Warton and Associates (1997) demonstrated that these streams (which are fed by surface water from the upper Brushy Creek basin) converge to form a groundwater flow path that likely follows the Cedar Park fault emerging at Blizzard R Bar B Spring approximately 3 miles to the southwest in the Colorado River Basin. Figure 15 presents a generalized cross section perpendicular to the Project Area illustrating this area of contrasting surface and sub-surface hydrologic gradients.

The second section extends from approximately McNeil Road to approximately the intersection with Loop 360. Along this section of the Project it appears that both the surface and subsurface drainage divides occur near or beneath the Project Area. Within this area, groundwater may pass from the Project Area either southwest into Bull Creek or northeast into the Lake Creek or Walnut Creek basins. These findings are supported by URS (2013), which found that a groundwater divide exists within the immediate vicinity of Balcones Woods Drive as it crosses the Project Area. Groundwater generally moves to the northeast of the Project Area on the east side of US 183N at this location (URS 2013).

The third section extends from approximately the Loop 360 intersection to the southern terminus of the project. In this area, the project is clearly down-gradient of any known JPS habitat (see Figure 14). Groundwater in this area may enter the confined zone of the northern Edwards Aquifer.

18 GROUNDWATER TECHNICAL REPORT for the Central Texas Regional Mobility Authority 183 North Mobility Project

Figure 14. Potentiometric Surface Model for a Portion of the Northern Segment of the Edwards Aquifer.

19 GROUNDWATER TECHNICAL REPORT for the Central Texas Regional Mobility Authority 183 North Mobility Project

Figure 15. Conceptual Model of Groundwater Flow in the Northern Segment of Edwards Aquifer Near the Project Area, Between RM 620 and Mc Neil Road.

20 GROUNDWATER TECHNICAL REPORT for the Central Texas Regional Mobility Authority 183 North Mobility Project

LITERATURE CITED

Barnes, V. 1981. University of Texas Bureau of Economic Geology Geologic Atlas of Texas map series, Austin Sheet. Black and Veatch and Daniel B. Stephens and Associates. 2010. Preconstruction Groundwater Assessment for the Jollyville Transmission Main. 64 pp. Brune, Gunnar, and Gail L. Duffin. 1983. Occurrence, Availability, and Quality of Ground Water in Travis County, Texas. Texas Department of Water Resources Report 276, 103 pp. Collins, E. W. 2005. Geologic Map of the West Half of the Taylor 30x60 Quadrangle: Central Texas Urban Corridor, Encompassing Round Rock, Georgetown, Salado, Briggs, Liberty Hill, and Leander. Bureau of Economic Geology, The University of Texas at Austin. ———. 1998. Geologic Map of the Round Rock 7.5 Minute Quadrangle, Bureau of Economic Geology, University of Texas at Austin. Davis, B., R. Hansen, N.L. McClintock, E.D. Peacock, M. Turner, C. Herrington, D. Johns, D.A. Chamberlain, D. Cerda, T. Pennington, L. Leopold, A. Vaughan, and D. Nuffer. 2001. Jollyville Plateau water quality and salamander assessment. Quality Report Series COA-ERM 1999-01. City of Austin, Watershed Protection Department, Environmental Resources Management Division, Water Resource Evaluation Section.

Duffin, G., and Musick, S.P. 1991. Evaluation of water resources in Bell, Burnet, Travis, Williamson and parts of adjacent counties, Texas. Texas Water Development Board, Report 326. Austin. Garner, L.E., Young, K.P., Rodda. P.U., Dawe, G.L., and Rogers, M.Y. 1976. Geologic Map of the Austin Area, Texas. In Environmental Geology of the Austin Area: An Aid to Urban Planning. Bureau of Economic Geology Report of Investigations No. 86, The University of Texas at Austin. Housh, T.B. 2007 Bedrock Geology of Round Rock and Surrounding Areas, Williamson and Travis Counties, Texas. Available at: https://www.lib.utexas.edu/geo/roundrockbedrockgeology/ bedrockgeologyoftheroundrockarea.pdf. Accessed 15 May 2014. Jenks, George F. 1967. The Data Model Concept in Statistical Mapping. In International Yearbook of Cartography 7:186–190. Jones, I.C. 2003. Groundwater availability model – Northern Segment of the Edwards Aquifer, Texas. Texas Water Development Board Report 358. Moore, C.H., 1964. Stratigraphy of the Fredericksburg Division, South-Central Texas. University of Texas, Bureau of Economic Geology Report of Investigations no. 52, 37 pp. Rodda, P.U. 1970. Geology of the Austin West Quadrangle, Travis County, Texas. University of Texas, Bureau of Economic Geology. Quadrangle Map No.38. Russell, W.H. 1993. The Buttercup Creek Karst, Travis and Williamson Counties, Texas: Geology, Biology, and Land Development. Report for the University Speleological Society. July 1993. Senger, R. K., E.W. Collins, and C.W. Kreitler. 1990. Hydrogeology of the Northern Segment of the Edwards Aquifer. Bureau of Economic Geology, Austin Region, University of Texas at Austin. Slade, R. M. 1985. Hydrogeology of the Edwards Aquifer in Bell, Williamson and Northern Travis counties, Texas. Austin Geological Society Field Trip Guidebook 8. Woodruff, C.M., Snyder, F., De La Garza, L., and Slade, R. coordinators.

21 GROUNDWATER TECHNICAL REPORT for the Central Texas Regional Mobility Authority 183 North Mobility Project

Snyder, F. 1985. Springs in the Northern Segment of the Edwards Aquifer, in Edwards Aquifer Northern Segment Travis, Williamson and Bell Counties. Austin Geological Society Field Trip Guidebook 8. Woodruff, C.M., Snyder, F., De La Garza, L., and Slade, R. coordinators. URS Corporation (URS). 2013. Monitoring Event Summary and Status Report (MESSR): Jack Brown Cleaners #19 (DC0008). Prepared for TCEQ Remediation Division. Austin, Texas. U.S. Fish and Wildlife Service (USFWS). 2013a. 50 CFR Part 17 Endangered and Threatened Wildlife and Plants; Determination of Endangered Species Status for the Austin Blind Salamander and Threatened Species Status for the Jollyville Plateau Salamander Throughout Their Ranges; Final Rule Federal Register Vol. 78 No. 151, 20 August, 2013. Woodruff, C.M. 1985. Jollyville Plateau – Geomorphic Controls on Aquifer Development in Edwards Aquifer Northern Segment Travis, Williamson and Bell Counties. Austin Geological Society Field Trip Guidebook 8. Woodruff, C.M., Snyder, F., De La Garza, L., and Slade, R. coordinators. Yelderman, J.C. 2013. Hydrogeology of the Northern Segment of the Edwards Balcones Fault Zone Aquifer in the Salado Creek Basin and Environs; a current understanding. Report submitted to the Clearwater Underground Water Conservation District.

APPENDIX A

POTENTIOMETRIC SURFACE POINTS

GROUNDWATER TECHNICAL REPORT for the Central Texas Regional Mobility Authority 183 North Mobility Project

Elevation Water Potentiometric Designation State Well ID Latitude Longitude (ft) Depth (ft) Surface (ft) Well Well 5834201 30.463611 -97.816666 1007 -100 907 Well 5834301 30.479721 -97.782221 940 -34 906 Well 5834305 30.476110 -97.764722 895 -39 856 Well 5834303 30.461666 -97.760000 870 -40 830 Well 219473 30.465278 -97.758889 843 -4 839 Well 205029 30.470833 -97.747222 847 -36 811 Well 205027 30.470556 -97.746111 846 -45 801 Well 153384 30.506667 -97.722778 807 -47 760 Well 306055 30.506944 -97.719167 806 -47 759 Well 38582 30.501667 -97.725556 800 -49 751 Well 39504 30.499444 -97.724167 803 -45 758 Well 5827718 30.502222 -97.721110 800 -90 710 Well 41061 30.500556 -97.725278 798 -38 760 Well 5835112 30.496388 -97.734166 867 -68 799 Well 41065 30.500556 -97.725833 795 -45 750 Well 5835110 30.498610 -97.721388 795 -49 746 Well 270972 30.493333 -97.721111 810 -80 730 Well 5835108 30.471666 -97.717221 782 -26 756 Well 5835102 30.471666 -97.717221 775 -25 750 Well 5835107 30.469721 -97.717499 783 -17 766 Well 5835105 30.467221 -97.718888 780 -18 762 Well 5835106 30.468332 -97.717777 780 -22 758 Well 5835406 30.458055 -97.722221 840 -58 782 Well 5835407 30.457500 -97.723054 845 -58 787 Well 5835404 30.456944 -97.722221 842 -59 783

Spring Spring 5827719- Avery Deer Spring 30.506388 -97.748610 825 0 825 Spring Davis Spring 30.48793 -97.76476 878 0 878 Spring Chapman Horse 30.48605 -97.75597 840 0 840 Spring Robinson Tank Spring 30.48003 -97.74769 820 0 820 Spring Davis Spring No. 2 30.48125 -97.74224 807 0 807 Spring Davis Spring No. 3 30.48153 -97.74226 807 0 807 Spring PC SPrings/SH 45 30.47985 -97.74195 815 0 815 Spring Avery Mill 30.50372 -97.75981 838 0 838 Spring Avery West 30.4951 -97.77987 856 0 856 Spring Hill Marsh 30.50758 -97.75509 826 0 826 Spring Oak Brook 30.51486 -97.74243 791 0 791 Spring Oak Brook 2 30.51895 -97.7355 778 0 778 Spring Oak Springs 30.52088 -97.70573 715 0 715 Spring Unnamed Spring 30.51412 -97.70088 709 0 709 Spring Unnamed Spring 30.46002 -97.72775 794 0 794 Spring Riviera Spring 30.47571 -97.81257 922 0 922

Other Other Quarry 30.50953 -97.70144 732 0 732 Other Creek Base level 30.48319 -97.72164 768 0 768 Other ONCOR Trench 30.4798 -97.75242 837 -7 830 Note that springs are located at the ground surface; therefore there is zero depth to underlying water.

A-1