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

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Groundwater Technical Report for the Central Texas Regional Mobility Authority 183 North Mobility Project 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 This page left intentionally blank ES-1 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. ES-2 This page left intentionally blank ES-3 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. ES-4 This page left intentionally blank ES-5 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 i 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. .................................................................................................
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