ABSTRACT

Perched aquifer systems are causing slope failures within the Hagerman Fossil Beds National Monument in northwestern Twin Falls County, Idaho. Six large slope failures have occurred since 1979, which are damaging natural resources and private property. In 1987 a slope failure destroyed a million-dollar irrigation pumping facility and nearly killed two workers. Numerous studies have been conducted since 1984 in an attempt to define the aquifer systems but none have resulted in a document that integrates all of the results. This study includes an analysis of all the previous data in addition to field mapping and investigations with the general objective to construct a hydrostratigraphic model for the site.

A geologic model of the study site was developed based on existing studies and literature, field investigations, drill logs and geophysical data. The proposed geologic model has six layers, three of the layers are aquifers and three are aquitards. Spatial extent of some layers has been interpreted between monitor wells based on geophysics.

A hydrologic model was developed using existing surface water, monitor well, water chemistry and geophysical data. Recharge to the perched aquifers occurs dominantly from irrigation on the plateau. A ground water pressure wave moves through the perched aquifer systems starting at the recharge area and moving down gradient to the discharge zones causing cyclic fluctuations in monitor well water levels along the flow path.

A hydrostratigraphic model was developed based on the geologic and hydrologic models. The model shows how three perched aquifer systems (upper, middle and lower) flow in the plateau and the implications of each system has for slope stability problems. Paleo stream channels control the upper and lower systems while the middle system is flowing primarily through fractured basalt. The flow regimes are dynamic; exhibiting both unconfined and confined characteristics.

The hydrostratigraphic model explains the spatial distribution of perched aquifer discharge zones and it will aid the development of a mitigative plan for slope stability problems. Results indicate recharge to the upper system is primarily near the canal inlet, while recharge to the middle basalt aquifer occurs in the area near the Fossil Gulch Pond. The lower system is receiving drainage from the middle system. Canal lining mitigative efforts will have the greatest impact to the middle aquifer system if applied near the Fossil Gulch Pond area. It is recommended to start ground water tracer tests, continue focused data collection and long term monitoring to establish trends and define specific flow regimes.

TABLE OF CONTENTS

Title Page i

Authorization to Submit Thesis ii

Abstract iii

Acknowledgments v

Table of Contents vii

List of Illustrations ix

List of Tables xii

Introduction 1

Description of the Problem 1

Purpose and Objectives 4

Geographic Setting 5

General Geology 8

Geologic Setting 8

Geologic Model 15

Hydrologic Model 23

Irrigation as a Recharge Source 23

Summary of Canal Leakage Studies 25

Monitor Wells 28

Ground Water Hydrographs and Contours 31

Ground Water Chemistry 34

Geophysics 35

Perched Aquifer Discharge 39

Conclusions 42

Hydrostratigraphic Model 43

Introduction 43

Upper Perched Aquifer System 43

Middle Perched Aquifer System 46

Lower Perched Aquifer System 47

Conclusions and Recommendations 52

Appendix A Discussion of Geoelectric Studies 55

Appendix B Discussion of Surface Water 68

Appendix C Discussion of Ground Water 75

Appendix D Discussion of Dissolved Nitrogen 100

References 104

LIST OF ILLUSTRATIONS

Figure 1 Location of Study 6

Figure 2 Sequence of Upper Cenozoic Rocks 8

Figure 3 Geologic Map of Study Site 11

Figure 4 Sedimentary Facies Map 12

Figure 5 Canal Base Geology 17

Figure 6 Seismic Line locations 19

Figure 7 Geologic Model 21

Figure 8 Volume of Water Pumped into Fossil Gulch Canal 25

Figure 9 Locations of Canal Leakage Studies 27

Figure 10 Locations of Monitor Wells 30

Figure 11 Hydrologic Model 32

Figure 12 Ground Water Contours and Recharge Pressure Wave 33

Figure 13 Trilinear Diagram of Water Chemistry 35

Figure 14 Misa-a-la-masse Results 36

Figure 15 Schlumberger Results 37

Figure 16 Weir and Flume Location 40

Figure 17 Hydrographs for Perched Aquifer Discharge Streams 41

Figure 18 Fence Diagram of Hydrostratigraphic Model 44

Figure 19 Block Diagram of Hydrostratigraphic Model 45

Figure 20 Photo of Perched Aquifer Discharge 49

Figure 21 Photo of Perched Aquifer Discharge 50

Figure 22 Photo of Perched Aquifer Discharge 51

Figure 23 Misa-a-la-masse Results Map 58

Figure 24 Wenner Location Map 60

Figure 25 Wenner Profile 61

Figure 26 Schlumberger Results Map - 100 foot Depth 66

Figure 27 Schlumberger Results Map - 150 foot Depth 67

Figure 28 Static Water Level Hydrographs for Monitor Wells 77

Figure 29 Monitor Well 9DCB1 (NPS-1) Construction and Geologic Log 82

Figure 30 Monitor Well 9CDD1 (NPS-2) Construction and Geologic Log 83

Figure 31 Monitor Well 9DCC2 (NPS-3) Construction and Geologic Log 84

Figure 32 Monitor Well 5DDD1 (NPS-4) Construction and Geologic Log 85

Figure 33 Monitor Well 7AAA1 (NPS-5) Construction and Geologic Log 86

Figure 34 Monitor Well 9CDD2 (NPS-6) Construction and Geologic Log 87

Figure 35 Monitor Well 9CBB1 Construction and Geologic Log 88

Figure 36 Monitor Well 9CCC1 Construction and Geologic Log 89

Figure 37 Monitor Well 9CDC1 Construction and Geologic Log 90

Figure 38 Monitor Well 9DCC1 Construction and Geologic Log 91

Figure 39 Monitor Well 17AAB1 Construction and Geologic Log 92

Figure 40 Monitor Well 17ABB1 Construction and Geologic Log 93

Figure 41 Static Water Levels for Monitor Wells NPS-1 and NPS-2 94

Figure 42 Static Water Levels for Monitor Wells NPS-3 and NPS-4 95

Figure 43 Static Water Levels for Monitor Wells NPS-5 and NPS-6 96

Figure 44 Static Water Levels for Monitor Wells 9CBB1 and 9CCC1 97

Figure 45 Static Water Levels for Monitor Wells 9CDC1 and 9DCC1 98

Figure 46 Static Water Levels for Monitor Wells 17AAB1 and 17ABB1 99

Figure 47 Nitrate Concentration Map 102

LIST OF TABLES

Table 1 Chronological Listing of Studies 2

Table 2 Table of Canal Leakage Volumes 28

Table 3 Monitor Well Information 29

Table 4 Comparison of Canal Leakage Rates 72

Table 5 Results of Water Balance Study 74