EXPLORING GEOLOGIC CONTROLS ON INFILTRATION AND GROUNDWATER RECHARGE ON AN EPHEMERAL RIVER: A COUPLED GEOPHYSICS AND MODELING APPROACH by Fern Beetle-Moorcroft A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science (Hydrology). Golden, Colorado Date _________________________ Signed: _______________________ Fern Beetle-Moorcroft Signed:________________________ Dr. Kamini Singha Thesis Advisor Golden, Colorado Date _________________________ Signed:________________________ Dr. Josh Sharp Professor and Director Hydrologic Science and Engineering Program ii ABSTRACT Ephemeral streams are the main surface-water source in arid climates, and streambed leakage in these systems is an important component of groundwater recharge. Subsurface geology influences the extent and location of streambed leakage and therefore changes streamflow, impacts groundwater levels, and has the potential to influence confined aquifer recharge. This study looks at exploring geological controls on recharge from an ephemeral river through scenario evaluation with numerical models, constrained by geophysical observations. Drone magnetometer, electrical resistivity, hammer seismic, and hydrologic data were collected along the Alamosa River in southeastern Colorado, USA to constrain and parameterize a MODFLOW-SFR model based on the field system consisting of three layers: 1) an alluvial aquifer, 2) a confining unit, and 3) a confined aquifer. Four scenarios were explored beyond a base case to quantify controls on subsurface geology: 1) adding a fault; 2) changing the alluvial aquifer’s hydraulic conductivity; 3) changing the thickness of the streambed; and 4) removing the confining unit. A fifth scenario, adding a pumping well, was used to explore the role of this human influence on ephemeral river systems. The drone magnetometer and resistivity data indicated that the middle portion of the study area was geologically distinct from the surrounding area, and the resistivity and seismic results indicate the presence of heterogeneity in the subsurface. Modeling scenarios that changed hydraulic conductivity values resulted in the most notable changes to the river’s hydrologic system, altering streamflow, leakage, and deep aquifer recharge. Streambed thickness proved to be an unimportant parameter. Results here suggest that the extent to which streambed leakage changes is proportional to the ratio of alluvial aquifer hydraulic conductivity (K1) to streambed conductivity (Ks), and that this in turn controls the impacts on streamflow. This research suggests that subsurface heterogeneities are a fundamental control on ephemeral rivers’ hydrogeologic system and are key to their resilience under climate change. iii TABLE OF CONTENTS ABSTRACT ................................................................................................................................... iii LIST OF FIGURES ....................................................................................................................... vi LIST OF TABLES ......................................................................................................................... ix ACKNOWLEDGEMENTS .............................................................................................................x CHAPTER 1 INTRODUCTION ...................................................................................................1 CHAPTER 2 EXPLORING GEOLOGIC CONTROLS ON INFILTRATION AND GROUNDWATER RECHARGE ON AN EPHEMERAL RIVER: A COUPLED GEOPHYSICS AND MODELING APPROACH ...........................3 2.1 Abstract ........................................................................................................3 2.2 Introduction ..................................................................................................4 2.3 Site Description ............................................................................................7 2.4 Methods......................................................................................................11 2.4.1 Hydrological Field Methods ....................................................11 2.4.2 Geophysical Field Methods .....................................................12 2.4.3 Modeling with MODFLOW ....................................................13 2.5 Results ........................................................................................................17 2.5.1 Hydrological Field Results ......................................................17 2.5.2 Geophysical Field Results........................................................17 2.5.3 Modeling Results .....................................................................20 2.5.3.1 Scenario 0: Base Case Model ....................................20 2.5.3.2 Scenario 1: Fault Introduced at 1980 m along the River Reach ................................................................24 2.5.3.3 Scenario 2: Increasing and Decreasing Layer 1 Hydraulic Conductivity (K1) along the Recharge Segment ......................................................................26 2.5.3.4 Scenario 3: Increasing and Decreasing Streambed Thickness (ST) along the Recharge Segment ............28 2.5.3.5 Scenario 4: Removal of the Confining Unit along the Recharge Segment ................................................29 2.5.3.6 Scenario 5: Pumping Well Introduced at 1980 m along the River Reach ................................................29 2.6 Discussion and Conclusions ......................................................................32 iv CHAPTER 3 FUTURE WORK ...................................................................................................37 REFERENCES ..............................................................................................................................39 APPENDIX A HOW TO BUILD A MODFLOW-SFR FILE ......................................................48 APPENDIX B GEOLOGICAL HISTORY OF THE SAN LUIS VALLEY ................................55 APPENDIX C RESISTIVITY AND SEISMIC REFRACTION DATA COLLECTION ............57 APPENDIX D MEASURING VOLUMETRIC FLOW RATE ....................................................59 v LIST OF FIGURES Figure 2.1 1 A) Site map locating the study area in the San Luis Valley, Colorado between the San Juan Mountains to the west and the Sangre de Cristo Mountains to the east; B) study area map just northwest of the town of Capulin shaded in yellow, showing the locations of hydrological and geophysical data collection: stilling wells as dark blue dots, head gates as grey dots, drone aeromagnetic survey lines in white, seismic lines in red, and the resistivity data in purple ...........................................................................................8 Figure 2.2 Simplified San Luis Valley cross sections detailing subsurface geology and generalized groundwater flow paths (modified from Hawley & Kernodle, 1999 and Wasiolek, 1995). Study area is designated by the star. Not to scale. ..............9 Figure 2.3 Conceptual model detailing the MODFLOW-SFR setup for the base case (scenario 0) and modeling scenarios including the fault location (scenario 1), variations in properties along the recharge segment (scenarios 2-4), and the pumping well location (scenario 5)........................................................................14 Figure 2.4 4 Drone magnetometer survey results and the location of the resistivity line. The colormap indicates total magnetic intensity (TMI) values measured in nanoteslas; warm colors indicate high TMI and cool colors show low TMI.........18 Figure 2.5 A) Resistivity profile and B) log10 values of the diagonal of the resolution matrix from west (0 m) to east (250 m). Higher resistivity values shown in yellow indicate more resistive geology. Values near 1 in (B) indicate where the model resolves the subsurface based on data. ..................................................19 Figure 2.6 Sensitivity of streamflow to parameters K1 and Ks, which were each varied -3 -5 between 1 x 10 and 1 x 10 m/s, for scenario 0, the base case. We varied Ks -3 -4 -5 assuming A) K1=1 x 10 m/s; B) K1=1 x 10 m/s; and C) K1=1 x 10 m/s. -3 -4 We then varied K1 assuming D) Ks=1 x 10 m/s; E) Ks=1 x 10 m/s; and F) -5 Ks=1 x 10 m/s. Lower K1 and Ks values resulted in higher streamflow, with K1 exerting a greater influence on streamflow than Ks ..........................................21 Figure 2.7 Scenario 0, the base case, and scenario 0 – perturbed, where K1 and Ks values -3 -5 were altered to match groundwater levels – K1= 8 x 10 m/s and Ks = 1 x 10 m/s; A) river stage, river base elevation, and groundwater head in Layer 1; B) streambed leakage. For scenario 0 – perturbed, leakage linearly decreases in response to decreasing streamflow; scenario 0, exhibits a bowing leakage pattern as a result of the river gaining minimally from groundwater, which reduces the net leakage in the middle of the recharge area ....................................22 Figure 2.8 Scenario 1 sensitivity of A) streamflow and B) streambed leakage to fault hydraulic conductivity Kf along the recharge segment. Groundwater head in model layers 1, 2, and 3 along the reach for C) high permeability fault (scenario 1.1) and D) low permeability fault (scenario 1.2). Scenario 1.1 results in increased leakage near the high permeability fault, which leads to a reduction in streamflow; the high permeability fault brings the model