Vadose Hydrology at Jinapsan Cave, Northern Guam Kaylyn K. Bautista John W. Jenson Mark Lander Timothy Righetti Technical Report #163 April 2018 ii ACKNOWLEDGMENTS This work was funded by the National Science Foundation (Grant #1451595), Strategic Environmental Research and Development Program (G14AC00103), the Pacific Islands Climate Science Center (G12A003), the United States Geologic Survey (G16 AP0048), and administered through the Water and Environmental Research Institute of the Western Pacific. ABSTRACT Eight years of monthly data (2008-2016) were analyzed from an active tropical limestone cave in Guam, the southernmost of the Mariana Islands, in the western Pacific Ocean. The purpose of this study was to characterize fast and slow vadose processes of aquifer recharge in the Plio-Pleistocene Mariana Limestone, which occupies about 75% of the surface of the Northern Guam Lens Aquifer (NGLA). Accurate understanding of aquifer recharge in the NGLA is important because the aquifer supplies 90% of the island's drinking water. This hydrogeologic study was conducted concurrent with paleoclimate research, in which correlative data on CO2 and other cave meteorological parameters were also collected. For this study, a ground survey grid was established on the surface above the cave, a vegetated talus slope beneath the >150-m cliff in the Mariana Limestone behind the cave. Cave and vadose zone 3-D models were constructed from the surface survey and a cave interior survey. Cross sections display talus slope (33°) features, inferred epikarst and vadose layer dimensions, cave floor slope (-34°), and structural and geomorphic features of the cave, including a brackish water-table pool at the cave bottom. GIS products include georeferenced cave boundary and cave room shapefiles. A plan-view map displays significant boulder talus and limestone-forest trees, cave entrance location, and the underlying cave boundary and fractures mapped on the cave ceiling. Thicknesses of the talus and vadose bedrock sections range from 1.3 to 17.0 meters and 1.7 to 46.4 meters, respectively. Drip rate and discharge rate data from 7 cave stations are presented in graphs showing varying responses between percolation and changes in rainfall during wet (Jun-Nov) and dry (Dec-May) seasons. Six stations (Trinity, Flatman, Station 1, Station 2, Stumpy, and Stumpy’s Brother) exhibited seasonal drip responses to wet-dry rainfall cycles. Amidala, the slowest drip, displayed mostly perennial dripping, with several overflow occurrences. Average drip rate, plotted on a log scale, divided stations based on order of magnitude into inferred hydrologic preferential pathway categories: fracture flow (fast; 103-104 drips/hr), fracture-fissure (fast; 102-103 drips/hr), small fissure flow (medium; 101-102 drips/hr), and matrix flow (slow; <101 drips/hr). Trinity is characterized as fracture flow; Flatman as a fissure-fracture flow; Station 1, Station 2, and Stumpy’s Brother as small fissure flow; and Stumpy and Amidala as matrix flow. iii TABLE OF CONTENTS LIST OF TABLES ...........................................................................................................v LIST OF FIGURES .........................................................................................................v CHAPTER I. INTRODUCTION 1.1 Introduction ........................................................................................1 1.2 Purpose ...............................................................................................1 1.3 The Northern Guam Lens Aquifer and Its Geologic Setting .............1 1.4 Hydrologic zones in carbonate island karst aquifers .........................3 1.5 Porosity in carbonate island karst aquifers ........................................5 1.6 Previous Studies .................................................................................5 II. SETTING: JINAPSAN CAVE 2.1 Location and Geologic setting ...........................................................7 2.2 Cave Description ................................................................................9 2.2.1 Navigating the Cave ............................................................12 III. METHODS 3.1 Previous Work ...................................................................................29 3.1.1 Sampling Stations ..............................................................36 3.2 Drip-count database ...........................................................................43 3.3 Cave survey ........................................................................................46 IV. RESULTS 4.1 Map of Jinapsan Cave surface plan and cross sections ......................52 4.2 Drip-water Data .................................................................................56 4.3 Drip rate magnitude and variability ...................................................60 4.3.1 Trinity ................................................................................66 4.3.2 Flatman .............................................................................68 4.3.3 Station 1 .............................................................................71 4.3.4 Station 2 .............................................................................75 4.3.5 Stumpy’s Brother ..............................................................78 4.3.6 Stumpy...............................................................................81 4.3.7 Amidala .............................................................................84 4.4 Drip rate variability ............................................................................87 V. DISCUSSION 5.1 Percolation Categorization .................................................................89 5.2 Fast Percolation ..................................................................................89 5.2.1 Trinity ..............................................................................89 5.2.2 Flatman ............................................................................91 5.2.3 Station 1 ...........................................................................92 5.2.4 Station 2 ...........................................................................93 5.2.5 Stumpy’s Brother .............................................................93 5.3 Slow Percolation ................................................................................94 5.3.1 Stumpy .............................................................................94 5.3.2 Amidala ............................................................................95 5.4 Relationship to the NGLA .................................................................96 iv VI. SUMMARY and CONCLUSION 6.1 Summary ............................................................................................99 6.2 Conclusion ........................................................................................99 REFERENCES ................................................................................................................100 APPENDICES A.1 – Jinapsan Cave Ground Surface and Cave Surveys ....................................104 A.2 – Previous Jinapsan Ground Surface and Cave Surveys...............................117 B – Jinapsan Cave drip-count database ...............................................................122 LIST OF TABLES 2.1 Summary of Jinapsan Cave approximate room area and volume, and corresponding figures in this section ...................................................................13 3.1 Summary of equations used to calculate observed and inferred values for drip rate (drips/hr) and dripwater flux (mL/day) .......................................................................... 45 4.1 Ceiling and floor distances between cave stations, respective estimated limestone bedrock and talus layer thicknesses, and hypothesized hydrologic pathway of each station...........................................................................................................56 4.2 Summary for entire record (wet and dry seasons) drip rate descriptive statistics (drips/hour) per station for 8-year record 2008-2016 ..........................................56 4.3 Summary for entire record of dripwater flux descriptive statistics (milliliters/day) per station .............................................................................................................57 4.4 Summary of wet season drip rate descriptive statistics per station ......................59 4.5 Summary of dry season drip rate descriptive statistics per station ......................59 4.6 Summary of storms on time series graphs ...........................................................62 4.7 Summary of days between drip response and rainfall events for all stations ......67 5.1 Cave drip stations characterized by discharge types ............................................90 LIST OF FIGURES 1.1 Map of Guam in the western Pacific, the northern limestone and southern volcanic areas divided by the Pago-Adelup Fault ................................................2 1.2 USEPA Map of the Northern Guam Sole Source Aquifer boundary ..................2 1.3 Schematic of a composite island from the CIKM ................................................3 1.4 Lens geometry and zones of the NGLA ..............................................................4 1.5 Conceptual model of the NGLA: Recharge, storage, transfer, and discharge .....4 1.6 Triple porosity model ...........................................................................................5