Vadose Hydrology at Jinapsan Cave, Northern Guam

Six years of monthly data were analyzed from an active tropical limestone cave in Guam, the southernmost Mariana Island in the western Pacific Ocean. The purpose of this study was to characterize rates and variability of vadose percolation in the Plio-Pleistocene Mariana Limestone, which occupies about 75% of the surface of the Northern Guam Lens Aquifer (NGLA). A ground survey grid was established on the surface above the cave, a vegetated talus slope beneath the >150-m-high cliff 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 features, inferred epikarst and vadose layer dimensions, cave floor slope, and structural and geomorphic features of the cave, including a brackish water-table pool at the cave bottom. 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 exhibited seasonal drip responses to wet-dry rainfall. One (the slowest) 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).