ABSTRACT
Recent site characterization activities at Andersen Air Force Base, Guam, has provided some interesting data challenging the expectations of rapid conduit flow within the karst groundwater flow regime. The main water bearing limestones of northern Guam consist of coralline reef limestones of the Barrigada and the Mariana Limestone Formations. The Barrigada lies on top of the underlying Tertiary aged volcanics. The Mariana Limestone overlies most of the surface of the northern plateau and onlaps the Barrigada Limestone as a vertical and transgressional faciès changing from a deep to a shallow water depositional sequence.
There are no surface streams on the northern plateau because of the porous nature of the soils and limestone bedrock. Rain water rapidly infiltrates through the limestone and supplies the freshwater aquifer; this aquifer is commonly referred to as the Northern Guam Lens. Conditions that effect the occurrence and extent of the lens are the result of the transient relationship between freshwater and saltwater. Other factors influencing the lens are; tidal fluctuations, storm surges, stratigraphic settings, and karstification of the limestone. Groundwater flows toward the sea, exiting at numerous surface seeps within the intertidal zone and suspected to also discharge from larger submarine springs along the protective reef fringe.
Evidence from boreholes drilling inland and near the coast provide evidence that cavern formation is found between the phreatic and vadose zone, and within the transition zone between the fresh and salt water interface. Caves investigated along the lower coastal terraces also provides evidence, by the steeply dipping entrances that have formed within the transition zone, that sea level was higher in the past, and/or tectonic activity vertically displaced the limestone and its relationship to the transition zone responsible for the active dissolution and the formation of caverns.
Although one might expect a rapid flow system in this karst aquifer, non-flashy responses on water levels in wells after rainfall, and current results from a dye tracing study, suggest groundwater movement is indicative of diffuse, advective flow. During a 15 month dye tracing program monitoring over 70 monitoring wells, production wells, and cave pools, groundwater flow velocity was measured on the order of 9 meters per day (30 ft/day) within the aquifer. However, flow within the epikarst/vadose zone can be several times the velocity within the aquifer and very difficult to predict the direction of movement.
