ABSTRACT
Recent hydrogeologic investigations on the northern portion of the island of Guam, Mariana Islands, have provided interesting data regarding expectations of rapid (conduit) flow within the fresh water lens. Evidence from cave exploration and boreholes drilled inland and near the coast suggest karstification has occurred between the phreatic and vadose zone at a depth of approximately 150 meters (500 feet) below ground surface, and within the transition zone between the fresh and salt water interface, near sea level. Tracing results, and non-flashy responses on water levels in wells, suggests ground water movement is representative of macro-porous-media flow, not conduit flow that is commonly assumed in most karst aquifers. Groundwater flow is further complicated by lateral and vertical variations of reef facies, partial or complete diagentic removal of primary porosity. Horizontal and vertical groundwater flow in the aquifer converges along the coastal area, and discharges as resurgences, or submarine springs. In contrast, flow within the vadose zone is rapid, and the direction of movement is highly unpredictable.
