The Northern Guam Lens Aquifer (NGLA) is comprised of Plio-Pleistocene limestone units deposited atop a volcanic basement. Typical heads range from about 3 to 5 feet above sea level, with the maximum thickness of the lens ranging up to about 150 feet. Continuously draining into the ocean, the water in the lens is replenished by rain, which averages about 100 inches per year but is highly variable on both long and short time and space scales. Guam draws some 80% of its drinking water from its freshwater lens in the aquifer, using some 45 millions gallons per day, a bit more than half of the most recent estimate of 70 to 80 million gallons per day of sustainable yield. In order to identify appropriate techniques and management practices to sustain aquifer development while protecting water quality, groundwater scientists and engineers need a better understanding of aquifer dynamics. In particular, it will be necessary to better understand the factors that control the rates and amounts of water taken into storage, the residence time of water in the aquifer, and the quantities that can therefore be extracted in given times and places without degrading the quality of the water. The responses of well levels to rainfall events provide important clues regarding the rate at which water descends through the vadose zone, the amount of time that it is retained in phreatic storage, and therefore the amount that may ultimately be available for exploitation by various techniques.

Variations in rainfall and sea level are the most direct and important causes of variations in well levels. Observations from three well sin the Agana Argillaceous Member of the NGLA indicate that the combined variations in sea level and rainfall in real time or near-real time account for up to 66% of the variance of water levels in the wells - the sea level accounting for the larger share of this variance near the coast, and the rain accounting for the larger share of the variance at well locations further inland. Multi-year variations of rainfall appear in the well levels at time lags up to nearly two years. Heavy 24-hour rainfalls of up to 3 inches may cause no immediate response of well levels if they occur at the end of prolonged dryness. Similar rain events cause immediate and large increases of well levels if they occur during prolonged wet periods. Rapid increases in well levels in response to heavy short-term rain events decay to background levels within a period of about 10 days. The observed response of the wells to variations in the rainfall and sea level suggest a complicated mix of diffuse and open pathways through a heterogeneous limestone medium. Future reports from this project will evaluate the responses of well [levels] in the non-argillaceous limestone units of the NGLA and address the degree to which the different lithologies and aquifer properties between the units may drive different aquifer responses to rainfall.