You have entered the website of the Water and Environmental Research Institute of the Western Pacific (WERI) at the University of Guam. We are currently six faculty and five staff and our mission is to seek solutions through research, teaching and outreach programs, to issues and problems associated with the location, production, distribution and management of freshwater resources in Guam, the CNMI and the FSM.
Atolls are composed of ring-shaped coral reefs and islands, enclosing or nearly enclosing a shallow lagoon. The small size, low elevation, unique geology, and isolation of atoll islands make island inhabitants vulnerable to water shortage during times of prolonged drought or the aftermath of damaging tropical storms. Under normal conditions freshwater demand is met by rooftop rain catchments, although under drought conditions the demand is supplemented by groundwater from hand-dug wells. However, the location, quality and quantity of the groundwater from the freshwater lens are not known in advance for most atoll islands, thus making effective long-term water resources planning difficult. In this study, a modified version of the USGS numerical model SUTRA was used to analyze the dimensions of the freshwater lens on atoll islands under varying geologic and climatic conditions.
Results from steady-state simulations show that the depth to the discontinuity between the upper and lower aquifer, presence of the reef flat plate, island width, recharge rate, and hydraulic conductivity of the upper aquifer all govern the thickness and volume of the lens. The discontinuity between the upper and lower aquifers acts as a limiting feature of the growth of the lens, while the reef flat plate acts as a confining layer, extending the lens under the plate on the ocean side of the island. An increase in island width and recharge increases the thickness of the lens to the depth of the discontinuity. The position of the island in reference to the prevailing winds has a large influence, as comparisons between numerical simulation results and observed lens thicknesses demonstrate that the hydraulic conductivity of the upper aquifer is 50 m day-1 and 400 m day-1 for islands located on the leeward and windward portions of the atoll, respectively.
Results from all simulations enabled the development of an algebraic model that predicts the depth of the freshwater lens for atoll islands as a function of recharge, island width, hydraulic conductivity of the upper aquifer, depth to the discontinuity, the existence of the reef flat plate, and time. The algebraic model may be valuable prediction tool for water resources managers of atoll islands.
