What is the USGS Study?
Saipan’s municipal water supply relies heavily on groundwater, which in most places consists of a freshwater lens floating on saltwater. Groundwater from some municipal wells can exceed certain drinking water standards. The Saipan groundwater availability study will provide information that is critical for the use and management of the island's groundwater resources.
Four Phases of the Study
Phase 1: Review Existing Data
Available data and literature are being compiled and reviewed to help create a conceptual model of Saipan’s groundwater system, and to identify data needs for subsequent phases. Data types include groundwater levels, groundwater salinity, groundwater withdrawals, aquifer tests, geologic maps, rainfall, ocean tides, stream discharge, and submarine groundwater discharge.
Phase 2: Collect Field Data
A field data-collection program is underway to gather additional groundwater-level and groundwater-salinity data. Groundwater-level data indicate the directions of groundwater flow and hydraulic properties of the aquifers and can be used to evaluate an aquifer’s response to changes in recharge and pumping from wells. As part of this study, groundwater levels are being monitored at selected wells across Saipan.
Salinity variations with depth and time are also being monitored in selected wells, including some wells that provide water to the municipal system.
Phase 3: Estimate Groundwater Recharge
Accurate estimates of groundwater recharge on Saipan are essential for assessing groundwater availability. Because recharge comes from precipitation that infiltrates the ground surface, it is affected by factors such as changes in climate and land cover. In this study, recharge will be estimated using a water-budget model that accounts for water inputs (rainfall, irrigation, septic-system discharge, and other inputs) and water outputs (runoff, evapotranspiration, and groundwater recharge). Recent improvements in water-budget models allow recharge assessments to be done with a higher degree of accuracy and spatial resolution than was previously possible. Also, the availability of climate-change projections allow estimates of how recharge may be different in the future. Urbanization and other land-cover changes may affect recharge.
Phase 4: Assess Groundwater-Availability
A groundwater model will be created to help assess groundwater availability on Saipan. A groundwater model is a computer simulation that is capable of incorporating the numerous and varied datasets that relate to groundwater availability. Data and other information from Phases 1, 2 , and 3 of this study will be used to create a groundwater model that replicates the aquifers of Saipan and the processes, such as saltwater intrusion and water-table decline, that can limit groundwater availability.