|
| WATER-RELATED ACTIVITIES OF USGS IN MICRONESIA AND AMERICAN SAMOA |
| Water-Resource Assessment and Technical Assistance, Commonwealth of the Northern Mariana Islands |
|
Location: Islands of Saipan, Tinian, and Rota Problem: Rapid economic development and population growth have increased the demand for potable water supplies on the islands of Saipan, Tinian, and Rota in the CNMI. The Commonwealth Utilities Corporation has a need for hydrologic information and technical assistance in support of their ongoing exploratory drilling and testing program as they strive to deliver 24-hour potable water to all residents. The long-term response of aquifers in the CNMI to additional ground-water development and to natural climatic variations is largely unknown. Proper planning and management of water resources require long-term information so that trends can be defined, problems discovered, and corrective actions taken. Information from the exploratory drilling and testing, coupled with regular measurements of rainfall, surface water, springflow, and ground-water systems provide a database from which to evaluate the effects of management and development, and assist in determining sustainability of future supplies. Residents of Saipan do not have a continuous potable water supply--many areas do not receive water 24 hours a day and most of the water that is produced does not meet drinking-water quality standards of the U.S. Environmental Protection Agency. Water-supply problems are intensified during the dry season and during recurring periods of drought. The Governor of the CNMI (Commonwealth of Northern Mariana Islands) has created, by Executive Order, a Water Task Force (WTF) to address the problems plaguing water supply and distribution on the Island of Saipan. The WTF has been charged with providing 24-hour water of the best quality possible to all areas on Saipan within two years. WTF members represent a cross section of government and private sector leaders and the Commonwealth Utilities Corporation (CUC), which is the principal water purveyor in the CNMI and has a long time cooperative relationship with the U.S. Geological Survey (USGS). Discussions between WTF members, CUC officials, and the USGS resulted in the WTF and the CUC requesting additional technical assistance from the USGS to oversee a project of well field optimization for the 14 well fields (about 130 wells) on the island. Objectives: The objectives of this project include: (1) provide technical assistance to the WTF and CUC on water-resources issues, (2) provide direction and technical oversight to the CUC’s exploratory drilling program, (3) operate and maintain a hydrologic data program consisting of ground-water, surface-water, and rainfall stations, and (4) prepare technical reports that document hydrologic information collected and analyzed during the study. Relevance and Benefits: An important part of the USGS Strategic Plan is to provide scientific information to manage the water resources of the Nation. A clearer understanding of the occurrence and movement of ground water, the effects of existing and future ground-water development, and the factors that may limit ground-water production will result from this project. Ultimately, a numerical model that can simulate saltwater/freshwater aquifer systems will allow testing of the effects of proposed ground-water development and thereby assist in developing efficient ground-water withdrawal practices while protecting the resource and the environment. Approach: Exploratory drilling and hydrologic data collection will be used to determine: (1) the stratigraphy of the islands, (2) the permeability of the hydrologic units that comprise the islands, (3) the distribution of water levels, (4) the extent and thickness of the freshwater lenses, and (5) water budgets for the islands. A Numerical model will be used to simulate ground-water occurrence and movement so the resource can be efficiently used and protected. Model simulations will examine how the ground-water resources respond to increased ground-water withdrawal and to periods of drought. Significant Findings: Aside from abundant rainfall, ground water constitutes most freshwater resources in the CNMI. Most recoverable fresh ground water occurs as a basal freshwater lens in permeable limestone coastal aquifer systems. On Saipan, about 130 municipal production wells produce about 11 million gallons of water per day, accounting for about 90 percent of the municipal water supply. Three developed springs and a rainwater catchment system at the airport make up the remaining 10 percent of the water supply. The thickness of the freshwater lens in the coastal aquifer system on Saipan ranges from about 20 to 60 feet and many wells produce water with high chloride concentrations. The most efficient means of developing a thin lens is to locate shallow wells where the lens is thickest and to maintain low, uniform pumping rates at each well. On Saipan, saltwater upcoming has contaminated wells that were placed where the lens is too thin, drilled too deep, or operated at pumping rates that withdraw too much water from a small area. Data show that rainfall events coincide with rapid water-level rises and decreases in chloride concentrations, an indication of rapid ground-water recharge and potential for contamination. Analysis of long-term ground-water withdrawal, chloride concentration, well distribution, well construction, and water-level data have resulted in information that the CUC has used to develop new wells fields in the Obyan and Kagman areas. The new well fields produce water with significantly lower chloride concentration than existing well fields in the coastal aquifer system. A conceptual model of the hydrogeology of Saipan has been developed from hydrologic data collected during the study and is being used to calibrate a ground-water model to simulate the effects of drought of the coastal aquifer systems that provide about 90 percent of the water supply on Saipan.
|
| ||||||||||