Defining and Evaluating Production Capacity for an Island Aquifer

The Northern Guam Lens Aquifer (NGLA) provides 90% of Guam’s drinking water. Recent modeling results provided insights into how the existing water-production system might respond to new development and natural changes in recharge, but local policy makers and water managers have also asked “What is the ultimate volume of water that could be sustainably withdrawn from the aquifer if we had the best possible production system?” Answering this question requires, first, specifying an idealized production system—which we define as A notional production system that would employ existing technology and realign well locations to utilize the most favorable hydrologic locations of the aquifer, in the interest of enhancing production volume, water quality—or both—within a specified standard of salinity. Second, it requires reliable knowledge of the natural limits imposed by recharge and aquifer properties. We present preliminary results from a modeling study directed at estimating total production rates that could be achieved by about the same number as in the present system, set at the same depth, and pumping at the same average rate as the existing system, but in which well locations have been selected to utilize the most productive parts of the aquifer. Natural limits are imposed by specifying the same recharge and aquifer properties employed in the model of the existing system. Although an ideal production system as defined above is not likely achievable in practice, an estimate of the total production that could be obtained by such a system, for a specified salinity standard, provides helpful insights for long-term planning and future decisions regarding sustainable management of the NGLA.