Hydrogeology of the Finegayan Basin, Northern Guam Lens Aquifer, Guam
Monday, December 4, 2017: 4:50 p.m.
The Finegayan Basin of the Northern Guam Lens Aquifer is already well developed, but is expected to undergo additional economic development in the near future, with expansion of US military activities on Guam. The purpose of this study was to better understand the natural plumbing that controls groundwater recharge, transmission, and discharge of the basin, preliminary to exploration for new groundwater production wells. A field survey showed that the single greatest concentration of freshwater discharge from the northwest coast issues from a coastal cave that lies precisely at the end of a major fault. An initial estimate suggested discharge from the cave of up to 5.3 Mgal/day, which would constitute of 32% of basin recharge. Given the 4.8 Mgal/day of withdrawal from the basin, discharge of 5.3 Mgal/day would further constitute 45% of the remaining total basin discharge of 11.7 Mgal/day. The calculated hydraulic gradient along the fault for 5.3 Mgal/day discharge, assuming hydraulic conductivity of 75,000 m/d along the fault, a minimum width of 10 m for the conductive zone, and an average thickness of 30 m for the freshwater lens, is 9 x 10-4. This is consistent with a regional hydraulic gradient of about 3 x 10-4 estimated from previous modeling studies. The estimate of 5.3 Mgal/day for this single discharge point also compares reasonably with a recent finite-element model estimate of 31 Mgal/day discharge from the coastal zone centered around the cave. Additional, more-sophisticated field measurements of discharge are recommended to test the accuracy of the 5.3 Mgal/day estimate for the cave discharge. This presentation offers a hypothesis for the influence of the fault on the drainage of the Finegayan Basin and makes concomitant recommendations for revision of the basin boundaries to reflect the hypothesized influence of the fault, based on the Carbonate Island Karst Model.