Water Resources Research Act Program

Principal Investigators: Dr. Yong Sang Kim

Abstract: Currently, Guam Waterworks Authority produces 90% of the 45 MGD potable water from its main source, the Northern Guam Lens Aquifer (NGLA). GWA is currently managing 100 to 120 deep vertical production wells in this aquifer to meet the demand. However, many of these production wells are deteriorating from age with more than 50 years in operation, thus lifespan is exceeded and maintenance is no longer economically viable, quality, and production has become unsustainable. US EPA, Guam EPA, GWA, and CUC had concurred to a plan of rehabilitating several aged wells. The rehabilitation plan was to drill and install new production wells next to selected wells within property bounds. This was determined considering historic performance of the wells, existing ease of access and available power utility, and the chance probability of replicating the performance of the old well nearby, using similar design specifications from the old well. The NGLA is a composite to complex island karst aquifer with a thick vadose zone, forming an uplifted plateau (> 200ft), and a highly anisotropic and permeable porous media. This aquifer bears a meteoric recharged freshwater lens atop saltwater, divided by a volcanic basement of 6 aquifer basins that form basal and parabasal zones with varying lens thickness in different regions. More than 4000 ft inland, the lens may be greater than 120ft thick, where production wells have been found to be economically viable and hydrologically yielding for municipal utility water supply. In this karst aquifer, however, the average production well rate, limited by updrawn saltwater contamination (USEPA, <250 mg/L), is 220 gpm, ranging100-500 gpm, and some cases (suprabasal zones) up to 1 MGD. Development of useful wells (> 150 gpm) in this aquifer poses some risks.The USEPA funding of the rehabilitation project yielded deep drilling and construction, with careful recording and documentation in tests and examination of seven new potential production wells. In the construction process, WERI served to contribute advice and recommendations, sharing data from the Guam Hydrologic Survey website (www.guamhydrologicsurvey.com) borehole database (WERI Technical Report 141), chloride and production database (WERI Technical Report 143), and the Northern Guam Lens Aquifer map (WERI Technical Report 142), working closely with the contractors (Allied Pacific Drilling Inc., APDI; AECOM) with exchange and examination of data. GEPA required the contractors to log and submit proper documentation, well video inspection, and well test (pump test production rate, water quality, drawdown) records. WERI, GWA, APDI, AECOM, and GEPA collaborated in recommending modification of plans such as deeper drilling to improve well production rate, and water quality (salinity < 250 mg/L). The collaboration and interagency team work have proven to be the right approach in all endeavors for a successful outcome in the project. This experience has also brought about things we can improve upon for future production well expansion. WERI deems that it is undoubtedly necessary and highly valuable to compile, organize, and share the lessons learned from this interagency experience to further improve groundwater development on this island, and for other similar aquifers.Objectives: The objectives of this project are:(1) Data acquisition from the GWA well rehabilitation project(2) Organize the data according to hydrogeologic issues, contamination, and well construction(3) Apply necessary analysis to categorize well development issues (4) Recommend rehabilitation procedure in a manual and checklist, methods for use of available data, and expert group communications (management, drillers, and experts)