Tracking climate variability through chloride concentration in municipal production well histories of the Yigo-Tumon basin, Northern Guam Lens Aquifer

PI: Nathen Habana, Assistant Professor of Groundwater Hydrology, University of Guam
Co-PI: Erin Miller, Environmental Science Graduate Program, University of Guam


Graphic profile view of the aquifer overlying volcanic basement rockUnderstanding the mechanics of water storage in the Northern Guam Lens Aquifer is critical to ensuring the sustainability of freshwater resources on Guam.

Background
    As Guam undergoes economic and strategic development, there is increasing demand on the natural water supplies to support population, economic, and infrastructure growth. Excessive chloride contamination, due to saltwater intrusion into the Northern Guam Lens Aquifer (NGLA), is the primary limit to water production rates, which in turn may limit the island's long term progress. Developing quality production wells, while at the same time limiting chloride contamination, will be a major concern and future challenge. Increasing production may test the limits of individual wells, and sustainable management will require some insights to prevent excessive chloride contamination.
    The NGLA is an unconfined fresh water lens aquifer with a complex multi-pathway recharge system. Vertical wells extract municipal drinking water from the freshwater lens, but are susceptible to saltwater intrusion when over-pumping of the lens draws underlying saline waters upward, resulting in chloride contamination. The threat of saltwater intrusion is lessened when wells are situated further inland where the aquifer overlies the volcanic basement rock. While these more resilient zones are preferable for production wells, they comprise less than 20% of the aquifer's total area. To support the island municipal water needs adequately, it becomes necessary to place wells in the unideal basal zone above the saltwater lens.
    Guam Waterworks Authority and the military are both endeavoring to reduce chloride contamination in production wells in the NGLA. Chloride concentration in exploratory boreholes is one of the early tests that determine a site's viability for production. Thus far, Guam's municipal and military groundwater extraction provides acceptable potable water to businesses and residents. However, Guam expects to continue economic and military growth, potentially straining the resources. Cooperative research, along with team efforts between the civilian and military sectors, will help to ensure the maintainability of our renewable water resource.

Project Summary
    Previous work has demonstrated that the freshwater lens in the NGLA responds to tides, typhoons, seasonal recharge, the El Nino Southern Oscillation (ENSO), and the Pacific Decadal Oscillation (PDO). Simard (2014) studied the geologic factors that contribute to saline up-draw in the NGLA, and also touched on potential hydrologic influences such as sea-level changes and documented the possible correlation of ENSO with changes in salinity in some wells.
    This project expands on Simard's work to update the production well and chloride concentration database for the NGLA. More specifically, the main goal is to examine causes and influences on salinity in production wells in the Yigo-Tumon Basin by exploring how the variability of natural climate cycles, like the wet/dry seasons and ENSO, relate to recharge, production, and salinity in the wells. Although each of the six basins in the NGLA is geologically and hydrologically unique, the study of the Yigo-Tumon Basin may have some implications for the other basins, thus providing critical information for developing sustainable management practices of groundwater resources on Guam, the ultimate purpose of this research.
    To improve our understanding of influences to aquifer salinity and limits to groundwater extraction, we begen with updating and reorganizing the NGLA chloride and production database, data, and charts. Then, complete time series were plotted of chloride and production of wells in the Yigo-Tumon basin against rainfall and other environmental data (e.g. sea-level, temperature), noting their associations with local climate phenomena like wet-dry cycles, extended drought, and major storms. We then identified relationships in relative magnitudes and timing between the salinity and environmental time series, and are working to suggest plausible causal explanations for them. Ultimately, we hope to identify and recommend management practices by which producers and regulators may mitigate or adapt to the effects of unfavorable natural conditions, and suggest best practices for maximizing production, while preserving quality, under favorable conditions.
    Note that the scope of this study is the active production wells in the Yigo-Tumon Basin. Of all the groundwater basins in the NGLA, the Yigo-Tumon Basin is the largest and most intensively developed, making it the logical choice for this study of natural influences in a single aquifer basin. This study is the first of its kind on Guam, completing an in-depth modeling and statistical analysis of a single groundwater basin's municipal production wells and the impacts they have on the groundwater system. It is also the first attempt to quantify the impacts of seasonality on water quality and study the Yigo-Tumon Basin in terms of the regulatory constraints implemented for water quality in a municipal groundwater system.