Isotopic and geochemical assessment of the sensitivity of groundwater resources of Guam, Mariana Islands, to intra- and inter-annual variations in hydroclimate

Assessing the sensitivity of groundwater systems to hydroclimate variability is critical to
sustainable management of the water resources of Guam, US territory. We assess spatial and
temporal variability of isotopic and geochemical compositions of vadose and phreatic
groundwater sampled from cave drip sites and production wells, respectively, to better
understand the vulnerability of the freshwater lens on Guam to variability in hydroclimate. We
independently evaluate the existing conceptual model of the Northern Guam Lens Aquifer that is largely based on physical, as opposed to geochemical, observations. Sampling was conducted from 2008 to 2015, over which rainfall gradually increased. Major ion geochemistry and Sr isotope values of groundwater show varying influence from soil, limestone bedrock, and
seawater. Geochemical modeling that can explain spatial variability in groundwater Na+ and
Mg2+ concentrations and Sr/Ca and 87Sr/86 Sr values indicates that groundwater compositions are dominantly controlled by mixing of freshwater with seawater and water-rock interaction.
Differences between amount-weighted annual average precipitation δ18 O values and groundwater
δ18 O values indicate a recharge bias toward the wet season, consistent with other tropical
carbonate island aquifer settings. Intra- and inter-annual variations in Na+ concentrations and
δ18 O values in groundwater reflect sensitivity of recharge to seasonal variations in rainfall
amount and changes in annual rainfall amounts. Our results indicate the influence of multiple
modes of recharge on groundwater compositions and spatial variability in the sensitivity of
groundwater to seawater mixing. This sensitivity of the freshwater lens points to the vulnerability
of groundwater resources to changes in recharge associated with climate, land-use change, and
increases in population.