HUNTING FOR HIGH-LEVEL GROUNDWATER ON TUTUILA, AMERICAN SAMOA

On the island of Tutuila, in the territory of American Samoa, there is ongoing concern for the sustainability of groundwater resources. The island has two distinct hydrologic regions: a pristine mountainous interior, and a developed plain which provides 90% of the island’s drinking water from a thin freshwater lens. In the plain, septic tanks, cesspools and piggeries lie proximal to drinking water wells in a highly permeable unconfined aquifer. The alternative to continued water development in the contamination prone plain is to seek high-level groundwater resources. Existing groundwater models cannot assess this resource as they are focused on the freshwater lens aquifer only and have not modeled high level groundwater as a separate system. In American Samoa drilling costs are high and utilities have tight budgets, thus it is imperative for water managers to be equipped with accurate groundwater models prior to subsurface investigation. Calibration is difficult, however, as no monitoring wells exist on Tutuila and production wells are not located within high level aquifers. This work will present a novel method for model calibration based on field observations and water sample geochemistry.

Perennial springs & seeps are generally understood to be expressions of the local water table and can be therefore be used to calibrate groundwater head. This methodology has not been employed before due to uncertainties in vertical accuracy, however, by geo-referencing points to a new high resolution (1m) DEM, elevations can be brought within acceptable uncertainty. In order to determine if spring waters are from aquifers instead of recent precipitation, samples are analyzed for radon gas. Radon is not significant in surface water or rain, however, it is easily dissolved into groundwater from recoil and diffusion from minerals. Isotopic ratios of water samples ²H/¹H & ¹⁸O/¹⁶O are also compared to those in local precipitation to seasonally bracket recharge timing, since winter precipitation has a lighter isotopic signature. Once calibrated the groundwater model can be used to investigate climate change effects on water resources, nutrient & pollution sources, and how land use affects groundwater quality.