Water from groundwater seepage and springs along the northwest coast of Guam was collected and analyzed bimonthly over an eight-month period to determine water quality characteristics. Additionally, water [samples from] three caves and one open sinkhole along the coast (though not connected at the surface with the sea) were similarly analyzed for comparative purposes. All samples were collected by grab sampling. The shoreline stations were sampled during lower low spring tides. Marine waters approximately five meters outward from the spring collection sites were also analyzed for inorganic nutrients, chlorophylls a, b, c, and carotenoids.

Water quality tests indicate that shoreline seepage averages six to seven percent seawater as based on chloride and sulfate concentrations. The larger springs have two to three percent seawater. Hence, very little dilution takes place with seawater prior to groundwater discharge at the surface. Nitrate-nitrogen concentrations fall within the same range observed for most well groundwater (i.e., 1 to 2 mg/L NO₃-N). Other inorganic nutrients (NH₃-N, NO₂-N, and PO₄-P) are also close to typical groundwater concentrations or only slightly higher. Coliform bacteria were absent or few in number in almost all samples excepting those collected near a sometimes overflowing sewage collection system.

Separate analyses of marine waters adjacent to the groundwater seepage areas found higher chlorophyll and carotenoids concentrations (chlorophyll a averaged 1.0 mg/m³) than observed in waters in the same area removed from shore. Nutrient concentrations, particularly NO₃-N, were frequently in excess of the established water quality standard (0.20 mg/L) for NO₃-N in these waters.