WERI Technical Report 143 is the last report of the hydrogeologic studies trilogy in support of a USGS groundwater model of the NGLA for the US Navy, Guam.

Salinity conditions within the NGLA are dynamic, and the snapshot of the aquifer obtained from the production well chloride samples and monitoring well specific conductance measurements provide limited but important insights into those dynamics. It is important to keep in mind that the NGLA consists of a heterogeneous triple porosity system (matrix, fracture, and conduit porosity), and groundwater conditions at one well may not apply even a few meters away at another well. As shown in Table 20, a hydraulic connection to brackish water or seawater is considered a probable cause for all production wells, but is especially applicable to those production wells with greater than 150 mg/l chloride. Another probable cause applicable to all production wells is non-seawater sources, as indicated by increasing chloride concentrations at production wells in the supra-basal groundwater zone. Other sources of chloride potentially affecting the NGLA include sea spray, salt aerosols forming condensation nuclei in rainfall and carried by winds, salinization of soils from irrigation and agricultural practices, and leakage from septic systems, industrial waste, and infrastructure containing chlorine-treated potable water. Read more >>