A preliminary study of the hydrogeology of northern Guam

Ayers, Jerry; Clayshulte, Russell

This report is a summary of work completed within the Hydrogeology Sector. Hydrogeology can be defined as the relationship between geological features of a sequence of rock units and the occurrence and movement of groundwater. Hydrogeologic investigations normally include a wide variety of exploratory techniques and analytical methods to determine magnitude and distribution of aquifer properties, type and location of aquifer boundaries, composition and physical characteristics of the porous media, structural fabric of rock units, and any other hydraulic aspects of the system that may affect groundwater movement. Field exploration techniques often employed in hydrogeologic investigations include surface geophysical methods (primarily seismic-refraction surveys and earth-resistivity soundings), bore-hole geophysical applications, geological mapping and detailed subsurface rock sampling (from exploratory drilling operations), pump and tracer tests, and periodic or continuous flow-system monitoring (both hydraulic and chemical aspects). Field data, once collected, are subject to a variety of analytical approaches which include megascopic and microscopic examination of rock samples, and statistical and mathematical data manipulations. Interpretations based on both field and analytical results lead to an understanding of the sometimes complex interaction between the solid rock environment and the enclosed hydrologic regime.