Geophysical Surveys For Ground Water Exploration In Northern Guam

Abstract

The general geology of northern Guam consists of reef limestones overlying altered volcanic pyroclastics. The limestones are highly porous and permeable which make them excellent aquifers. The volcanics which underlie the limestones consist primarily of pyroclastic volcanics which were transported to their present location by a gravity slide from the side of a submarine volcano, which was located west of the island of Guam. The volcanics are highly deformed as a result of the sliding. These originally glass-rich (no crystalline structure) pyroclastics were diagenetically altered at low temperature to a clay-rich rock. The permeability of the altered pyroclastics is very low and the unit acts as an aquitard, consequently, very little water can be produced from the volcanics. The fresh ground water in this portion of the island occurs in the limestones in two modes: . parabasal - lying on top of the impermeable volcanics; and, l basal - a lens of fresh water floating on denser salt water. In the basal mode, the ground water approximately follows the Ghyben-Herzberg principle which states that the salt water/fresh water interface occurs below sea level at a depth 40 times the elevation of the top of fresh water. For example, a head of 2 ft above sea level for fresh ground water in the basal mode means the salt water/fresh water interface is at an elevation of 80 ft below sea level. Heads of 2.5 to 3.0 ft above sea level are typical for fresh water occurrences in the basal mode in northern Guam, which places the elevation of the fresh water/salt water interface between 100 ft and 120 ft below sea level. Therefore, most ground water occurs below sea level, although some ground water resources perched on volcanics may exist at higher elevations. The limestone/volcanic and fresh water/saline water interfaces are important components in the ground water regime of northern Guam. Thus, it is necessary to map these interfaces to explore for ground water resources, and to understand how the ground water regime will be affected by pumping of ground water. Time Domain Electromagnetic (TDEM) soundings were employed in the surveys because the three units to be delineated (limestones saturated with fresh water; limestones saturated with salt water; and, volcanics) have characteristic ranges of electrical resistivity. The boundaries of these units can be inferred from the geoelectric section. Sample case histories of TDEM surveys in northern Guam will be shown.