Geophysical Surveys For Ground Water Exploration In Northern Guam

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