Integrated Hydrologic Study of the Inarajan and Tinaga River Basins, Inarajan, Guam

Geomatrix has conducted a hydrogeologic assessment of the Inarajan and Tinaga River basins in southern Guam to develop an integrated conceptual hydrologic model of groundwater and surface water interactions in support of potential landfill design and construction. Currently, few hydrologic studies have focused on southern Guam. The model indicates that surface water and groundwater levels are very dynamic in the study area. During below average precipitation years, surface water flow is maintained primarily by groundwater in the dry season and a more balanced contribution of groundwater and precipitation runoff in the rainy season. During above average precipitation years, summer storms interrupt the dry season, resulting in greater surface water flows during the summer months, creating less defined wet and dry seasons. Topography and, to a lesser extent soil type, exhibit the greatest influence on the interactions between surface water and groundwater in the study area. With the onset of the rainy season, headwaters areas for local rivers, exhibiting flatter topography, wetland vegetation, and laterite soils exhibit more rapid rises in groundwater levels than upland areas with more irregular and steeper topography and saprolite soils.