Water Resources Research Act Program

Principal Investigators: Dr. Nathan Habana

Abstract: A concerning major pathway for contaminant transport into limestone aquifers is through sinkholes. The Northern Guam Lens Aquifer (NGLA) has a surface plateau formed from uplifted and tilted geologic formations of limestone bedrock. This aquifer is also the island’s vital source of fresh water. It has a deep vadose zone (80-120 m) that filters much of the percolating infiltrated rainfall. However, during storms that often bring intense rainfall, infiltration rate is exceeded, and surface runoff occurs. The porous karst terrain on the plateau has surface depressions in a mesh of enclosed basins, basins within basins, and cascading basins. These basins serve as sites where surface runoff may pond and serve as cumulative and secondary sources of contaminants into the catchment low points. Some of these surface depressions have turned out to be sinkholes (collapsed and doline) and fractures that conduct or drain large volumes of runoff into the freshwater aquifer. Proper land development must take into consideration the pathways in which anthropogenic contaminants may be transported to water sources and drawn up in near production well drawdown zones or in down gradient production wells.Guam Environmental Protection Agency (GEPA) is keen on regulating development over hydrologically significant sinkholes, fractures, and surface depressions. They require hydrogeological assessment and site inspection in these sensitive areas. However, initial assessment via existing maps of closed contour depressions used to determine potential sinkholes were derived from old topography and has proven to have major inaccuracies and limitations. The development of a new map using the latest lidar based digital elevation (1 m raster resolution) and GIS hydrologic spatial analysis should greatly improve the island’s stormwater management practices over the aquifer. The map analysis provided by this proposed project will provide an important aquifer map series that will be used to assess groundwater protection zones. The need for an improved sinkhole analysis has been an important concern discussed in the WERI Guam Advisory Council Meetings. Governmental policy makers, officials from Guam Coastal Management, GEPA, and Guam Waterworks Authority (GWA), and those involved in the US Army Corps of Engineers, Silver Jackets, Stormwater Management Program all agree that this is a high priority research need.Methods: This proposed project will consist in the advanced application of Environmental Systems Research Institute (ESRI) ArcView GIS Spatial Analysis Tools to model the hydrology, of the NGLA surface features. Hydrologic tools will be applied to the available high-resolution ( 1 meter) Light Detection and Ranging (LIDAR) based Digital Elevation Model (DEM). Basin delineation, Flow Accumulation, Flow Direction, Fill, Sink , and Map Algebra tools will be applied to delineate sink hole configurations. We will configure the resulting raster gradients and contrasts to reveal the best perspective of the sinkhole terrain. We will also delineate parent basins using the results from the flow direction grids. One final product will be an overlay of the sink hole defined features over existing hydrogeologic features on the already published NGLA Map.Specific objectives of the proposed project include: 1)Apply hydrologic spatial analysis on the latest digital LIDAR based elevation map,2)Organize pertinent hydrologic spatial information and overlay hydrogeologic boundaries from the NGLA map including identified sinkholes, faults, and coastal springs,3)Characterize terrain, features such as karrenfelds, dolines, or collapsed sinkholes4)Organize a map grid layout, succeeding enlarged map pages, and a large format map plate,5)Provide training for regulating agencies (GEPA, and Department of Land Management (DLM)) on how to use the map and useful GIS files that will be made available.