DIGITAL WATER-TABLE MAPPING IN DELAWARE

Water-table elevation and depth to ground water are important variables in many engineering, hydrogeologic, and environmental management and regulatory evaluations. Depth to water information is crucial for many environmental risk and site assessments. The importance of having readily available depth to water data was acknowledged in the 1950s and 1960s with the creation and publication of a statewide water-table elevation map, which was included in the Hydrologic Atlas series published by the United States Geological Survey (USGS). In recent years, the increasing usage of geographic information systems (GIS) in environmental management and land-use decision making has led to several attempts to get the Hydrologic Atlas maps into a suitable digital form. As a result of these efforts, there has been an increasing recognition of the need to update these maps. In order to facilitate this need, we developed a water-table elevation surface by using a multiple linear regression equation to calculate the surface based on two specific variables. The first variable is the minimum water-table surface, which is estimated from the elevations of surface water features. The second variable is the depth to the minimum water table from land surface. These two variables along with two numerical coefficients, calculated from a statistical analysis performed on groundwater levels, are inserted into a regression equation to produce the water-table elevation surfaces for dry, normal, and wet conditions. The estimated water-table elevation surfaces are in the form of GIS grids with 30-m horizontal and 1-ft vertical resolutions. Depth to water surfaces are then calculated by subtracting the water-table elevation grids from the 30-m land surface DEM. A water-table elevation surface will be completed for the entire Coastal Plain area of Delaware (~5,000-square kilometers). Currently approximately 4,000-square kilometers have been completed.