This digital product contains gridded estimates of water-table (wt) elevation and depth to water (dtw) under dry, normal, and wet conditions for Kent County, Delaware. Files containing the point data used to create the grids are also included. This work is the final component of a larger effort to provide estimates of water-table elevations and depths to water for the Coastal Plain portion of Delaware. Mapping was supported by the Delaware Department of Natural Resources and Environmental Control and the Delaware Geological Survey.

These grids were produced with the same multiple linear regression (MLR) method as Andres and Martin (2005). Briefly, this method consists of: identifying dry, normal, and wet periods from long-term observation well data (Hb14-01, Jd42-03, Mc51-01, Md22-01); estimating a minimum water table (Sepulveda, 2002) by fitting a localized polynomial surface to elevations of surface water features (e.g., streams, swamps, and marshes); and, computing a second variable in the regression from water levels observed in wells. A separate MLR equation was determined for dry, normal, and wet periods and these equations were used in ArcMap v.9 (ESRI, 2004) to estimate grids of water-table elevations and depths to water. Kent County was divided into three regions (south, central, north). A minimum water-table surface was calculated for each of these areas and were merged together to create a single minimum water-table surface for the entire county. This grid was filtered and smoothed to eliminate edge effects that occurred at the boundaries between each of the three regions. Water-table elevation and depth to water grids for dry, normal, and wet conditions were then calculated for the county as a whole.

The occurrences of earthquakes in northern Delaware and adjacent areas of Pennsylvania, Maryland, and New Jersey are well documented by both historical and instrumental records. Over 550 earthquakes have been documented within 150 miles of Delaware since 1677. One of the earliest known events occurred in 1737 and was felt in Philadelphia and surrounding areas. The largest known event in Delaware occurred in the Wilmington area in 1871 with an intensity of VII (Modified Mercalli Scale). The second largest event occurred in the Delaware area in 1973 (magnitude 3.8 and maximum Modified Mercalli Intensity of V-VI). The epicenter for this event was placed in or near the Delaware River. Sixty-nine earthquakes have been documented or suspected in Delaware since 1871.

Ground-water recharge potential maps show land areas characterized by their abilities to transmit water from land surface to a depth of 20 feet. The basic methods for mapping ground-water recharge potential are presented in Delaware Geological Survey Open File Report No. 34 (Andres, 1991) and were developed specifically for the geohydrologic conditions present in the Coastal Plain of Delaware. The digital data for this layer comes from DGS Digital Data Product DP 02-01, Digital Ground-Water Recharge Potential Map Data For Kent and Sussex Counties, Delaware: A. S. Andres, C. S. Howard, T. A. Keyser, L. T. Wang, 2002.

Ground water comprises nearly all of the water supply in Kent County, Delaware. The confined aquifers of the area are an important part of this resource base. The aim of this study is to provide an up-to-date geologic framework for the confined aquifers of Kent County, with a focus on their stratigraphy and correlation. Seven confined aquifers are used for water supply in Kent County. All occur at progressively greater depths south-southeastward, paralleling the overall dip of the sedimentary section that underlies the state. The two geologically oldest, the Mount Laurel and Rancocas aquifers, are normally reached by drilling only in the northern part of the county. The Mount Laurel aquifer is an Upper Cretaceous marine shelf deposit composed of clean quartz sands that are commonly glauconitic. It occurs at around 300 ft below sea level in the Smyrna Clayton area and is typically just less than 100 ft thick. Southward, toward Dover, it passes into fine-grained facies that do not yield significant ground water. The Rancocas aquifer is a Paleocene to Eocene marine unit of shelf deposits consisting of glauconite-rich sands with shells and hard layers. It occurs as high as 100 ft below sea level in northwestern Kent County and deepens southeastward, rapidly changing facies to finer-grained, nonaquifer lithologies in the same direction.

Ground-water recharge potential maps support decision-making and policy development in land use, water-resources management, wastewater disposal systems development, and environmental permitting in state, county, and local governments. Recently enacted state law requires that counties and towns with more than 2,000 residents provide protection to areas with excellent recharge potential in comprehensive land use plans. Approximately 14 percent of Kent County and 8 percent of Sussex County have areas with excellent recharge potential. Ground-water recharge potential maps show land areas characterized by the water-transmitting capabilities of the first 20 feet below land surface. Ground-water recharge potential mapping in Kent and Sussex counties was done using geologic mapping techniques and over 6,000 subsurface observations in test borings, wells, borrow pits, natural exposures, and ditches. Hydraulic testing of more than 200 wells shows that the four recharge potential categories (excellent, good, fair, poor) can be used as predictors of the relative amounts and rates at which recharge will occur. Numerical modeling shows that recharge rates in areas with excellent recharge potential can be two to three times greater than rates in fair and poor recharge areas. Because of the association of recharge potential map categories with hydraulic properties, map categories are indicators of how fast contaminants will move and how much water may become contaminated. Numerical modeling of contaminant transport under different recharge potential conditions predicts that greater masses of contaminants move more quickly and affect greater volumes of water under higher recharge potential conditions than under lower recharge potential conditions. This information can be used to help prioritize and classify sites for appropriate remedial action.