Rising and highstands of sea level during the middle to late Pleistocene deposited swamp to nearshore sediments along the margins of an ancestral Delaware Bay, Atlantic coastline, and tributaries to an ancestral Chesapeake Bay. These deposits are divided into three lithostratigraphic groups: the Delaware Bay Group, the Assawoman Bay Group (named herein), and the Nanticoke River Group (named herein). The Delaware Bay Group, mapped along the margins of Delaware Bay, is subdivided into the Lynch Heights Formation and the Scotts Corners Formation. The Assawoman Bay Group, recognized inland of Delaware’s Atlantic Coast, is subdivided into the Omar Formation, the Ironshire Formation, and the Sinepuxent Formation. The Nanticoke River Group, found along the margins of the Nanticoke River and its tributaries, is subdivided into the Turtle Branch Formation (named herein) and the Kent Island Formation.

Delaware Bay Group deposits consist of bay-margin coarse sand and gravel that fine upward to silt and silty sand. Beds of organic-rich mud were deposited in tidal marshes. Near the present Atlantic Coast, the Delaware Bay Group includes organic-rich muds and shelly muds deposited in lagoonal environments.

Assawoman Bay Group deposits range from very fine, silty sands to silty clays with shells deposited in back-barrier lagoons, to fine to coarse, well-sorted sands deposited in barriers and spits.

Nanticoke River Group deposits consist of coarse sand and gravel that fine upward to silty clays. Oyster shells are found associated with the clays in the Turtle Branch Formation. Organic-rich clayey silts were deposited in swamps and estuaries. Well-sorted fine sands to gravelly sands were deposited on beaches and tidal flats on the flanks of the ancestral Nanticoke River and its tributaries.

The Lynch Heights, Omar, and Turtle Branch Formations are age-equivalent units associated with highstands of sea level,which occurred at approximately 400,000 and 325,000 yrs B.P. (MIS 11 and 9, respectively). The Scotts Corners, Ironshire, Sinepuxent, and Kent Island Formations are age-equivalent units associated with highstands of sea level, which occurred between 120,000 and 80,000 yrs B.P. (MIS 5e and 5a, respectively).

Radiocarbon dates from 231 geologic samples from the offshore, coastal, and upland regions of Delaware have been compiled along with their corresponding locations and other supporting data. These data now form the Delaware Geological Survey Radiocarbon Database. The dates range from a few hundred years to approximately 40,000 yrs (40 ka) BP (before present). All dates younger than about 18,000 yrs have been calibrated using the method of Stuiver and Reimer (1993). A plot of the dates versus the elevations of the samples shows four distinct groupings: those associated with the rise of sea level during the Holocene, those from the uplands, those in modem stream valleys, and those older than the detectable range of present radiocarbon techniques. A fifth group of samples in the 20-38 ka range and from below present sea level are ambiguous and were previously used as evidence for a mid-Wisconsinan high sea stand (Milliman and Emery, 1968).

This digital product contains gridded estimates of water-table (wt) elevation and depth to water (dtw) under dry, normal, and wet conditions for New Castle County, Delaware excluding the Piedmont. 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 (Db24-01, Hb14-01); 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. Separate MLR equations were 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. New Castle County was divided into a northern section and a southern section with the C&D Canal being the natural line of demarcation. A minimum water-table surface was then calculated for both the northern and southern sections of New Castle County. However, dividing the county, as well as the water-level data, into two sections did not result in sufficient regression coefficients for use in the estimation process. Therefore, the data (minimum water-table surface and water-level data) were merged together and the water-table elevation and depth to water grids for dry, normal, and wet conditions were then calculated for the county as a whole.

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.

This digital product contains gridded estimates of water-table (wt) elevation and depth to water (dtw) under dry, normal, and wet conditions for Sussex 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 (Nc45-01, Ng11-01, Qe44-01); 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. Grids produced in this project were merged with those previously completed for eastern Sussex and smoothed to minimize edge effects.

The Nanticoke Watershed Water-Quality Database (NWWWQDB) is used to
store, manage, and retrieve water-quality data generated by the “Nanticoke River
Watershed” project. The database contains information on sampling stations, samples,
and field and laboratory analyses, queries to extract and analyze data, forms to input and
edit data, a main menu to navigate to forms and specific queries, and a few formatted
report templates. The database is in Microsoft Access 2003 format. Table, field, and table
relationship metadata are stored in the database as properties of those objects. The
software's metadata reporting options can be used to view the information.

The Delaware Inland Bays Water-Quality Database (DIBWQDB) is used to store,
manage, and retrieve water-quality data generated by the “Nutrient Inputs as a Stressor
and Net Nutrient Flux as an Indicator of Stress Response in Delawares’ Inland Bays
Ecosystem” (CISNet) and the “Inland Bays Tributary Total Maximum Daily Load”
(IBTMDL) projects. It contains information on sampling stations, samples, and field and
laboratory analyses, queries to extract and analyze data, forms to input and edit data, a
main menu to navigate to forms and specific queries, and a few formatted report
templates. The database is in Microsoft Access 2003 format. Table, field, and table
relationship metadata are stored in the database as properties of those objects. The
software's metadata reporting options can be used to view the information.

The stratigraphy of the Coastal Plain of Delaware is discussed with emphasis placed upon an appraisal of the stratigraphic nomenclature. A revised stratigraphic column for Delaware is proposed. Rock stratigraphic units, based mainly on data from certain key wells, are described and the published names which have been or which might conceivably be applied to those units are reviewed. In each case a name is chosen and the reasons for the choice are stated. The relationships between the column established for Delaware and the recognized columns for adjacent states are considered. The rock units of the Coastal Plain of New Jersey, Delaware, and Maryland form an interrelated mass. However, profound facies changes do occur, particularly in the dip direction, but also along the strike. Thus, attempts to extend units established in the outcrop belt almost indefinitely into the subsurface have been unsatisfactory.

Exploration for sand resources for beach nourishment has led to an increase in the amount of geologic data available from areas offshore Delaware's Atlantic Coast. These data are in the form of cores, core logs, and seismic reflection profiles. In order to provide a geologic context for these offshore data, this cross section has been constructed from well and borehole data along Delaware's Atlantic coastline from Cape Henlopen to Fenwick Island. Placing the offshore data in geologic context is important for developing stratigraphic and geographic models for predicting the location of stratigraphic units found offshore that may yield sand suitable for beach nourishment. The units recognized onshore likely extend offshore to where they are truncated by younger units or by the present seafloor.

Three multichannel, common-depth-point (CDP), seismic reflection profiles were run off Delaware's coast for the Delaware Geological Survey. Their purposes were (1) to determine the depth to the unconformity (post-rift unconformity) at the base of the nearshore submerged Coastal Plain sedimentary rocks and (2) to relate onshore with offshore
geology as interpreted from the U. S. Geological Survey's network of regional seismic profiles. In addition, the nearshore profiles reveal considerable detail about the nature of the Neogene lithostratigraphic units and aquifers within them that supply water to the coastal communities of Delaware and Maryland (Miller, 1971; Weigle and Achmad, 1982).

Thousands of homeowners in Delaware currently rely on individual wells and water systems to provide water. In addition, hundreds of new wells and systems are constructed each year to provide water for those not served by public water systems. Methods used to construct water wells in Delaware are discussed in DGS Information Series No. 2 (Domestic Water Well
Construction). Domestic water systems are described herein.

Because of its "renewability" water is unique among earth resources that sustain and enhance life. No other mineral resource that we extract on a long-term and continuous basis can be counted on for at least some degree of replenishment within a human lifetime. This attribute allows a great deal of flexibility in management of the resource. In Delaware local rainfall, approximately 40" to 44" per year, renews part or all of our water supply on a regular basis. However, not all of the rain that falls is available for use. From this total rainfall must be subtracted the water that evaporates (about 20"/ year), the amount that is used by plants (about 3"/year), and the amount that runs overland to surface streams during storms (about 4"-5"/year). The remainder, approximately 13" to 15" is Delaware's bank of water for the year. This water is stored in a system of ground-water reservoirs, or aquifers, that underlie most of the State. Not only do these ground-water reservoirs provide water to wells but they also maintain the flow in surface streams during times of no rainfall. Streamflow between rainfall events is nothing more than the discharge of
excess ground water.

The storage and movement of ground water depends on the types of rocks and associated
interconnected spaces in which the water occurs. The Piedmont Province in northernmost
Delaware is underlain by crystalline rocks. Because of the massiveness and hardness of such
rocks, they yield little or no interstitial water to wells. Water is stored in and moves through fractures, cracks, and solution cavities. The amount of water available depends on the number and size of openings, and the degree to which they are interconnected. Wells drilled in the Piedmont range from 100 to 400 feet in depth and yields are highly variable over very short distances.

In the Coastal Plain, the rest of the State, ground water is stored and transmitted in spaces between adjacent rock particles. As much as 30 percent of the rock mass may be saturated. Unconsolidated rocks are analogous to a bathtub filled with sand into which water is poured. The Coastal Plain consists of sandy water-bearing units referred to as aquifers interlayered between non-water-bearing units. Wells constructed for domestic use range in depth from 15 feet to 500 feet. Yields are generally much greater than those obtained from the crystalline rocks of the Piedmont. In general, minimum well yields of 3 to 5 gallons per minute are adequate for most domestic water supply systems.

The unconfined portion of the Columbia aquifer is a key hydrologic unit in Delaware, supplying water to many agricultural, domestic, industrial, public, and irrigation wells. The aquifer is recharged through infiltration of precipitation and is the source of fair-weather stream flow and water in deeper confined aquifers. The aquifer occurs in permeable sediments ranging in age from Miocene to Recent. Over most of Delaware, the top of the unconfined or water-table portion of the Columbia aquifer occurs at depths less than 10 feet below land surface. Because of the permeable character of the aquifer and its near-surface location, the unconfined aquifer is highly susceptible to contamination.

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.