First State Geology offers news on Delaware geology and water resources, on recent DGS publications, and on DGS staff activities as an online newsletter publication. Interested parties may be placed on the First State Geology mailing list by completing the online signup form, sending an email request to email@example.com, or simply giving us a call. It also is made available as a PDF on the DGS website.
Recent and Historical Groundwater Level Data. Data accessible on this page are a subset of DGS holdings. Click on the chart link to display a hydrograph or the data link to download all observations for the period of record.
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.
One hundred seventy-nine monuments help to mark Delaware's boundaries with Maryland, Pennsylvania, and New Jersey. Although there are only four major boundaries, there are seven boundary lines that make up the confines of the State. They are the east-west boundary, or Transpeninsular Line; the north-south boundary, or the Tangent Line, Arc, and North lines;; the Delaware-Pennsylvania boundary, including the Top of the Wedge Line and the 12-mile Circle; and the Delaware-New Jersey boundary including the 1934 Mean Low Water Line and the Delaware Bay Line. Only the Transpeninsular, Tangent, Arc, North, 12-mile Circle, and 1934 Mean Low Water lines are monumented. The Delaware Bay Line is defined by the navigational
channel. The boundaries described here evolved through long, complex histories (see references). They are based largely on adjudication in England of conflicting claims by the Penns and the Calverts for the Pennsylvania and Maryland colonies.
Map and data listing of all earthquakes with an epicenter within the State of Delaware.
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.
RI71 Internal Stratigraphic Correlation of the Subsurface Potomac Formation, New Castle County, Delaware, and Adjacent Areas in Maryland and New Jersey
This report presents a new time-stratigraphic framework for the subsurface Potomac Formation of New Castle County, Delaware, part of adjacent Cecil County, Maryland, and nearby tie-in boreholes in New Jersey. The framework is based on a geophysical well-log correlation datum that approximates the contact between Upper and Lower Cretaceous sediments. This datum is constrained by age determinations based on published and unpublished results of studies of fossil pollen and spores in samples of sediment cores from boreholes in the study area. Geophysical log correlation lines established above and below the datum approximate additional chronostratigraphic surfaces. The time-stratigraphic units thus defined are not correlated parallel to the basement unconformity, as in previous practice, but instead onlap it in an updip direction. In future studies, the sedimentary facies of the Potomac Formation within each time-stratigraphic layer may be mapped and analyzed as genetically related contemporaneous units. This new stratigraphic framework will allow better delineation of the degree of lateral connection between potential aquifer sands, thus enhancing understanding of aquifer architecture.
This publication formally establishes the Old College Formation, a lithostratigraphic unit located along the Fall Zone of Delaware. It is named for sediments encountered in numerous drill holes on, and adjacent to, the Old College campus of the University of Delaware in Newark, Delaware. The Old College Formation consists of micaceous, brown to reddish-brown, fine to coarse sand with scattered gravelly sand overlain by sandy silt beds. The Old College Formation has a distinctive suite of abundant heavy minerals including sillimanite, staurolite, and magnetite. Provenance of the sand is local, derived from erosion of Piedmont rocks along and just to the west of the Fall Zone. The unit is the result of alluvial fan deposition on a pediment-like surface extending from the Fall Zone to the adjacent Coastal Plain. The Old College Formation is a surficial unit that overlies Piedmont saprolite, the Cretaceous Potomac Formation, and the Pleistocene Columbia Formation. No fossil data are available for the unit. Stratigraphic and geomorphic positions indicate that it ranges from 500,000 to 1,000,000 years old; slightly younger than the Columbia Formation.
Because of the rapid development occurring in coastal Delaware and the importance of ground water to the economy of the area, definition of formal lithostratigraphic units hosting aquifers and confining beds serves a useful purpose for resource managers, researchers, and consultants working in the area. The Pocomoke and Manokin are artesian aquifers pumped by hundreds of domestic and dozens of public wells along the Atlantic coast in Delaware and Maryland. These aquifers are being increasingly used for public water supply. Two formal lithostratigraphic units, the Cat Hill Formation and Bethany Formation, are established to supercede the Manokin formation and Bethany formation, respectively. In Delaware, these lithostratigraphic units host important aquifers—the Manokin, which occurs in the Cat Hill Formation, and the Pocomoke, which occurs in the Bethany Formation. Composite stratotypes of these units are identified in five drillholes located near Bethany Beach, Delaware.
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.
Water supply in the rapidly developing Lewes and Rehoboth Beach areas of coastal Sussex County in Delaware is provided by more than 80 individual public water wells and hundreds of domestic wells. Significant concerns exist about the future viability of the ground-water resource in light of contamination threats and loss of recharge areas. As part of Delaware's Source Water and Assessment Protection Program, wellhead protection areas (WHPAs) were delineated for the 15 largest public supply wells operated by three public water systems. The WHPAs are derived from analysis of results of dozens of steady-state ground-water flow simulations. The simulations were performed with a Visual MODFLOW-based 6-layer, 315,600-node model coupled with GIS-based data on land cover, ground-water recharge and resource potentials, and other base maps and aerial imagery. Because the model was operated under steady-state conditions, long-term average pumping rates were used in the model. The flow model includes four boundary types (constant head, constant flux, head-dependant flux, and no flow), with layers that represent the complex hydrogeologic conditions based on aquifer characterizations. The model is calibrated to within a 10% normalized root mean squared error of the observed water table.
RI61 The Occurrence and Distribution of Several Agricultural Pesticides in Delaware’s Shallow Ground Water
In June 1996, the U. S. Environmental Protection Agency (USEPA) proposed a regulation to require individual states to develop Pesticide Management Plans (PMPs) to protect their ground-water resources from pesticide contamination. The USEPA designated the predominantly agricultural pesticides atrazine, alachlor, cyanazine, metolachlor, and simazine as the first five that would require a PMP.
The surficial Pliocene and Quaternary sedimentary deposits of the Atlantic Coastal Plain of Delaware comprise several formal and informal stratigraphic units. Their ages and the paleoenvironments they represent are interpreted on the basis of palynological and lithologic data and, to a lesser degree, on geomorphology.
The stream-gaging network in Delaware is a major component of many types of hydrologic investigations. To ensure that the network is adequate for meeting multiple data needs by a variety of users, it must represent the range of hydrologic conditions and land-use types found in Delaware, and include enough stations to account for hydrologic variability. This report describes the current stream-gaging network in Delaware and provides an evaluation of its representativeness for the State.
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.
Water samples were collected from 63 wells in southern New Castle County to assess the occurrence and distribution of dissolved inorganic chemicals in ground water. Rapid growth is projected for the study area, and suitable sources of potable drinking water will need to be developed. The growth in the study area could also result in degradation of water quality. This report documents water quality during 1991-92 and provides evidence for the major geochemical processes that control the water quality.