Borehole Oh25-02, located about 3 miles southwest of Lewes, Delaware, ends at a total depth of 1,337 ft in a mid-Oligocene glauconitic silt unit. It penetrated 317 ft of glauconitic sands and silts between the base of the Calvert Formation at a depth of 1,020 ft and total depth. A hiatus at 1,218 ft separates an outer neritic lower Miocene interval (Globorotalia kugleri Zone) above it from a deep upper bathyal mid-Oligocene (G. opima opima Zone) section below; the lower section is characterized by abundant large uvigerinid benthic foraminiferal species representing the transition from Uvigerina tumeyensis to Tiptonina nodifera. Similar uvigerinid assemblages identify the mid-Oligocene unit in boreholes near Bridgeville and Milford, Delaware; Cape May, New Jersey; and Ocean City, Maryland. Updip from these boreholes, the Calvert Formation, of latest Oligocene-middle Miocene age in Delaware, unconformably overlies middle Eocene glauconitic sands of the Piney Point Formation. The juxtaposition of the downdip mid-Oligocene rocks against the updip middle Eocene rocks can best be explained by a fault between the two regions.
The results of water-budget and flow-net model calculations indicate that the rate of fresh ground-water discharge into Rehoboth and Indian River bays is in the range of 21 to 43 million gallons per day. The estimates should be used only as gross indicators of actual conditions because of data gaps and the simplifying assumptions used in the models. However, the estimated discharge rates are significant and useful studies of the water budget of the Bays.
RI41 Hydrogeology and Geochemistry of the Unconfined Aquifer, West-Central and Southwestern Delaware
The unconfined aquifer is the major source of water supply in west-central and southwestern Delaware. The aquifer, which is composed of quartz sand, gravel, clay, and silt, ranges in thickness from 20 to 200 feet. The water table ranges from land surface to about 20 feet below land surface. Analyses of water from wells distributed throughout the area were used to study processes controlling the chemical quality of the water in the unconfined aquifer.
The Delaware Academy of Science has been instrumental in informing Delaware citizens about science and utilization of local resources. Since 1970 the annual meeting of the Delaware Academy of Science has been used as a time for presentation of ongoing research in various areas of science in the Delaware region. The proceedings of these meetings have resulted in publication of transactions of the Delaware Academy of Science. The 1976 annual meeting focused on aspects of the geology of Delaware. Members of the Delaware Geological Survey and the Geology Department at the University of Delaware contributed papers in their specific disciplines. This volume presents an overview of studies of geological features and processes of evolution of the geology of Delaware. Although this collection of papers does not represent an all-inclusive study of the subject, the selections included in this volume highlight past, present, and future trends in the study of Delaware's geology. It is hoped that the combined bibliographies of all the papers will provide a comprehensive view of the literature for further investigation into the geology of Delaware.
Digital watershed and bay polygons for use in geographic information systems were created for Rehoboth Bay, Indian River, and Indian River Bay in southeastern Delaware. Polygons were created using a hierarchical classification scheme and a consistent, documented methodology that enables unambiguous calculations of watershed and bay surface areas within a geographic information system. The watershed boundaries were delineated on 1:24,000-scale topographic maps. The resultant polygons represent the entire watersheds for these water bodies, with four hierarchical levels based on surface area. Bay boundaries were delineated by adding attributes to existing polygons representing water and marsh in U.S. Geological Survey Digital Line Graphs of 1:24,000-scale topographic maps and by dissolving the boundaries between polygons with similar attributes. The hierarchy of bays incorporates three different definitions of the coastline: the boundary between open water and land, a simplified version of that boundary, and the upland-lowland boundary. The polygon layers are supplied in a geodatabase format.
The effect of rapid growth in the Hockessin and Pleasant Hill areas in northern Delaware has caused concern about possible declines in ground-water recharge to the underlying Cockeysville Formation. The Cockeysville is a major source of ground water (aquifer) in the Hockessin area from which about 1.5 million gallons of water per day is withdrawn for public water supply, even though it receives recharge over a relatively small area of 1.6 square miles. The Cockeysville in the Pleasant Hill area is currently used as a source at water supply for individual domestic users and one school. Results of ground-water exploration in the Pleasant Hill area suggest that the Cockeysville is capable of yielding several hundreds of gallons per minute to individual wells for water supply. A two-year investigation was undertaken to map the extent of the Cockeysville Formation and address questions of long-term ground-water yields. the sources of recharge, and the effects of additional development on ground-water supplies. Results of various field studies were integrated to determine the basic geologic framework and those elements that particularly affect ground-water supply.
B16 Ground-Water Resources of the Piney Point and Cheswold Aquifers in Central Delaware as Determined by a Flow Model
A quasi three-dimensional model was constructed to simulate the response of the Piney Point and Cheswold aquifers underlying Kent County, Delaware to ground-water withdrawals. The model included the Magothy, Piney Point, Cheswold, and unconfined aquifers, and was calibrated using historical pumpage and water-level data. Model calibration was accomplished through the use of both steady-state and transient-state simulations.
B14 Hydrology of the Columbia (Pleistocene) Deposits of Delaware: An Appraisal of a Regional Water-Table Aquifer
The Columbia (Pleistocene) deposits of Delaware form a regional water-table aquifer, which supplies about half the ground water pumped in the State. The aquifer is composed principally of sands which occur as channel fillings in northern Delaware and as a broad sheet across central and southern Delaware. The saturated thickness of the aquifer ranges from a few feet in many parts of northern Delaware to more than 180 feet in southern Delaware. Throughout 1,500 square miles of central and southern Delaware (75 percent of the State's area), the saturated thickness ranges from 25 to 180 feet and the Columbia deposits compose all or nearly all of the water-table aquifer.
B13 Geology, Hydrology, and Geophysics of Columbia Sediments in the Middletown-Odessa Area, Delaware
Columbia sediments in the Middletown-Odessa area are composed of boulders, gravels, sands, silts and clays. These sediments are exposed in four gravel pits where their structures and textures were studied. Subsurface geology was interpreted on the basis of the well-log data from 40 holes drilled in the area of study. Columbia sediments were laid upon a surface made up of the greensands of the Rancocas Formation (Paleocene – Eocene age). The contact between the Rancocas and Columbia Formations is an erosional unconformity.
Sussex County is in the Atlantic Coastal Plain. Its relatively flat, featureless topography is characterized by two terrace-like surfaces; the lower one rises from sea level to about 40 feet above sea level, and the higher one rises inland from 40 to about 60 feet above sea level. Peculiar landforms of low relief, broad ovals, similar to the "Carolina bays," and to the "New Jersey basins" are common on the sandy flat divides in Sussex County. Hydrologically, they are sites of much ground-water discharge, by evapotranspiration, from meadow and marsh of lush vegetation.
Northern Delaware, the area above the Chesapeake and Delaware Canal in New Castle County, is an area of rapidly growing population and expanding industry. In some places the demand for water has reached or exceeded the capacity of the existing facilities creating apparent water shortages. Many agencies, both public and private, are attempting to alleviate these shortages; studies are being made and reports prepared for immediate action as well as long-term planning. It is the purpose of this report to examine on a long-range basis the water resources of the northern Delaware area. This examination indicates that the surface-water and groundwater resources of the area far exceed the 72.8mgd (million gallons per day) used during 1955. The amount of ground water potentially available in the area is estimated to be at least 30 mgd and the amount of surface water potentially available depends principally on the amount of storage that may be feasible economically. Storage of 3 million gallons per square mile would provide an allowable draft rate of 140 mgd with a deficiency at average intervals of ten years, while storage of 30 million gallons per square mile would raise the allowable draft to 250 mgd, which is about half of the mean annual discharge. In addition to the fresh-water resources, saline water from the Delaware River and its tidal estuaries is available in almost unlimited quantity for cooling, fire fighting, some types of washing, and other purposes.
Well and aquifer coefficients have been determined for a crystalline rock aquifer system that provides part of the water supply of the City of Newark, Delaware. Conventional analytical methods can be used to derive coefficients for crystalline rocks in the Newark area if the limitations of such methods are recognized and if the local hydrologic framework is known.
Field reconnaissance, geologic mapping, and photogeologic interpretations aided collectively in the identification of 30 potential high-yield well sites in the crystalline rocks of Delaware's western Piedmont. Fracture traces discernable on panchromatic and color infrared photography were identified in the study area. Well locations were selected on individual traces and on fracture trace intersections. Six test wells averaging 468.5 feet in depth were drilled at selected sites. Test analyses indicate that production wells at these sites would have a combined potential estimated at 1.0 to 1.1 million gallons per day of water. A thorough knowledge of the hydrogeologic framework is key to successful ground-water exploration and development. Subsurface fracturing is of prime importance in governing the water-yielding properties in the crystalline rocks. The surface traces of vertical or near-vertical zones of subsurface rock fracture were identified and used as an aid in high-capacity well siting.
Beaverdam Branch, the Nanticoke River, Sowbridge Branch, and Stockley Branch drain small basins in the Delaware Coastal Plain that are characterized by similar climate, topography, geology, and land use. Withdrawals of ground water and surface water are very small, there is little urbanization, and other man-made effects, which include minor regulation on Sowbridge Branch and construction of drainage ditches in the Nanticoke basin, probably have had minimal effect on the natural hydrologic regimen. These are virtually natural-flow streams, which, because of similar basin characteristics, have nearly identical rates of evapotranspiration and runoff. During the 10-year period, 1959-68, precipitation averaged 40-42 inches annually, runoff averaged 16-17 inches annually, and evapotranspiration averaged 23-25 inches annually.