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.
Southern New Castle County has a land area of 190 square miles in north-central Delaware. It is predominantly a rural area with a population of about 9,000 people who are engaged chiefly in agriculture. By and large, the residents are dependent upon ground water as a source of potable water. This investigation was made to provide knowledge of the availability and quality of the ground-water supply to aid future development. The climate, surface features, and geology of the area are favorable for the occurrence of ground water. Temperatures are generally mild and precipitation is normally abundant and fairly evenly distributed throughout the year. The topography of the area is relatively flat and, hence, the streams have low gradients. The surface is underlain to a considerable depth by highly permeable unconsolidated sediments that range in age from Early Cretaceous to Recent.
The purpose of this investigation was to obtain data on and study the factors affecting the salinity of the Delaware River from Philadelphia, Pa., to the Appoquinimink River, Del. The general chemical quality of water in the estuary is described, including changes in salinity in the river cross section and profile, diurnal and seasonal changes, and the effects of rainfall, sea level, and winds on salinity. Relationships are established of the concentrations of chloride and dissolved solids to specific conductance. In addition to chloride profiles and isochlor plots, time series are plotted for salinity or some quantity representing salinity, fresh-water discharge, mean river level, and mean sea level. The two major variables which appear to have the greatest effect on the salinity of the estuary are the fresh-water flow of the river and sea level. The most favorable combination of these variables for salt-water encroachment occurs from August to early October and the least favorable combination occurs between December and May.
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.
Delaware has an abundant supply of ground water of a quality suitable for most purposes. About 30 million gallons of water a day was pumped from the ground in 1954. It is estimated that this is roughly 1/16 of the optimum yield. This water is derived from nine groups or series of water-bearing units and is obtained from wells which yield as much as 1,100 gallons per minute. Thousands of wells serve agriculture, industry, municipalities, and domestic users. Geographically, Delaware is situated along the Atlantic coast of the United States in two physiographic provinces: the Piedmont and the Coastal Plain. The Piedmont is a belt of rolling foothills of the Appalachian Mountains. It is separated from the Coastal Plain by the Fall Line, a narrow zone of rapids or falls along which rivers and creek descend rapidly from the mature valleys of the Piedmont to the sluggish tidal estuaries of the coastal area. The Coastal Plain is a flat or gently undulating plain of relatively low altitude, which borders the Atlantic Ocean and its estuarine embayments.
B2 Geology and Ground-Water Resources of the Newark Area, Delaware with a Section on the Surface Water Resources
This report describes the geological and lithological conditions in the Newark area, and the occurrence, quantity, and quality of the available ground-water supply. Newark is located on the Fall Line, the boundary between the rolling hills of the Piedmont on the north and the gentle slopes of the Coastal Plain on the south. Because the Piedmont is underlain by dense crystalline rocks and their weathered clayey soils, which are of low water-bearing capacity in contrast to the more permeable silts and sands of the Coastal Plain, the exploration for ground water was confined to the Coastal Plain south and southeast of Newark.
Geophysical logging techniques have been used in Delaware for many years as a means of identification and correlation of Coastal Plain formations. Criteria for the recognition of those formations having distinctive types of logs are presented. Formation factors have been calculated using multiple-point resistivity logs, temperature logs, and ground-water quality data and range from 1.2 to 6.8 for various formations underlying the State. Formation factors in turn are used to estimate water quality in later test holes.
This poster shows three different map views of the water table as well as information about how the maps were made, how the depth to water table changes with seasons and climate, and how the water table affects use and disposal of water. The map views are of depth to the water table, water-table elevation (similar to topography), and water-table gradient (related to water flow velocity).
Emphasis is placed herein on the years of Dr. Groot's leadership of the Survey. The remarkable work of James C. Booth in the last century is acknowledged but has elsewhere been entered in history. Some continuing activities of the Survey after 1969 are noted together with comments of an experienced observer; this current period may someday receive the attention of a recorder having the enhanced perspective of time.
The following report of the geological survey of the state of Delaware, conducted in the years 1837 and 1838, embraces all the observations and examinations which were made during the continuance of the survey, including those contained in the first and second annual reports, already laid before the legislature.
This is a brief story about water and the ways in which the Delaware Geological Survey helps insure that you will always have a plentiful supply of this precious natural resource.
Additional sources of ground water have been located in the Piedmont Province as a result of a ground-water exploration program conducted by the Delaware Geological Survey at the University of Delaware in cooperation with the City of Newark. Drilling sites for relatively high-yielding wells were located through the use of geophysical investigations, air-photo interpretation, field mapping, and review of existing data.
The increasing population of the State of Delaware is placing severe strains on the quality of ground water in the water-table aquifer by disposing of septic-tank effluent in the soil. At the same time the water resources of this aquifer are being used in greater amounts. The permeable water-table aquifer, containing reserves of 331 million gallons per day, is very vulnerable to contamination by objectionable or toxic fluids and dissolved substances placed on or in the soil.
The results of an intensive ground-water study on University of Delaware lands in the Newark area revealed additional sources of available ground water. Geophysical techniques, air-photo interpretation, studies of existing data, field mapping, test drilling, and pump tests were used as the bases for guiding additional well development. The study, conducted by the Delaware Geological Survey, was a cooperative effort between the University of Delaware and the City of Newark in response to mutual water supply problems. A potential ground-water yield of about 500 gpm was discovered on the University Laird Tract in the Piedmont Province. Ground water available from other locations in the Coastal Plain portion of the study area may total about 175 gpm. However, careful well development and proper well spacing will be necessary to obtain optimum yields.
The need for locating additional sources of ground water for the Delaware Atlantic seashore, a predominantly recreation-oriented area, is indicated by an expanding population in the belt between Philadelphia, Pennsylvania and Washington, D.C., combined with increasing leisure time. Present water use in the shore area is approximately 4 million gallons per day and will reach 9.3 million gallons per day by the year 2000. A new geologic interpretation of the occurrence of deep aquifers in the Delaware Atlantic seashore area is presented. Recent data from deep wells has enabled the construction of a more accurate geologic framework upon which the hydrologic data are superimposed. Correlation of Miocene sands concludes that the Manokin aquifer lies at greater depths in southeastern Delaware than previously thought.
Information on ground-water quality in Delaware has become critical for three reasons: (1) increased water demand, (2) need for a better understanding of ground-water flow patterns, (3) need for a "base" against which future quality changes can be measured. Analyses of about 150 water quality samples from wells show that Delaware's fresh ground waters are suitable for most purposes. High iron content may occur, however, in wells tapping the Columbia and the Potomac formations. Overall, total dissolved solids in Delaware aquifers are relatively low except in the Cheswold and Frederica aquifers (Miocene), and possibly parts of the Piney Point Formation (Eocene).
This report discusses the occurrence of ground water in relation to certain problems in highway construction and maintenance. These problems are: the subdrainage of roads; quicksand; the arrest of soil creep in road cuts; the construction of lower and larger culverts necessitated by the farm-drainage program; the prevention of failure of bridge abutments and retaining walls; and the watercement ratio of sub-water-table concrete. Although the highway problems and suggested solutions are of general interest, they are considered with special reference to the State of Delaware, in relation to the geology of that State. The new technique of soil stabilization by electroosmosis is reviewed in the hope that it might find application here in road work and pile setting. Field application by the Germans and Russians is reviewed.