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.
A multiple linear regression method was used to estimate water-table elevations under dry, normal, and wet conditions for the Coastal Plain of Delaware. The variables used in the regression are elevation of an initial water table and depth to the initial water table from land surface. The initial water table is computed from a local polynomial regression of elevations of surface-water features. Correlation coefficients from the multiple linear regression estimation account for more than 90 percent of the variability observed in ground-water level data. The estimated water table is presented in raster format as GIS-ready grids with 30-m horizontal (~98 ft) and 0.305-m (1 ft) vertical resolutions. Water-table elevation and depth are key facets in many engineering, hydrogeologic, and environmental management and regulatory decisions. Depth to water is an important factor in risk assessments, site assessments, evaluation of permit compliance data, registration of pesticides, and determining acceptable pesticide application rates. Water-table elevations are used to compute ground-water flow directions and, along with information about aquifer properties (e.g., hydraulic conductivity and porosity), are used to compute ground-water flow velocities. Therefore, obtaining an accurate representation of the water table is also crucial to the success of many hydrologic modeling efforts. Water-table elevations can also be estimated from simple linear regression on elevations of either land surface or initial water table. The goodness-of-fits of elevations estimated from these surfaces are similar to that of multiple linear regression. Visual analysis of the distributions of the differences between observed and estimated water elevations (residuals) shows that the multiple linear regression-derived surfaces better fit observations than do surfaces estimated by simple linear regression.
This map shows the saturated thickness of the water-table aquifer. This aquifer consists of the deposits of the Columbia Formation and those portions of the Magothy and Englishtown-Mt. Laurel formations, and Rancocas Group that are hydraulically connected with the Columbia deposits (see Groot, Demicco, and Cherry, 1983). For example, large, saturated thicknesses in the zone trending northeast-southwest near Townsend reflect the addition of the sands of the Rancocas Group to the total thickness of the sands of the overlying Columbia Formation.
The purposes of the study described in this report are (1) to determine the total amount of fresh ground water (chloride content less than 150 milligrams/liter) available in New Castle County south of the Chesapeake and Delaware Canal, and (2) to map the geographic distribution of available fresh ground water on the basis of areas delineated by one minute of latitude and one minute of longitude (such areas measure essentially one square mile). The investigation has been based solely on data available in various publications and in the files of the Delaware and United States Geological Surveys.
Numerical indicators, or indices, are widely used to measure the status of complex relationships. As such, indices have become accepted by researchers and the public in such disparate fields as economics, air quality, and weather. In this paper we explore the formulation of an indicator of water conditions in northern Delaware, propose formulas that may be applicable, and test those proposals against long-term records of basic data. The need for a simple indicator of water supply conditions in Delaware, and especially in New Castle County, has become increasingly apparent. The Delaware River Basin Commission (DRBC) has applied an index to the Delaware River Basin, which includes a portion of Delaware. The Governor's Drought Advisory Committee has sought an objective means of determining when water supply conditions might warrant conservation measures. Discussions of the subject have also been held within the State Comprehensive Water Management Committee. We are pleased to acknowledge the constructive comments of these groups and of other colleagues with whom we have discussed this work. George R. Phillips of the Delaware Department of Natural Resources and Environmental Control (DNREC) was especially helpful in analyzing the practical implications of using the index presented in this paper. John R. Mather, Delaware State Climatologist, provided Palmer Drought Severity Index values with the cooperation of the National Weather Service. This report was reviewed by Richard N. Benson and John H. Talley of the Delaware Geological Survey (DGS).
The nature and occurrence of subsurface resources, whether ground water, minerals, or petroleum, are controlled by the geologic history and framework of any particular area. Several years ago the staff of the Delaware Geological Survey began an informal assessment of the potential resources of southern Delaware and demonstrated the lack of basic data on the deep subsurface in this area. This assessment was later summarized by Benson (1976) with particular emphasis on the possibilities for petroleum occurrence.
Southern New Castle County is dependent on ground water for nearly all of its water supply. The area has been undergoing development from predominately agricultural land use to urban/suburban land use (Delaware Water Supply Coordinating Council [WSCC], 2006). With this development comes a need to more accurately predict the availability of ground water to reduce the potential of overusing the resource. This report has 3 plates listed as separate files.
The Delaware Geological Survey conducted a review of existing ground-water quality data collected from shallow (less than 100 feet deep) domestic water-supply wells and small public water-supply wells (serving fewer than 100 residents) to determine the extent to which toxic and carcinogenic compounds are present in the shallow ground water serving domestic water supply wells. These data were generated by several agencies including the Delaware Geological Survey, U.S. Geological Survey, Delaware Department of Natural Resources and Environmental Control, Delaware Division of Public Health Office of Drinking Water, and the Delaware Department of Agriculture Pesticide Management Program.
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.
OFR44 Storm-Water and Base-Flow Sampling and Analysis in the Delaware Inland Bays Preliminary Report of Findings 1998-2000
This report provides initial research results of a storm-water and base-flow sampling and analysis project conducted by the University of Delaware College of Marine Studies (CMS) and the Delaware Geological Survey (DGS). Base-flow samples were collected from six tributary watersheds of Delaware’s Inland Bays on 29 occasions from October 1998 to May 2000. Water samples were filtered in the field to separate dissolved nutrients for subsequent analysis, and a separate sample was collected and returned to the laboratory for particulate nutrient determinations. On each sampling date, temperature, conductivity, pH, and dissolved oxygen concentrations were determined at each sampling station. Stream discharge measurements at each of these sites were made by the U.S. Geological Survey (USGS) under a joint-funded agreement with the Delaware Department of Natural Resources and Environmental Control (DNREC) and the DGS. Together, the nutrient and discharge data were used to determine the total and unit (normalized to watershed area) nutrient loading from base flow to the Inland Bays from each of these watersheds on a quarterly and annual basis. At the same six stations, storm water was collected during eight storms from May 1999 to April 2000. Storm-water loadings of nutrients from each watershed were calculated from the concentrations of nutrients in water samples collected at fixed time intervals from the beginning of the storm-water discharge period until recession to base flow. These data provide DNREC with a more complete picture of the seasonal dependence of nutrient loading to the Bays from which to establish goals for total maximum daily loads in the Inland Bays watershed.
Inspection of water analyses on file at the Delaware Geological Survey revealed that 25 percent of the shallow wells yield water with nitrate concentrations approaching or in excess of the Delaware State Board of Health and U. S. Public Health Service limit of 45 parts per million (ppm). Nitrate concentrations greater than 45 ppm seem to be detrimental to the health of infants during their first few months of life; adults drinking the same water are not affected but breast-fed infants of mothers drinking such water may become ill. The illness ("blue baby sickness" or methemoglobinemia) results from the conversion of nitrate to nitrite by nitrite-forming bacteria in the upper part of the digestive tract of some infants and the further conversion of hemoglobin to methemoglobin which is incapable of transporting oxygen; the result is oxygen starvation. Little is known about the low level effect of undetected methemoglobinemia on infants.
Data from three streamflow water-quality stations were statistically analyzed to determine the relationships of the major inorganic chemical constituents to specific conductance and to stream discharge. The results show that ion concentrations varied directly with the flow and with specific conductance. A set of regression equations defining these relationships were derived for each of the three stations: Brandywine Creek at Wilmington, St. Jones River at Dover, and Nanticoke River near Bridgeville.
Distilled water spiked with heavy metal cations was passed at a rate of 2-4 ml/min through a filter composed of greensand containing about 80 percent glauconite. The capability of the greensand to trap metal cations is increased by prolonging the contact time between the leachate and the greensand. Flushing the charged greensand filter with water does not cause significant release of cations back into solution, suggesting that polluted greensand might be disposed in landfills without adding pollutants to either ground or surface water in the vicinity.
OFR35 Estimate of Nitrate Flux to Rehoboth and Indian River Bays, Delaware, through Direct Discharge of Ground Water
Agricultural fertilizer application, animal (poultry) waste, and wastewater disposal practices of the past 40 years have resulted in widespread nitrate contamination of ground water in coastal Sussex County, Delaware. Discharge of contaminated ground water to Rehoboth and Indian River bays is suspected of being a significant contributor to elevated nutrient concentrations in these surface water bodies, resulting in excessive phytoplankton growth and other related problems.
This report documents the development of a methodology for mapping ground-water recharge areas in Delaware's Coastal Plain. It is anticipated that the methodology presented herein will evolve as it is applied to other areas in the State and as computerized geographic information systems become more widely available. This report deals with methodology; the recharge area maps generated in the course of the research are available for review at the DGS.
OFR33 Ground-Water Level and Chemistry Data from the Coastal Sussex County, Delaware, Ground-Water Quality Survey
This report contains the supporting ground-water level and chemistry data and the data collection methodology for Delaware Geological Survey Report of Investigations No. 49, "Results of the Coastal Sussex County, Delaware, Ground-Water Quality Survey" (Andres, 1991). Because raw data are commonly requested, they are published here in open-file format to accommodate those needing it for further analysis.
Ground water is Delaware's most important natural resource. Our aquifers, which are present everywhere in Delaware, provide more than 75 million gallons each day for all uses. Nearly all of the water used in Delaware south of the Chesapeake and Delaware Canal is obtained from aquifers, both water-table and artesian. An appreciable quantity of water is also obtained from aquifers in northern New Castle County. Ground water has generally been of good quality, been used with little or no treatment, and has been readily available at low cost.
OFR28 Potential for Ground-Water Recharge in the Coastal Plain of Northern New Castle County, Delaware
This map was constructed primarily to indicate the possibilities for artificial recharge into both the surficial sediments of Quaternary age (exclusive of soils) and the older, immediately underlying sediments. However it can also be used to determine where natural recharge might be entering the ground most readily in those areas relatively free from impermeable cover. The surficial sediments include micaceous sands and gravels in the vicinity of the Fall Line derived from underlying crystalline rocks, Holocene marsh deposits, Delaware River sediments, and the Columbia Formation of Pleistocene age. The Columbia Formation is composed of poorly sorted sands with some gravels, silts and occasional clays. The unit is one of the most important ground-water reservoirs in New Castle County.
This map shows the saturated thickness of the Columbia Formation. The Columbia Formation covers most of the Coastal Plain of Delaware. Because it consists primarily of coarse sand, it is important to the hydrology of the area. It is an important groundwater reservoir and in most places water must pass through it to reach deeper units. The water budget of the Columbia Formation also influences runoff and baseflow components of streamflow. The saturated thickness was determined through interpretation of data in publications and files of the Delaware Geological Survey, United States Geological Survey, and the Water Resources Center of the University of Delaware. The thicknesses shown on the map represent the best judgment of the authors based on available data. Detailed investigations of specific sites will require additional data.