The Cypress Swamp Formation is the surficial geologic unit in south-central Sussex County, Delaware. Detailed hydrologic observations made as part of four separate studies between 1995 and 1999 show that the Cypress Swamp Formation consists of a complex assemblage of moderately permeable sands and low permeability organic and inorganic silts and clays that form a heterogeneous shallow subsurface hydrologic system that is between about 5 and 15 feet thick. Aquifer tests show that hydraulic conductivity ranges between 0.55 and 40 ft/day, with an arithmetic mean of 13 feet/day.
The Cypress Swamp of Sussex County, Delaware, is underlain by a body of late Pleistocene- to Holocene-age unconsolidated sediments. They form a mappable geologic unit herein named the Cypress Swamp Formation. Deposits of the formation can be found outside the current boundaries of the Cypress Swamp and record the erosion and redistribution of older Pleistocene coastal and Pliocene sedimentary units.
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.
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.
Several common herbicides used on corn and soybeans were detected in ground water at two agricultural sites in Delaware as part of a study of the distribution of herbicides in shallow ground water and the environmental factors affecting their occurrence.
The results of this investigation of the Columbia aquifer in coastal Sussex County, Delaware, provide some of the data necessary to evaluate the condition of the area's primary source of fresh water. Chemical analyses of water samples from domestic, agricultural, public, and monitoring wells document the effects of past and present land use practices. Groundwater flow paths and flow systems are inferred from flow-net analysis, ground-water chemistry, and isotopic composition.
RI45 Effects of Agricultural Practices and Septic-System Effluent on the Quality of Water in the Unconfined Aquifer in Parts of Eastern Sussex County, Delaware
The unconfined aquifer is a major source of water supply in eastern Sussex County, Delaware. It also is an important source of water for surface-water bodies and deeper, confined aquifers. The aquifer consists mainly of permeable sand and gravel; its shallow water table is susceptible to contamination by nitrate and other chemical constituents associated with agricultural practices and effluent from septic systems.
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.
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.