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.
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 Calvert Formation, deposited in a shallow sea during the late Oligocene and early to middle Miocene (15-27 million years ago), contains a very rich fossil microflora, both in terms of number of specimens and number of species. Most abundant are pollen of oak, pine, and hickory, but exotic taxa (those that no longer occur in Delaware) are present in all samples of this formation. They include pollen of Engelhardia type, Manilkara, Planera (water elm), Alangium(?), and palms. All of these exotics are genera of subtropical or tropical regions, some occurring now in Central America, Florida, and east Asia. The climate during the deposition of the Calvert Formation was probably subtropical and moist.
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.
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.
Subsurface temperatures were measured in instrumented boreholes for about one and one-half years at depths down to 10 feet below land surface at four locations in the State. In New Castle County, temperatures were measured periodically in the field about twice a month at three sites, and, in Sussex County, they were automatically recorded every 15 minutes at one site. The depths of interest are generally in the unsaturated zone and are subject to both daily temperature fluctuations and longer seasonal changes.
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.
Water-level records from 19 observation wells in Delaware for the period January 1978 - December 1987 provide the bases for analyses of water-level fluctuations. Water levels in shallow water-table wells generally rise from November to March when recharge exceeds discharge and decline during the warm growing season from May through September. Although water levels in individual water-table wells fluctuated by as much as 11.72 feet during the 10-year period studied, the water-table system remained in a state of dynamic equilibrium and exhibited no significant changes in aquifer storage.
Onshore and offshore geological and geophysical data were used to investigate the lithostratigraphy, seismic stratigraphy, and depositional history of the late Tertiary age post-Choptank Chesapeake Group rocks in Sussex County, Delaware and adjacent counties in Maryland. The results of this investigation suggest that the St. Marys (?) Formation and the sandy interval of which the Manokin aquifer is a part, are distinct lithostratigraphic units. The Manokin formation is proposed as an informal lithostratigraphic unit that refers to the sandy interval of which the Manokin aquifer is a part. On a regional scale, the section containing the Ocean City and Pocomoke aquifers and adjacent and intervening confining beds is best treated as a single undifferentiated lithostratigraphic unit. The Bethany formation is proposed as an informal lithostratigraphic unit that refers to this section.
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.