DGS Reports of Investigations

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Analysis And Summary Of Water-Table Maps For The Delaware Coastal Plain
(Newark, DE: Delaware Geological Survey, University of Delaware, 2008) Martin, M.J.; Andres, A.S.
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.
The Cat Hill Formation And Bethany Formation Of Delaware
(Newark, DE: Delaware Geological Survey, University of Delaware, 2004) Andres, A.S.
Because of the rapid development occurring in coastal Delaware and the importance of ground water to the economy of the area, definition of formal lithostratigraphic units hosting aquifers and confining beds serves a useful purpose for resource managers, researchers, and consultants working in the area. The Pocomoke and Manokin are artesian aquifers pumped by hundreds of domestic and dozens of public wells along the Atlantic coast in Delaware and Maryland. These aquifers are being increasingly used for public water supply. Two formal lithostratigraphic units, the Cat Hill Formation and Bethany Formation, are established to supercede the Manokin formation and Bethany formation, respectively. In Delaware, these lithostratigraphic units host important aquifers—the Manokin, which occurs in the Cat Hill Formation, and the Pocomoke, which occurs in the Bethany Formation. Composite stratotypes of these units are identified in five drillholes located near Bethany Beach, Delaware.
The Cypress Swamp Formation, Delaware
(Newark, DE: Delaware Geological Survey, University of Delaware, 2000) Andres, A.S.; Howard, C.S.
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.
Estimate Of Direct Discharge Of Fresh Ground Water To Rehoboth And Indian River Bays
(Newark, DE: Delaware Geological Survey, University of Delaware, 1987-06) Andres, A.S.
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.
Estimating Evapotranspiration for 2016 Growing Season Using Landsat 8 Data and Metric Model in Sussex County, Delaware
(Newark, DE: Delaware Geological Survey, University of Delaware, 2023-08) He, C.; Andres, A.S.; Brinson, K.R.; DeLiberty, T.L.
Evapotranspiration (ET) is a major part of the water cycle. Reliable measurements or estimates of ET can greatly improve quantitative forecasts and hindcasts of water demand by crops, horticulture, and natural vegetation, and also help to manage and conserve water resources. Direct measurement of ET requires not only specific devices such as eddy covariance instruments, but also well-trained research personnel to collect accurate data. As a result, a variety of indirect methods for estimating ET have been developed in recent decades. Among them, remote sensing methods have proved cost-effective in providing accurate regional and global coverage of ET. In Sussex County, Delaware’s leading county in crop production, the ET distribution for the 2016 growing season was estimated using the Mapping Evapotranspiration at high Resolution with Internalized Calibration method, an energy-balance based ET mapping tool that utilizes satellite images and weather data. The estimated result was compared with field measurements using an eddy covariance instrument. The total estimated ET during Sussex County’s growing season (May-September) in 2016 accounts for 77 to 87 percent of historical-averaged annual ET in this region. The model-simulated seasonal ET for agricultural land is about 33 percent higher than urban/suburban areas and about 22 percent lower than forested areas. This study shows that when forestlands are converted to urban/suburban uses, significant amounts of water are diverted from ET and are then available to run off and/or infiltrate. Given that urban/suburban land has impervious surfaces in the forms of rooftops, roads, driveways, parking lots, sidewalks, etc., much of the water not lost to the atmosphere through ET presumably becomes part of the surface runoff portion of the water budget, thus underscoring the need for adequate storm-water management systems for urban/suburban lands. The results also imply that the practice of ET-based irrigation scheduling could be valuable in Sussex County and throughout the 20 percent of Delaware farmland that is irrigated.
Estimation Of The Water Table For The Inland Bays Watershed, Delaware
(Newark, DE: Delaware Geological Survey, University of Delaware, 2005) Andres, A.S.; Martin, M.J.
A geographic information system-based study was used to estimate the elevation of the water table in the Inland Bays watershed of Sussex County, Delaware, under dry, normal, and wet conditions. Evaluation of the results from multiple estimation methods indicates that a multiple linear regression method is the most viable tool to estimate the elevation of the regional water table for the Coastal Plain of Delaware. The variables used in the regression are elevation of a minimum water table and depth to the minimum water table from land surface. Minimum 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 output as a GIS-ready grid with 30-m (98.43 ft) horizontal and 0.305-m (1 ft) vertical resolutions.
Geologic And Hydrologic Studies Of The Oligocene - Pleistocene Section Near Lewes, Delaware
(Newark, DE: Delaware Geological Survey, University of Delaware, 1990-10) Andres, A.S.; Benson, R.N.; Ramsey, K.W.; Talley, J.H.
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.
Geology Of The Seaford Area, Delaware
(Newark, DE: Delaware Geological Survey, University of Delaware, 1996) Andres, A.S.; Ramsey, K.W.; Groot, J.J.
This report supplements the map "Geology of the Seaford Area, Delaware" (Andres and Ramsey, 1995). The map portrays surficial and shallow subsurface stratigraphy and geology in and around the Seaford East and Delaware portion of the Seaford West quadrangles. The Quaternary Nanticoke deposits and Pliocene Beaverdam Formation are the primary lithostratigraphic units covering upland surfaces in the map area. Recent swamp, alluvial, and marsh deposits cover most of the floodplains of modern streams and creeks. The Miocene Choptank, St. Marys, and Manokin formations occur in the shallow subsurface within 300 ft of land surface. The Choptank, St. Marys, and Manokin formations were deposited in progressively shallower water marine environments. The Beaverdam Formation records incision of underlying units and progradation of a fluvial-deltaic system into the map area. The geologic history of the Quaternary is marked by weathering and erosion of the surface of the Beaverdam and deposition of the Nanticoke deposits by the ancestral Nanticoke River. Depositional environments in the Nanticoke deposits include fresh water streams and ponds, estuarine streams and lagoons, and subaerial dunes.
Ground-Water Recharge Potential Mapping In Kent And Sussex Counties, Delaware
(Newark, DE: Delaware Geological Survey, University of Delaware, 2004) Andres, A.S.
Ground-water recharge potential maps support decision-making and policy development in land use, water-resources management, wastewater disposal systems development, and environmental permitting in state, county, and local governments. Recently enacted state law requires that counties and towns with more than 2,000 residents provide protection to areas with excellent recharge potential in comprehensive land use plans. Approximately 14 percent of Kent County and 8 percent of Sussex County have areas with excellent recharge potential. Ground-water recharge potential maps show land areas characterized by the water-transmitting capabilities of the first 20 feet below land surface. Ground-water recharge potential mapping in Kent and Sussex counties was done using geologic mapping techniques and over 6,000 subsurface observations in test borings, wells, borrow pits, natural exposures, and ditches. Hydraulic testing of more than 200 wells shows that the four recharge potential categories (excellent, good, fair, poor) can be used as predictors of the relative amounts and rates at which recharge will occur. Numerical modeling shows that recharge rates in areas with excellent recharge potential can be two to three times greater than rates in fair and poor recharge areas. Because of the association of recharge potential map categories with hydraulic properties, map categories are indicators of how fast contaminants will move and how much water may become contaminated. Numerical modeling of contaminant transport under different recharge potential conditions predicts that greater masses of contaminants move more quickly and affect greater volumes of water under higher recharge potential conditions than under lower recharge potential conditions. This information can be used to help prioritize and classify sites for appropriate remedial action.
Investigation of Submarine Groundwater Discharge at Holts Landing State Park, Delaware: Hydrogeologic Framework, Groundwater Level and Salinity Observations
(Newark, DE: Delaware Geological Survey, University of Delaware, 2017-05) Andres, A.S.; Michael, H.A.; Russoniello, C.J.; Fernandez, C.; He, C.; Madsen, J.A.
Monitoring wells and groundwater sensors were installed and monitored in and around Holts Landing State Park on the Indian River Bay, eastern Sussex County, Delaware, between October 2009 and August 2012. Data from test drilling, geophysical logging, geophysical surveys, and well testing characterized the hydrogeological framework and spatial and temporal patterns of water pressure, temperature, and salinity in the shallow, unconfined Columbia aquifer. The work revealed a plume of freshened groundwater extending more than 650 ft into the bay from the shoreline. Groundwater salinities intermediate between baywater and inland groundwater are present both offshore and on land adjacent to the bay and tidal tributaries. The fresh groundwater plume, as observed in wells and borehole geophysical logs, decreases in thickness from more than 40 ft nearest the shoreline to less than 20 ft farthest from the shoreline. Saline water is found above and below the plume and the freshwater-saltwater interface is spatially complex. Characterization of the hydrogeologic framework was critical to explaining the distribution of fresh groundwater. Fresh water is trapped near the bay bottom by an overlying confining bed composed of the low permeability sediments of a Holocene paleovalley fill sequence and the Beaverdam Formation. This complex, heterogeneous geological framework also causes multiple stacked interfaces in one location at the study site. Groundwater levels, temperatures, and specific conductivity respond to climatic, seasonal, and storm-related weather forcing patterns as well as to forces caused by astronomical tides. The relative importance of these forces to groundwater levels, the flux of fresh groundwater, and groundwater salinity varies with location. Ranges in groundwater levels are more than 6 ft at an inland location and are clearly controlled by seasonal recharge patterns. Extreme weather events have a secondary effect on groundwater levels. In comparison, ranges of groundwater levels are much smaller in near shore and offshore wells, and are more closely related to tidal forces. As a result of this difference in ranges of groundwater levels, seasonal variations in water levels at inland locations are the primary variable controlling bayward-directed groundwater gradients, fresh groundwater flux, and groundwater salinity distribution. Shorter duration weather and tidal events have a secondary role. The freshwater-saltwater interface and associated mixing zone moves upward and/or landward during extended periods of low freshwater flux into the bay, and downward and/or bayward during extended periods of higher freshwater flux.
Investigation of Submarine Groundwater Discharge at Holts Landing State Park, Delaware: Hydrogeologic Framework, Groundwater Level and Salinity Observations
(Newark, DE: Delaware Geological Survey, University of Delaware, 2017-05) Andres, A.S.; Michael, H.A.; Russoniello, C.J.; Fernandez, C.; Madsen, J.A.
Monitoring wells and groundwater sensors were installed and monitored in and around Holts Landing State Park on the Indian River Bay, eastern Sussex County, Delaware, between October 2009 and August 2012. Data from test drilling, geophysical logging, geophysical surveys, and well testing characterized the hydrogeological framework and spatial and temporal patterns of water pressure, temperature, and salinity in the shallow, unconfined Columbia aquifer. The work revealed a plume of freshened groundwater extending more than 650 ft into the bay from the shoreline. Groundwater salinities intermediate between baywater and inland groundwater are present both offshore and on land adjacent to the bay and tidal tributaries. The fresh groundwater plume, as observed in wells and borehole geophysical logs, decreases in thickness from more than 40 ft nearest the shoreline to less than 20 ft farthest from the shoreline. Saline water is found above and below the plume and the freshwater-saltwater interface is spatially complex. Characterization of the hydrogeologic framework was critical to explaining the distribution of fresh groundwater. Fresh water is trapped near the bay bottom by an overlying confining bed composed of the low permeability sediments of a Holocene paleovalley fill sequence and the Beaverdam Formation. This complex, heterogeneous geological framework also causes multiple stacked interfaces in one location at the study site. Groundwater levels, temperatures, and specific conductivity respond to climatic, seasonal, and storm-related weather forcing patterns as well as to forces caused by astronomical tides. The relative importance of these forces to groundwater levels, the flux of fresh groundwater, and groundwater salinity varies with location. Ranges in groundwater levels are more than 6 ft at an inland location and are clearly controlled by seasonal recharge patterns. Extreme weather events have a secondary effect on groundwater levels. In comparison, ranges of groundwater levels are much smaller in near shore and offshore wells, and are more closely related to tidal forces. As a result of this difference in ranges of groundwater levels, seasonal variations in water levels at inland locations are the primary variable controlling bayward-directed groundwater gradients, fresh groundwater flux, and groundwater salinity distribution. Shorter duration weather and tidal events have a secondary role. The freshwater-saltwater interface and associated mixing zone moves upward and/or landward during extended periods of low freshwater flux into the bay, and downward and/or bayward during extended periods of higher freshwater flux.
Kent County Groundwater-Monitoring Project: Results Of Hydrogeological Studies
(Newark, DE: Delaware Geological Survey, University of Delaware, 2023-02) Andres, A.S.; McQuiggan, R.W.; He, C.; McKenna, T.E.
In 2019, the Delaware Geological Survey, in cooperation with the Delaware Department of Natural Resources and Environmental Control, completed a groundwater-monitoring, infrastructure-construction, and data-collection project in Kent County, Delaware. This work, recommended by the Governor’s Water Supply Coordinating Council and funded by a capital appropriation from the state, addressed data gaps for the shallower aquifers commonly pumped by water-supply wells that serve domestic, public, irrigation, and commercial users and provided additional data to characterize the relationships between the aquifers and streamflow. The aquifers investigated in this study are, from shallowest (closest to the surface) to deepest, the Columbia, Milford, Frederica, Federalsburg, Cheswold, Piney Point, Rancocas, and Mt. Laurel. The groundwater-monitoring infrastructure and data created during this project will facilitate follow-up projects targeted to specific issues for the water resources of Delaware. The Piney Point aquifer has a characteristic uncommon among other aquifers in the Coastal Plain of Delaware, in that it receives recharge only through slow, diffuse leakage through overlying and underlying confining beds. As a result, pumping of the Piney Point aquifer in the Dover area has reduced water levels more than 80 feet over the past 50 years in several wells in the Dover area. Given current rates of decline, static water levels in long-term observation well Id55-01 will reach the top of the aquifer within 30 years. Pumping water levels in two supply wells operated by Dover Water are projected to reach the top of the aquifer within 10 years if current rates of decline continue. Given that the Piney Point aquifer matrix contains glauconite, a compressible clay pellet, there is significant risk for aquifer compaction and reductions in well yield should water levels continue to decline. Water-level and water-quality data from nested wells (e.g., multiple wells at the same site finished at different depths) in the Milford, Frederica, Federalsburg, and Cheswold aquifers are recharged primarily in areas where they are in close hydraulic connection with the overlying water table aquifer. Similarities in hydrographs, potentiometric surface maps from these aquifers, and time series of head differentials between the Frederica, Federalsburg, and Cheswold aquifers indicate that they function as a single, leaky, layered aquifer. Pumping has reduced water levels in the Frederica, Federalsburg, and Cheswold aquifers below sea level over large areas of Kent County, and has caused flow directions to change from a general southeasterly direction in pre-development times to flow directed toward pumping centers. Water quality data that show no significant correlation between dissolved solids and well depth support the interpretation that flow directions have changed in response to pumping. Long-term declines in annual minimum total flow and baseflow at streamflow gaging stations in the Beaverdam Branch and Marshyhope Creek watersheds and associated long-term increases in annual precipitation, number of growing days, irrigated acres, and number of irrigation wells in those basins are consistent with the interpretation that the combined effects of irrigation pumping and climate change are reducing groundwater discharge to those streams. Results of testing major groundwater constituents in the water-table portion of Columbia aquifer are consistent with previous studies in Delaware, with calcium and sodium the major cations, and different mixtures of the anions chloride, nitrate, and bicarbonate depending on land use and composition of the aquifer near each well. Major constituents of groundwater in the Milford, Frederica, Federalsburg, and Cheswold aquifers over most of Kent County are dominated by calcium, bicarbonate and sulfate.
Results Of Hydrogeologic Studies Of The Cypress Swamp Formation, Delaware
(Newark, DE: Delaware Geological Survey, University of Delaware, 2002) Andres, A.S.; Howard, C.S.
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.
Results Of The Coastal Sussex County, Delaware Ground-Water Quality Survey
(Newark, DE: Delaware Geological Survey, University of Delaware, 1991-02) Andres, A.S.
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.
Simulation of Groundwater Flow and Contaminant Transport in Eastern Sussex County, Delaware With Emphasis on Impacts of Spray Irrigation of Treated Wastewater
(Newark, DE: Delaware Geological Survey, University of Delaware, 2015-08) He, C.; Andres, A.S.
This report presents a conceptual model of groundwater flow and the effects of nitrate (NO3-) loading and transport on shallow groundwater quality in a portion of the Indian River watershed, eastern Sussex County, Delaware. Three-dimensional, numerical simulations of groundwater flow, particle tracking, and contaminant transport were constructed and tested against data collected in previous hydrogeological and water-quality studies. The simulations show a bimodal distribution of groundwater residence time in the study area, with the largest grouping at less than 10 years, the second largest grouping at more than 100 years, and a median of approximately 29 years. Historically, the principal source of nitrate to the shallow groundwater in the study area has been from the chemical- and manure-based fertilizers used in agriculture. A total mass of NO3- -nitrogen (N) of about 169 kg/day is currently simulated to discharge to surface water. As the result of improved N-management practices, after 45 years a 20 percent decrease in the mass of NO3- -N reaching the water table would result in an approximately 4 percent decrease in the mass of simulated N discharge to streams. The disproportionally smaller decrease in N discharge reflects the large mass of N in the aquifer coupled with long groundwater residence times. Currently, there are two large wastewater spray irrigation facilities located in the study domain: the Mountaire Wastewater Treatment Facility and Inland Bays Wastewater Facility. The effects of wastewater application through spray irrigation were simulated with a two-step process. First, under different operations and soil conditions, evaporation and water flux, NO3- -N uptake by plants, and NO3- -N leaching were simulated using an unsaturated flow model, Hydrus-1D. Next, the range of simulated NO3- -N loads were input into the flow and transport model to study the impacts on groundwater elevation and NO3- -N conditions. Over the long term, the spray irrigation of wastewater may increase water-table elevations up to 2.5m and impact large volumes of groundwater with NO3-. Reducing the concentration of NO3- in effluent and increasing the irrigation rate may reduce the volumes of water impacted by high concentrations of NO3-, but may facilitate the lateral and vertical migration of NO3-. Simulations indicate that NO3- will eventually impact deeper aquifers. An optimal practice of wastewater irrigation can be achieved by adjusting irrigation rate and effluent concentration. Further work is needed to determine these optimum application rates and concentrations.
Simulation of Groundwater Flow in Southern New Castle County, Delaware
(Newark, DE: Delaware Geological Survey, University of Delaware, 2011) He, C.; Andres, A.S.
To understand the effects of projected increased demands on groundwater for water supply, a finite-difference, steady-state, groundwater flow model was used to simulate groundwater flow in the Coastal Plain sediments of southern New Castle County, Delaware. The model simulated flow in the Columbia (water table), Rancocas, Mt. Laurel, combined Magothy/Potomac A, Potomac B, and Potomac C aquifers, and intervening confining beds. Although the model domain extended north of the Chesapeake and Delaware Canal, south into northern Kent County, east into New Jersey, and west into Maryland, the model focused on the area between the Chesapeake and Delaware Canal, the Delaware River, and the Maryland- Delaware border. Boundary conditions for these areas were derived from modeling studies completed by others over the past 10 years.
Southern New Castle – Northern Kent Counties Groundwater Monitoring Project: Results of Subsurface Exploration and Hydrogeological Studies
(Newark, DE: Delaware Geological Survey, University of Delaware, 2018-11) Andres, A.S.; Coppa, Z.J.; He, C.; McKenna, T.E.
The Delaware Geological Survey, in cooperation with the Department of Natural Resources and Environmental Control, completed a groundwater-monitoring, infrastructure-construction, and data-collection project in southern New Castle and northern Kent Counties, Delaware. This work, recommended by the Water Supply Coordinating Council and funded by a capital appropriation from the state, addressed data gaps for the shallower aquifers commonly pumped by water-supply wells that serve domestic, public, irrigation, and commercial users and provided additional data to characterize the relationships between the aquifers and streamflow. The aquifers investigated in this study are, from top to bottom, the Columbia, Rancocas, Mt. Laurel, and Magothy. The groundwater-monitoring infrastructure and data created during this project will continue to serve the management and research needs for water resources of Delaware, and lead to additional follow-up projects and technical reports.
Stratigraphy And Depositional History Of The Post-Choptank Chesapeake Group
(Newark, DE: Delaware Geological Survey, University of Delaware, 1986-08) Andres, A.S.
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.
Thickness And Transmissivity Of The Unconfined Aquifer Of Eastern Sussex County, Delaware
(Newark, DE: Delaware Geological Survey, University of Delaware, 2006) Andres, A.S.; Klingbeil, A.D.
The unconfined portion of the Columbia aquifer is a key hydrologic unit in Delaware, supplying water to many agricultural, domestic, industrial, public, and irrigation wells. The aquifer is recharged through infiltration of precipitation and is the source of fair-weather stream flow and water in deeper confined aquifers. The aquifer occurs in permeable sediments ranging in age from Miocene to Recent. Over most of Delaware, the top of the unconfined or water-table portion of the Columbia aquifer occurs at depths less than 10 feet below land surface. Because of the permeable character of the aquifer and its near-surface location, the unconfined aquifer is highly susceptible to contamination.
Wellhead Protection Area Delineations For The Lewes-Rehoboth Beach Area, Delaware
(Newark, DE: Delaware Geological Survey, University of Delaware, 2003) Andres, A.S.; Duffy, C.A.; Costas, E.M.
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.

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