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Browsing by Author "McKenna, T.E."

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Characterization Of The Potomac Aquifer, An Extremely Heterogeneous Fluvial System In The Atlantic Coastal Plain Of Delaware
(Newark, DE: Delaware Geological Survey, University of Delaware, 2004) McKenna, T.E.; McLaughlin, P.P.; Benson, R.N.
Fluvial sands of the subsurface Cretaceous Potomac Formation form a major aquifer system used by a growing population in the northern Coastal Plain of Delaware. The aquifer is extremely heterogeneous on the megascopic scale and connectivity of permeable fluvial units is poorly constrained. The formation is characterized by alluvial plain facies in the updip section where it contains potable water. While over 50 aquifer tests indicate high permeability, the formation is primarily composed of fine-grained silt and clay in overbank and interfluvial facies. Individual fluvial sand bodies are laterally discontinuous and larger-scale sand packages appear to be variable in areal extent resulting in a labyrinth style of heterogeneity. The subsurface distribution of aquifers and aquitards has been interpreted within a new stratigraphic framework based on geophysical logs and on palynological criteria from four cored wells. The strata dip gently to the southeast, with generally sandy fluvial facies at the base of the formation lapping onto a south-dipping basement unconformity. The top of the formation is marked by an erosional unconformity that truncates successively older Potomac strata updip. Younger Cretaceous units overly the formation in its downdip area. In the updip area, the formation crops out or subcrops under Quaternary sands.The fine-grained facies include abundant paleosols that contain siderite nodules and striking mottling that commonly follows ped faces and root traces. These paleosols may serve as regional aquitards. This geologic complexity poses a challenge for determining the magnitudes and directions of ground-water flow within the aquifer that are needed for making informed decisions when managing this resource for water supply and contaminant remediation.
Characterization of Tidal Wetland Inundation in the Murderkill Estuary
(Newark, DE: Delaware Geological Survey, University of Delaware, 2018-03) McKenna, T.E.
A parameterization of tidal marsh inundation was developed for the 1,200 hectares of tidal marsh along the 12-km reach of the tidal Murderkill River between Frederica and Bowers Beach in Kent County, Delaware. A parsimonious modeling approach was used that bridges the gap between the simple and often used “bathtub model” (instantaneous inundation based on tides in Delaware Bay), and the more complex modeling of shallow overland that results in the wetting and drying of tidal marshes. For this project, and many other modeling studies that include large areas of marsh, a complex modeling approach of marsh inundation is not warranted due to the lack of data on the dynamics of wetting and drying. A simple parameterization of the wetland inundation process coupled with more complex hydrodynamic and water-quality models can provide sufficient results for estimating the extent of hydrologic and biogeochemical interactions between a marsh and a river. The parameterization can also be used to evaluate anomalies in conservation of water mass and tidal phase offsets that can result from hydrodynamic models that do not explicitly model the dynamic flow and storage of water in tidal wetlands. In the parameterization, the marsh was divided into marsh tracts (n=31) based on hydrologic character and position along the river. A cumulative probability distribution of wetland elevation was calculated for each marsh tract from a digital elevation model. These cumulative probability distributions served as a simplification of the critical information contained in the raster data sets of marsh tracts and elevation. Each marsh tract was related to an adjacent river reach; the area in the tract that was below the stage of its related river reach was instantaneously inundated. Marsh tracts were aggregated into two sets of marsh groups (n=22 and n=4) for analysis and visualization of elevation, hydroperiod, and hydraulic loading. The parameterization was successfully implemented in a collaborative modeling study that created a set of mass loading functions to represent the import and export of chemical species to and from the wetlands. The parameterization was also used to evaluate conservation of water mass and phase offsets in tidal discharge due to the dynamic storage of water in intertidal areas. Marsh elevations had a normal distribution with a mean elevation of 0.72 m and standard deviation of 0.19 m based on analysis of LiDAR data collected for this study. These values have a potential positive bias of 0.1 to 0.2 m resulting from the LiDAR beam not penetrating through the marsh vegetation. Nominal relief on the marsh at the scale of the study area was about 0.6 m (0.4 to 1 m absolute elevation using the NAVD88 datum). From Bowers Beach upstream to Frederica there was a decrease in marsh elevation with the mean elevation decreasing from 0.86 m to 0.60 m. This observation is consistent with measured accretion rates at four sites in the study area that document higher accretion rates upstream near Frederica (0.74 cm/yr) relative to downstream near Bowers (0.33 cm/yr). Upstream marshes are flooded more frequently and for longer duration than downstream marshes so there is more opportunity for accretion to occur.
Digital Watershed And Bay Boundaries For Rehoboth Bay, Indian River Bay, And Indian River
(Newark, DE: Delaware Geological Survey, University of Delaware, 2007) McKenna, T.E.; Andres, A.S.; Lepp, K.P.
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.
Evaluating Impacts of Sea-Level Rise on Groundwater Resources in The Delaware Coastal Plain
(Newark, DE: Delaware Geological Survey, University of Delaware, 2023-06) He, C.; McKenna, T.E.
Due to low elevation and a shallow water table, the Delaware Bay coast is highly vulnerable to sea-level rise. Numerical simulations of rising sea levels, groundwater flow, and salt transport through year 2100 indicate significant impacts on land use due to a rising water table and localized impacts due to saltwater intrusion in the surficial aquifer. Impacts from changes in watertable depths were defined as the conditions where the water table rose above two critical depths: 0 meters (termed saturation, waterlogging, or inundation) and 0.5 meters (effective rooting depths of major local crops). Scenarios modeled were for 0.5, 1.0, and 1.5 meters rise by year 2100. Simulations used SEAWAT4, a three-dimensional, variable-density groundwater flow model. We constructed synthetic conceptual and numerical models with a single rectangular-shaped watershed with an upland, one river, and bay-parallel and inland salt marshes. Parameters for the models were based on the characteristics of ten Delaware Bay watersheds. We transferred water-table depths from simulations to real-world watersheds by mapping model coordinates to a curvilinear grid system within each watershed, which allowed for comparison of areas adversely impacted by sea-level rise by comparing water-table depths to the critical depths. The simulation results predict that sea-level rise causes significant impacts from a rising water table by year 2100. Over 60 percent of the impacted area in all scenarios was cropland. The model results also indicate that the saltwater front under the riverbed migrates landward as far as 4.8 kilometers from its initial location, but is limited to a small area near and parallel to the river and marsh boundaries.
Groundwater Monitoring Procedures Part 1: Equipment and Procedures for Manual and Automated Field Measurement of Groundwater Levels in Dedicated Monitoring Wells
(Newark, DE: Delaware Geological Survey, University of Delaware, 2018-08) Andres, A.S.; He, C.; McKenna, T.E.
The Delaware Geological Survey (DGS) has measured, managed, and distributed groundwaterlevel data for several decades using widely accepted procedures and practices, many of which were derived from interactions with staff of the USGS, consulting firms, and other state agencies. Many of the individual methods and procedures have been described in DGS reports, however, written documentation for these tasks have not been assembled in a single published document. The need for such a document has become more apparent with the development of standards for participation in the National Ground-Water Monitoring Network (SOGW, 2009). This document describes methods used by the DGS for routine manual and automated measurement of groundwater levels in dedicated monitoring wells. Alternative methods used for manual measurement of water levels in other types of wells are noted in this document to provide reference for historical measurements but not described in detail. These methods are excerpted and modified from procedures described by federal agencies and national standards organizations (e.g., ASTM, D4750-2007; Drost, 2005; USEPA, 2007). In this document, the term water levels will be used interchangeably with groundwater levels. Please refer to these and other appropriate documents for additional guidance or contact DGS staff with specific questions. Practices pertaining to data processing and management, metadata, and quality assurance procedures for electronic data are rapidly evolving. Additional sections on these topics will be added to this document as time and resources permit.
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.
Locating Ground-Water Discharge Areas In Rehoboth And Indian River Bays And Indian River, Delaware Using Landsat 7 Imagery
(Newark, DE: Delaware Geological Survey, University of Delaware, 2008) Wang, L.T.; McKenna, T.E.; DeLiberty, T.L.
Delaware’s Inland Bays in southeastern Sussex County are valuable natural resources that have been experiencing environmental degradation since the late 1960s. Stresses on the water resource include land use practices, modifications of surface drainage, ground-water pumping, and wastewater disposal. One of the primary environmental problems in the Inland Bays is nutrient over-enrichment. Nitrogen and phosphorous loads are delivered to the bays by ground water, surface water, and air. Nitrogen loading from ground-water discharge is one of the most difficult to quantify; therefore, locating these discharge areas is a critical step toward mitigating this load to the bays. Landsat 7 imagery was used to identify ground-water discharge areas in Indian River and Rehoboth and Indian River bays in Sussex County, Delaware. Panchromatic, near-infrared, and thermal bands were used to identify ice patterns and temperature differences in the surface water, which are indicative of ground-water discharge. Defining a shoreline specific to each image was critical in order to eliminate areas of the bays that were not representative of open water. Atmospheric correction was not necessary due to low humidity conditions during image acquisition. Ground-water discharge locations were identified on the north shore of Rehoboth Bay (west of the Lewes and Rehoboth Canal), Herring and Guinea creeks, the north shore of Indian River, and the north shore of Indian River Bay near Oak Orchard.
Results Of Trenching Investigations Along The New Castle Railroad Survey-1 Seismic Line, New Castle, Delaware
(Newark, DE: Delaware Geological Survey, University of Delaware, 2002) McLaughlin, P.P.; Baxter, S.J.; Ramsey, K.W.; McKenna, T.E.; Strohmeier, S.A.
Five trenches were excavated to a depth of 5 to 8 ft along the path of an abandoned railroad grade near the city of New Castle to investigate potential near-surface faults that may be related to earthquake activity in northern Delaware. Seismic reflection profiles along this line suggested the existence of significant faulting in the area, which lies along a postulated fault trend in eastern New Castle County. Subsequent drilling, however, failed to substantiate displacement interpreted for faults in the sedimentary section. Detailed examination of exposures in the trenches also failed to reveal the existence of near surface faults. Together these findings suggest that there has been minimal or no modern near-surface fault activity in this area of New Castle County.
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.

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