The geology and hydrology of the area between Wrangle Hill and Delaware City, Delaware, have been the focus of numerous studies since the 1950s because of the importance of the local groundwater supply and the potential environmental impact of industrial activity. In this report, 490 boreholes from six decades of drilling provide dense coverage, allowing detailed characterization of the subsurface geologic framework that controls groundwater occurrence and flow.

The region contains a lower section of tabular Cretaceous strata (Potomac, Merchantville, Englishtown, Marshalltown,and Mount Laurel Formations in ascending order) and a more stratigraphically complex upper section of Pleistocene-to-modern units (Columbia, Lynch Heights, and Scotts Corners Formations, latest Pleistocene and Holocene surficial sediments and estuarine deposits). The lowermost Potomac Formation is a mosaic of alluvial facies and includes fluvial channel sands that function as confined aquifer beds; however, the distribution of aquifer-quality sand within the formation is extremely heterogeneous. The Merchantville Formation serves as the most significant confining layer. The Columbia Formation is predominantly sand and functions as an unconfined aquifer over much of the study area.

To delineate the distribution and character of the subsurface formations, densely spaced structural-stratigraphic cross sections were constructed and structural contour maps were created for the top of the Potomac Formation and base of the Columbia Formation. The Cretaceous formations form a series of relatively parallel strata that dip gently (0.4 degrees) to the southeast. These formations are progressively truncated to the north by more flatly dipping Quaternary sediments, except in a narrow north-south oriented belt on the east side of the study area where the deeply incised Reybold paleochannel eroded into the Potomac Formation.

The Reybold paleochannel is one of the most significant geological features in the study area. It is a relatively narrow sandfilled trough defined by deep incision at the base of the Columbia Formation. It reaches depths of more than 110 ft below sea level with a width as narrow as 1,500 ft. It is interpreted to be the result of scour by the sudden release of powerful floodwaters from the north associated with one or more Pleistocene deglaciations. Where the Reybold paleochannel cuts through the Merchantville confining layer, a potential pathway exists for hydrological communication between Columbia and Potomac aquifer sands.

East of the paleochannel, multiple cut-and-fill units within the Pleistocene to Holocene section create a complex geologic framework. The Lynch Heights and Scotts Corners Formations were deposited along the paleo-Delaware River in the late Pleistocene and are commonly eroded into the older Pleistocene Columbia Formation. They are associated with scarps and terraces that represent several generations of sea-level-driven Pleistocene cut-and-fill. They, in turn, have been locally eroded and covered by Holocene marsh and swamp deposits. The Lynch Heights and Scotts Corners Formations include sands that are unconfined aquifers but complicated geometries and short-distance facies changes make their configuration more complex than that of the Columbia Formation.

Ground-water levels are basic information needed for evaluating water conditions and for basic and applied research. For these efforts, water levels are being measured statewide in wells completed in multiple aquifers. Some wells are measured for specific projects, such as the Coastal Aquifers Salinity Project and the Water Conditions program, while other wells are measured so that staff can maintain long term records of ground-water levels for evaluation of trends. Table contains summary data from wells having 100 or more water level observations.

Groundwater levels are basic information needed for evaluating water conditions and for basic and applied research. For these efforts, water levels from various aquifers are being measured statewide. Some wells are measured for specific reasons, such as for the Coastal Aquifers Salinity Project and the Water Conditions Report, while other wells are measured so that staff can maintain long-term records of groundwater levels for evaluation of trends.

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.

Compilation and review of data used for model input revealed gaps in hydraulic properties, pumping, aquifer and confining bed geometry, and water-level data. The model is a useful tool for understanding hydrologic processes within the study area such as horizontal and vertical flow directions and response of aquifers to pumping, but significant data gaps preclude its use for detailed analysis for water resources management including estimating flow rates between Delaware and adjacent states. The calibrated model successfully simulated groundwater flow directions in the Rancocas and Mt. Laurel aquifers as expected from the conceptual model. Flow patterns in the Rancocas and Mt. Laurel aquifers are towards local streams, similar to flow directions in the Columbia (water table) aquifer in locations where these aquifers are in close hydraulic connection.

Water-budget calculations and simulated heads indicate that deep confined aquifers (Magothy and Potomac aquifers) receive groundwater recharge from shallow aquifers (Columbia, Rancocas, and Mt. Laurel aquifers) in most of the study domain. Within shallow aquifers, groundwater moves toward major streams, while in the deep aquifers, groundwater moves
toward major pumping centers.

This report presents a new time-stratigraphic framework for the subsurface Potomac Formation of New Castle County, Delaware, part of adjacent Cecil County, Maryland, and nearby tie-in boreholes in New Jersey. The framework is based on a geophysical well-log correlation datum that approximates the contact between Upper and Lower Cretaceous sediments. This datum is constrained by age determinations based on published and unpublished results of studies of fossil pollen and spores in samples of sediment cores from boreholes in the study area. Geophysical log correlation lines established above and below the datum approximate additional chronostratigraphic surfaces. The time-stratigraphic units thus defined are not correlated parallel to the basement unconformity, as in previous practice, but instead onlap it in an updip direction. In future studies, the sedimentary facies of the Potomac Formation within each time-stratigraphic layer may be mapped and analyzed as genetically related contemporaneous units. This new stratigraphic framework will allow better delineation of the degree of lateral connection between potential aquifer sands, thus enhancing understanding of aquifer architecture.

In recent years there has been a renewed interest by both amateur and professional paleontologists in the rich upper Cretaceous exposures along the Chesapeake and Delaware Canal, Delaware (Fig. 1). Large quantities of fossil material, mostly clams, oysters, and snails have been collected as a result of this activity. Recent dredging (1978, 1981) by the United States Army Corps of Engineers has helped expose a rich vertebrate fossil assemblage. It includes representatives from the classes Reptilia, Osteichthyes, and Chondrichthyes. An extensive literature search has revealed that a wealth of information exists which would aid in the identification of the vertebrate fossils of Delaware.

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.