RI79 Simulation of Groundwater Flow and Contaminant Transport in Eastern Sussex County, Delaware With Emphasis on Impacts of Spray Irrigation of Treated Wastewater
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.
This vector data set contains the rock unit polygons for the surficial geology in the Delaware Coastal Plain covered by DGS Geologic Map Series No. 18 (Bethany Beach and Assawoman Bay Quadrangles). The geologic history of the surficial units of the Bethany Beach and Assawoman Bay Quadrangles is that of deposition of the Beaverdam Formation and its subsequent modification by erosion and deposition related to sea-level fluctuations during the Pleistocene. The geology reflects this complex history onshore, in Indian River Bay and Assawoman Bay, and offshore in the Atlantic Ocean. Erosion during the late Pleistocene sea-level lowstand and ongoing deposition offshore and in Indian River Bay during the Holocene rise in sea level represents the latest of several cycles of erosion and deposition. An additional dataset of datapoints used to generate rock unit polygons for the surficial geology in the Delaware Coastal Plain covered by DGS Geologic Map Series No. 18 (Bethany Beach and Assawoman Bay quadrangles) exists for use in conjunction with this dataset.
The geologic history of the surficial units of the Bethany Beach and Assawoman Bay Quadrangles is that of deposition of the Beaverdam Formation and its subsequent modification by erosion and deposition related to sea-level fluctuations during the Pleistocene. The geology reflects this complex history onshore, in Indian River Bay and Assawoman Bay, and offshore in the Atlantic Ocean. Erosion during the late Pleistocene sea-level lowstand and ongoing deposition offshore and in Indian River Bay during the Holocene rise in sea level represents the latest of several cycles of erosion and deposition.
The composition, thickness, and geophysical log signature of the Bethany Formation vary with location and depth. In general, the Bethany Formation is a sequence of clayey and silty beds with discontinuous lenses of sand (Andres, 1986; Ramsey, 2003). The most common lithologies are silty, clayey fine sand; sandy, silty clay; clayey, sandy silt; fine to medium sand; sandy, clayey silt, and medium to coarse sand with granule and pebble layers. Thin gravel layers occur most frequently in updip areas and are rarer in downdip areas. Sands are typically quartzose. Lignite, plant remains, and mica are common, grains of glauconite are rare. In the Lewes area, Ramsey (2003) describes the Bethany Formation as consisting of gray, olive gray, bluish-gray clay to clayey silt interbedded with fine to very coarse sand. Lignitic and gravelly beds are common.
RI75 Stratigraphy and Correlation of the Oligocene to Pleistocene Section at Bethany Beach, Delaware
The Bethany Beach borehole (Qj32-27) provides a nearly continuous record of the Oligocene to Pleistocene formations of eastern Sussex County, Delaware. This 1470-ft-deep, continuously cored hole penetrated Oligocene, Miocene, and Pleistocene stratigraphic units that contain important water-bearing intervals. The resulting detailed data on lithology, ages, and environments make this site an important reference section for the subsurface geology of the region.
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 microflora of the Bethany formation and the lower part of the Beaverdam Formation is characterized by a Quercus-Carya assemblage, very few non-arboreal pollen, and Pterocarya and Sciadopitys as exotic constituents. This assemblage has much in common with that of the Brandywine Formation of Maryland and the Eastover Formation of Virginia which are of late Miocene or early Pliocene age. The environment of deposition of the Bethany was probably deltaic, and that of the lower Beaverdam fluviatile.
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.
The surficial geology of the Lewes and Cape Henlopen quadrangles reflects the geologic history of the Delaware Bay estuary and successive high and low stands of sea levels during the Quaternary. The subsurface Beaverdam Formation was deposited as part of a fluvial-estuarine system during the Pliocene, the sediments of which now form the core of the Delmarva Peninsula. Following a period of glacial outwash during the early Pleistocene represented by the Columbia Formation found to the northwest of the map area (Ramsey, 1997), the Delaware River and Estuary developed their current positions. The Lynch Heights and Scotts Corners Formations (Ramsey, 1993, 1997, 2001) represent shoreline and estuarine deposits associated with high stands of sea level during the middle to late Pleistocene on the margins of the Delaware Estuary. In the map area, the Lynch Heights Formation includes relict spit and dune deposits at the ancestral intersection of the Atlantic Coast and Delaware Bay systems, similar in geomorphic position to the modern Cape Henlopen.
These vector and raster data sets contain the rock unit polygons for the surficial geology in ESRI shapefile and TIF format for the Delaware Coastal Plain covered by DGS Geologic Map No. 12 (Lewes-Cape Henlopen area).