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.
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.
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.
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.
Population and accompanying water use are expected to increase by 34 percent in southeastern Delaware between 1975 and 2000. To assess the capability of the aquifers in that area to supply the required amount of ground water, a study of those aquifers was started in 1976. Interpretation of geologic sections developed from drilling and geophysical data showed that the confining beds between the Manokin, Ocean City, and Pocomoke aquifers of Neogene age are thin and discontinuous in some parts of the area. Possible fault zones coinciding with deep tectonic features may also contribute to interconnection of these aquifers. Hydrographs of water levels in the aquifers show differential drawdown during periods of heavy pumping, but levels return to a common altitude during unstressed periods. Because of these characteristics, the Manokin, Ocean City, and Pocomoke aquifers are considered to be a single confined aquifer, in most places.