DGS Reports of Investigations

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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.
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.
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.
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.
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.

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