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DGS Holts Landing report

Delaware Geological Survey issues report on submarine groundwater discharge at Holts Landing State Park

The Delaware Geological Survey (DGS) has released a new technical report titled “Investigation of submarine groundwater discharge at Holts Landing State Park, Delaware: Hydrogeologic framework, groundwater level, temperature, and salinity observations.”

The report was prepared by A. Scott Andres and Changming He of DGS, and Holly Michael, Christopher J. Russoniello, Cristina Fernandez and John A. Madsen of the University of Delaware’s Department of Geological Sciences.

DGS Report of Investigations No. 80 documents the development of a detailed study of subsurface hydrogeology, adaptive drilling technology and interactions between aquifers and Indian River Bay.

Previous studies have detected the presence of fresh groundwater in the aquifer beneath Indian River Bay near Holts Landing State Park. Investigators installed wells and instrumentation in the aquifer on shore and under the bay in an effort to more fully characterize the aquifer, the causes for the presence of fresh water beneath the bay bottom, and the impacts of weather and seasonal changes on the fresh water-salt water interface.

These wells also were used by other researchers and graduate students who evaluated the transport and discharge of nitrate from groundwater to the bay. This report documents the results of this study in which a barge-mounted drill rig was used to install four single-well and four multi-level wells offshore in water depths of 1 foot to 5 feet. These wells were equipped with custom-developed seals to permit sampling at multiple times and be instrumented with automated sensors, yet be protected from wave and fishing pressures.

Multiple wells were also installed on land and equipped with automated sensors. Water level, temperature, and salinity were monitored in all of these wells between 2010 and 2012, a period that included Hurricane Irene.

Several of the wells showed the presence of fresh water in the aquifer under the bay at distances up to 650 feet from shore. Freshwater was present because a muddy confining unit located just below the bay bottom implied groundwater discharge into the bay.

Data collected by automated sensors showed that the dominant force affecting the position of the fresh water – salt water interface in the aquifer was seasonal trends in groundwater recharge, while storm and high tide events had short-term affects.

The information in this report will be useful for planning future water supplies and wastewater disposal, and for management of water-dependent environmental resources.

The report fulfills part of the DGS’ mission to understand hydrologic systems and to advise, inform and educate Delawareans about the results of such investigations so they can serve as a resource for scientists, engineers, planners, emergency managers, and the public

Report of Investigations No. 80 is available in PDF format from the DGS website at publications. For additional information, contact DGS at 302-831-2833 or via email at delgeosurvey@udel.edu.

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