Reports
Turbidity plumes formed during fossil oyster shell dredging in the northern Chesapeake Bay, July-September 1999
2000, Wikel, G., Panageotou, W., and Halka, J.P.
File Reports, Coastal and Estuarine Geology, File Report 2000-01_Wikel
Introduction
Water column suspended sediment concentrations have been documented to increase in the vicinity of fossil oyster shell dredging activities in the northern Chesapeake Bay. Areas in which elevated concentrations of suspended sediment exist are called turbidity plumes. Turbidity increases during dredging as a result of bottom excavation and near-surface hydraulic discharge of sediment separated from the shell. The mined shell is utilized as cultch material by the Maryland Fisheries Service for setting of oyster spat in more saline environments of the Chesapeake Bay.
In August 1998, Maryland Geological Survey (MGS) conducted a study of turbidity plumes formed during fossil oyster shell dredging. This study assessed turbidity and suspended sediment concentrations in the water column and the extent of plume elongation given certain tidal conditions. Based on this assessment, the Maryland Department of the Environment (MDE), as part of the State Water Quality Certification (WQC) process, requested that a more focused study be conducted during the summer of 1999.
The WQC issued by MDE mandates a detailed plan and course of action for monitoring water quality parameters in the immediate vicinity of any dredging activity. The WQC permits fossil oyster shell dredging in designated areas under the condition that the resulting turbidity in open water does not violate state water quality regulations. The WQC requires that turbidity does not exceed 150 Nephelometric Turbidity Units (NTU) at any one time or 50 NTU as a monthly average outside a location-specific mixing zone, a predetermined area that circumscribes the turbidity source (COMAR 26.08.02.03-3A).
In the summer of 1999, C.J. Langenfelder & Son, Inc. hydraulically dredged fossil oyster shell beds in Area F, the region east of Pooles Island and the Chesapeake & Delaware (C&D) Canal approach channel (Figure 1). MGS conducted a detailed study monitoring the turbidity occurring at the edge of the mixing zone during dredging operations. In addition to the further investigation of plume transport and dynamics, the main objectives of the study were:
- To document turbidity levels at the edge of the predetermined mixing zone encompassing the shell dredging operation at near-surface and mid water column depths.
- To sample turbidity at predicted peak flood and ebb tidal currents to develop a relationship between current speed and turbidity levels.
- To compare turbidity levels to the dredge production rate and assess if a relationship exists between production and turbidity levels.
