The Persistence of Anthropogenic Turbidity Plumes in a Shallow Water Estuary

Increased turbidity and light attenuation (Kd) in the suspended sediment plumes created by hydraulic clam dredging were examined in Chesapeake Bay, MD, U.S.A. Turbidity andKdvalues were measured along transects in areas with and without dredge plumes. The turbidity andKdof individual plumes were tracked as they returned to background levels using both Lagrangian and Eulerian techniques. Existing aerial photographs and a geographical information system (ARC/INFO) were used to examine plume sizes and dredge boat locations in relation to bathymetry. Hydraulic clam dredging produced plumes with significantly higher turbidity and light attenuation compared to background values. Plume characteristics were determined primarily by bottom sediment type and water depth. The greatest increase in turbidity and light attenuation occurred when dredges operated in shallow water (<1•0 m) where bottom sediments had increased amounts of silt and clay. Plume turbidity andKddissipated exponentially over time. The initial change in concentration of suspended sediments was rapid as the coarse sediments settled to the bottom. The rate of plume decay slowed as diffusion and resuspension acted on the finer sediments remaining in suspension. Some Lagrangian rates of plume dissipation were faster than Eulerian rates due to resuspension in shallow waters. Examination of aerial photographs indicated that 72% of the dredge boats digitized in the Chester River were operating in less than 2 m water. The area of a plume measured per boat in the Chester River was highly variable, ranging from 0•01 to 0•64 m2.