Monitoring Dispersion of Dredged Material Following Riverine Deep Trough Disposal

Proposed placement of dredged material in a deep trough of the Potomac River below Alexandria, Virginia led to environmental concerns that the deposited material would disperse to nearby shallow bottom habitats. Between December 15, 1999 and February 5, 2000, approximately 328,000 cubic yards of dredged material, representing 212 split-hull barge loads, were placed in the trough. Monitoring efforts included pre-, during, and post-disposal deployment of bottom-mounted optical backscatter sensors (OBS), and two sediment profiling imagery (SPI) surveys, one four months following placement and a second a year later. Examination of the OBS data indicated that pulses of extremely turbid (>1,000 NTU) water moved through the study area prior to the onset of disposal operations, and intermittently throughout the study period. Bottom waters in the channel thalweg were generally much more turbid than bottom waters over adjacent shoals. These high background turbidities tended to mask any elevated turbidities linked to the disposal events. The few turbidity pulses that were discerned to be associated with disposal events were relatively small (< 250 NTU) and ephemeral (< 1 hr). SPI images collected four months after cessation of disposal and a year later revealed "signatures" of sandy dredged material within the trough. No evidence of significant dispersion of dredged material outside of the trough was seen. Following cessation of disposal, benthic assemblages in the vicinity of the site appeared to be undisturbed. Although long-term data describing bathymetric changes within the trough are needed to evaluate the stability of the deposited material, no significant long-term (1.5 year) movement of materials out of the trough was observed.