Studying the River

The sediment deposits in the lower Grasse River have been impacted by the historic discharges of PCBs from the Alcoa facility. Concurrent with land-based clean-up efforts, Alcoa has conducted numerous studies to evaluate whether the PCB-containing river sediments remain intact during high river flow events. These studies included:

  • Measurement of river velocities;
  • Comparison of sediment bed elevation measurements collected at different times;
  • Development of a hydrodynamic model to predict river velocities during high flow events;
  • Study of suspended solids and PCB levels during high flow events;
  • Measurement of erosion potential and grain size distribution of the sediments;
  • Development of a sediment transport model to predict erosion during high flow events;
  • Radiochemistry analysis of finely-sliced sediment cores; and
  • Comparison of surface sediment, fish, and water column PCB levels before and after an extreme high flow event that occurred in January 1998.

The results of these studies demonstrate that high flow events (such as a 100-year storm event) have only a small effect on how river sediments move because relatively low velocities exist in the river even under higher flow conditions. Recreational boating activity (e.g., impacts from boat propellers) on the river was also determined to have only a small impact on the river sediments.

It is important to note these physical characteristics that are particular to this section of the Grasse River because they play a critical role in determining the best way to remediate the river in order to reduce human health risks.

Grasse River Cross Section