LEGACY DAM POOL SEDIMENTS LESS RISKY THAN ASSUMED IN A LOW-HEAD DAM IN RICHMOND, IN

The city of Richmond, IN plans to remove a low-head dam to increase ecological connectivity, remove a potential safety hazard, and improve water quality of the East Fork Whitewater River. However, the industrial history of the Whitewater Gorge makes it crucial to assess the dam pool sediments for contaminants prior to removal.

We conducted a pre dam removal study to map and characterize the nature and contaminant concentrations of the sediment wedge retained by the dam. In order to provide baseline data to assess geomorphic changes following dam removal, we surveyed a longitudinal profile of the Whitewater River and 12 cross sections using total stations and GNSS, and conducted bathymetric surveys of the dam pool and deep pools downstream of the dam using CHIRP sonar.

Our survey determined that the dam pool is approximately 370m long and contains approximately 70m³ of sand and gravel. Cores of dam pool sediment collected for chemical and grain size analysis found the sediment accumulated behind the weir dam to be bimodal, where a poorly sorted pebble population is mixed with a moderately sorted coarse sand population. Samples located outside of the main channel were commonly capped with moderately well-sorted, fine sand.

Analysis of 12 sediment samples for metals, pesticides, PCBs, polycyclic aromatic hydrocarbons, total petroleum hydrocarbons, and cyanide found trace amounts of metals and hydrocarbons, but no legacy pesticides or PCBs detected. These findings suggest that dam removal does not pose a significant risk to human health or the environment.