The exchange and transport of mercury-attached fine sediments in storage reservoirs of the South River, VA

To remediate legacy mercury contamination of the South River in Virginia, banks are being stabilized to eliminate erosion of mercury-rich bank sediments. This approach eliminates mercury loading to the channel, but mercury-rich silt- and clay-sized sediments from historic loading remain stored within hyporheic gravels of the streambed and in channel margin deposits. The current remediation strategy assumes that the river will gradually remove mercury-rich fine sediments stored within the channel perimeter, but the time required to achieve this natural cleansing is unknown. We quantify rates of fine-sediment remobilization and its pathways of downstream transport using scour chains, geomorphic mapping, and by monitoring rates of fine-sediment erosion and deposition with gravel-filled bed mesocosms. Data from bed mesocosms indicate that fine sediments are readily eroded from the streambed surface except for those protected by overlying gravels, thus the removal from the hyporheic zone predominantly occurs through the scour of the bed. On annual timescales, scour depths of ~10-20 cm are approximately correlated with the active layer depth computed using the Wong and Parker (2007) approach, suggesting that the hyporheic zone will be fully reworked every few years. After erosion from the streambed, silt and clay sized sediments are transported downstream, but are likely to be redeposited within a few kilometers. Storage probabilities are related to the volume of silt and clay exchanged with storage reservoirs. Only about 1% of the hyporheic zone consists of silt and clay, representing 0.02% of the annual suspended sediment load, but silt and clay are stored in pools at low discharges. Channel margin and floodplain deposits store ~ 1% of the annual load per kilometer; storage in the channel margin is particularly important in protected areas of low river slope, where sediments may remain in storage from months to decades. Overbank flows, occurring on annual timescales, store sediment for decades to millennia. Following bank stabilization, silt and clay with adsorbed mercury stored in the channel is likely to be mobilized within a few years, but will be redeposited in pools, channel margins, and on the floodplain instead of being carried directly to downstream depocenters.