Quantifying hyporheic zone exchange and downstream transport of mercury-contaminated fine sediment, South River, Virginia, U.S.A.

Abstract:

Mercury was released into the bedrock-alluvial South River between 1929 and 1950 from a manufacturing facility in Waynesboro, Virginia. Current remediation efforts strive to eliminate the transport of mercury to the channel from legacy mercury stored in floodplains. While this strategy eliminates future sources of mercury to the South River, it does not remove mercury stored within the channel, leaving the river to gradually cleanse its own bed through erosion of mercury adsorbed to fine-grained particles stored within the hyporheic zone. However, the rate of this process is unknown. Our study has two goals: 1) to determine the rate of removal of mercury adsorbed to fine-grained sediment stored in the hyporheic zone, and 2) to predict the spatial and temporal evolution of this “cleansing” process as it proceeds downstream. To quantify rates of hyporheic zone particle exchange, we are monitoring steeply (0.0031) and gently (0.0025) sloping reaches. Water level recorders document changes in stage, and HEC-RAS modeling quantifies local hydraulic conditions. Scour chains document release of stored fine sediments through bed scour. Paired bed mesocosms document deposition and erosion in the absence of scour. The mesocosms consist of buckets filled with gravel buried flush with the ambient streambed, with holes to allow for groundwater flow. Buckets are filled with 3 different sizes of gravel derived from the South River’s bed material. Paired installations include one mesocosm filled with just gravel, and another with pores filled with pottery-grade kaolinite. The kaolinite-filled mesocosm provides data on rates of hyporheic zone fine-sediment erosion, while the mesocosm without kaolinite provides data on sediment accumulation. To adequately sample flow events of varying magnitude with different sized gravel sediment across changing morphology, at least two years of monitoring will be needed. Initial results suggest that 1) limited bed scour occurs on the South River, and 2) erosion of hyporheic zone fine sediment below the surface gravel layer requires significant high flows. Data from hyporheic zone monitoring will be incorporated into a sediment routing model that will assess timescales of hyporheic zone cleansing as remediation progresses downstream.