Effects of Hyporheic Water Fluxes and Sediment Grain Size on the Concentration and Diffusive Flux of Heavy Metals in the Streambed

The hyporheic zone regulates physicochemical processes in surface-groundwater systems and can be an important source of heavy metals in fluvial systems. This study assesses the pore water concentrations and diffusive fluxes of heavy metals with respect to the vertical water exchange flux (VWEF) and sediment grain size. Water and sediment samples were collected on August 2016 from upstream Site 1 and downstream Site 2 along the Juehe River in Shaanxi Province, China. Streambed vertical hydraulic conductivity (Kv) and the VWEF were estimated via the standpipe permeameter test method and Darcy's law. The heavy metal concentrations in the pore water were measured and the diffusive fluxes were calculated using Fick's first law. The VWEF patterns were dominated by upward flow, and Site 1 featured higher values of Kv and VWEF. Higher Cu and Zn concentrations occurred near the channel centre with coarse sand and gravel and greater upward VWEFs because coarser sediment and greater upward VWEFs cause stronger metal desorption capacity. Additionally, Cu and Zn at the two sites generally diffused from pore water to surface water, potentially due to the upward VWEF. The VWEF and sediment grain size are likely crucial factors influencing the heavy metal concentrations and diffusive fluxes.