Modeling hydro-biogeochemical transformation of chromium in hyporheic zone: Effects of spatial and temporal resolutions

摘要

Effects of spatial and temporal resolutions (SR and TR) on modeling hydro-biogeochemical transformation of chromium (Cr) are important in simulating reactive transport processes. The current study was conducted in the hyporheic zone (HZ) at the Hanford Site of the U.S. Department of Energy, which has been known for its highly heterogeneous sediments and transient hydrodynamics. Distributions of hydraulic conductivity and sediment-associated Fe concentration were averaged at a group of SRs, while measured hourly water levels were further moving averaged at daily and monthly TRs. Fe concentration is selected for assembling geochemical heterogeneity due to its important role in redox transformation of Cr at the site. Three Fe distributions, with Fe concentrated on small, medium, and large sediment grains, respectively, were also considered. A series of flow and reactive transport simulations configured with different combinations of SRs, TRs, and Fe distributions were conducted. Simulated results revealed that Cr(VI) discharged to river is underestimated if SR decreases. For both the hydrodynamics and the discharge of Cr(VI) to river, difference caused by SR can be amplified with the decrease of TR. Biogeochemical transformation of Cr is more dependent on SR while hydrodynamics is on TR. The stronger control of SR than TR on biogeochemical transformation of Cr is resulted from more sensitive increase of Fe(II) with decreasing SR than with decreasing TR. Effect of SR is highly sensitive to variations of SR with Fe on medium-size grains while is persistent to a much smaller SR with Fe on small-size grains. Results from the current study are expected to benefit modelers on the selections of the spatial-temporal resolutions for in-house modeling and field sampling, which may also have implications for upscaling.