Modeling the effects of tidal resuspension and deposition on early diagenesis of contaminants

In order to better understand the effects of tidal resuspension on contaminant cycling in coastal waters, coupled physical and biogeochemical 1-D time and depth dependent numerical models were developed. The physical model calculates sediment resuspension and deposition in response to imposed tidal current forcing and passes time series of erosion rate, deposition rate, suspended sediment load, and shear velocity to the biogeochemical water quality model. The water quality model simulates interactions between a water column, a resuspendable floc layer, and the underlying sediments through multiple resuspension and deposition cycles induced by regular tides. It includes advection and diffusion of both particulate and dissolved contaminant phases, organic carbon biodegradation, and linear-reversible contaminant sorption. Pyrene and particulate organic carbon cycling were simulated simultaneously for this study. The model was used to explore the responses of relatively clean sediments to contaminated input loadings under different levels of hydrodynamic forcing. Model results confirm influences of resuspension found in previous field and model studies, and show some unexpected effects on porewater fluxes. The results indicate that tidal resuspension 1) accelerates organic carbon degradation rate in the water column; 2) affects porewater fluxes by increasing mass transport, but decreasing the concentration difference between the water column and sediments, with unexpected results; 3) decreases accumulation (burial fluxes) in the sediments; 4) prolongs particle residence time in the water column and allows greater diagenesis and sorption/desorption to take place. Sensitivity analyses using a Monte Carlo simulation technique indicate that sediment porosity is the most critical parameter for the model, indicating further development and testing. Enhancing the prognostic skill of the model will require comparison to specific data sets from laboratory experiments and field measurements, but it is clear that resuspension may influence the sites and rates of organic matter mineralization in shallow environments and therefore affect distributions of particulate organic carbon and organic contaminants.