Quantifying biologically and physically induced flow and tracer dynamics in permeable sediments

Abstract

Abstract. Insight in the biogeochemistry and ecology of sandy sediments crucially depends on a quantitative description of pore water flow and the associated transport of various solutes and particles. We show that widely different problems can be modelled by the same flow and tracer equations. The principal difference between model applications concerns the geometry of the sediment-water interface and the pressure conditions that are specified along this boundary. We illustrate this commonality with four different case studies. These include biologically and physically induced pore water flows, as well as simplified laboratory setups versus more complex field-like conditions: [1] lugworm bio-irrigation in laboratory set-up, [2] interaction of bioirrigation and groundwater seepage on a tidal flat, [3] pore water flow induced by rotational stirring in benthic chambers, and [4] pore water flow induced by unidirectional flow over a ripple sequence. The same two example simulations are performed in all four cases: (a) the time-dependent spreading of an inert tracer in the pore water, and (b) the computation of the steady-state distribution of oxygen in the sediment. Overall, our model comparison indicates that model development for sandy sediments is promising, but within an early stage. Clear challenges remain in terms of model development, model validation, and model implementation.

The negligible effect of bed form migration on denitrification in hyporheic zones of permeable sediments

“…The stationary ripple simulations (U c = 0) result in symmetrical profiles of solute distribution, with downwelling oxic water at the ripple trough and upwelling anoxic water at the ripple peak (Figures 4 and 5, rows 1 and 2, first column). These profiles are similar to those presented in previous field , laboratory [Precht et al, 2004;Ziebis et al, 1996], and model [Bardini et al, 2012[Bardini et al, , 2013Cardenas et al, 2008;Cook et al, 2006;Meysman et al, 2007] studies. The introduction of bed form celerity begins to distort these flow fields.…”

Quantifying denitrification in rippled permeable sands through combined flume experiments and modeling

“…While this may affect the interpretation of the model for prediction of denitrification in a natural environment, it was imperative to this study that the domain matched the experimental conditions. In any case, it is not uncommon for both flume (Precht et al 2004) and model (Meysman et al 2007) experiments in permeable sand to have such a boundary condition. A full study considering a range of different sediment types would set up a more realistic, continuous boundary, although it is not clear how much of an effect this would have on denitrification rates.…”

“…In recent years, there have been several efforts to describe pore-water flow fields and associated biogeochemical processes using theoretical computations based on simple parameters, such as flow rate, pressure, and permeability (Meysman et al 2006(Meysman et al , 2007. The approach generally consists of first calculating the subsurface flow field; and then overlaying a reactive transport model, based on measured and/or estimated chemical kinetic rates (Cook et al 2006;Cardenas et al 2008).…”

The “salt wedge pump”: Convection‐driven pore‐water exchange as a source of dissolved organic and inorganic carbon and nitrogen to an estuary

“…The DIC : DO ratios associated with molecular diffusion and pore-water advection were 2.5 and 83.0, respectively. While molecular diffusion operates in the upper few millimeters of sediments, pore-water advection operates over a much deeper region (Huettel et al 2003;Meysman et al 2007). As all the oxygen is likely to be exhausted within the upper few millimeters of sediments, anaerobic processes such as sulfate reduction likely drive the extremely high DIC : DO ratios associated with porewater advection.…”