Modelling the long-term accumulation and migration of heavy metals in Sustainable Urban Drainage Systems: a sensitivity analysis
Résumé
The increasing use of SUDS for stormwater management raises some concerns about the fate of ubiquitous runoff contaminants in soils and their potential threat to groundwater. A reactive transport model may be a valuable tool to assess their long-term behaviour in these systems and derive guidelines about the conception and maintenance of such devices, but so far, very few modelling studies have been undertaken in this particular context. This paper presents several modelling options and corresponding hypotheses, and discusses their application to a theoretical " source control " technique. The inter-event variability of a pollutant's mean concentration in runoff affects the sorbed concentration in the first centimetres of soil, and slightly influences its global migration dynamics. The shape of the sorption isotherm (linear, Langmuir of Freundlich models) induces noticeable differences in the predicted contamination profiles. Given the typical time constants of sorption processes, taking into account rate-limited mechanisms causes differences in solute concentrations lower than 4% of the concentration in runoff, and this difference decreases with increasing inflow rate and dispersivity of the soil. The latter parameter was found to have a significant effect on both contaminant profiles and fluxes, but experimental studies show that it may vary within more than two orders of magnitude, and that laboratory-scale measurements tend to underestimate the in-situ values. The uncertainty in the contamination patterns caused by a biased estimation of the dispersion parameter is likely to be larger than the accuracy gained with a more complex modelling framework.
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