Reactive contaminant infiltration under dynamic preferential flow.

• Heterogeneous soils, transient flow, and multicomponent biodegradation simulated. • Soil heterogeneity may reduce contaminant leaching without increasing travel times. • Effects of heterogeneity are stronger when reactant mixing limits biodegradation. • Effects of heterogeneity on leaching not significantly affected by transient flow. • Mean residence times and initial biodegradation rate are informative on leaching. Biodegradation is an important mechanism of contaminant removal from soils. We use numerical simulations to study the contaminant transport in heterogeneous soils subject to transient flow conditions and anaerobic multicomponent biodegradation. These processes and their interactions affect contaminant travel times, the extent of reactant mixing, solute and microbial biomass distributions in the soil, and biodegradation outcomes. Especially when flux variations are large, the combination of soil heterogeneity and transient flow gives rise to dynamic preferential flow zones, which affects reactant mixing and biodegradation outcomes. Results show that soil heterogeneity may reduce contaminant leaching due to enhanced reactant mixing, especially when biodegradation is more limited by reactant mixing. Furthermore, unlike under steady-state flow, under transient flow soil heterogeneity does not substantially reduce contaminant residence times. As preferential flow zones change dynamically, the spatio-temporally averaged transport, mixing, and biodegradation experienced by various parts of contaminant plumes become homogenized. Therefore, knowing the initial biodegradation rate of a contaminant upon infiltration, and its mean residence time in the soil, enables a relative sensitivity analysis. This allows biodegradation outcomes of various scenarios to be approximately ranked, even under soil heterogeneity and transient flow, using information that is straightforward to measure or estimate in the field. [ABSTRACT FROM AUTHOR]