Comparison of steady-state and transient flow conditions on reactive transport of contaminants in the vadose soil zone

This paper investigates transport and fate of reactive compounds in the unsaturated subsoil (vadose zone) using numerical simulations of steady-state and transient flow scenarios. The objective is to investigate whether steady-state flow simulations accurately describe reactive contaminant transport under transient conditions in the field. The focus lies on organic compounds; advection, kinetic sorption and biodegradation are considered as relevant processes. In model scenarios the impact of steady-state and transient seepage water flow on solute concentrations at the groundwater table was evaluated. Sorption and degradation kinetics relative to the advective transport velocity were characterized by dimensionless Damköhler numbers for degradation (Dad) and sorption (Das). Compound properties representing two frequent pollutants (Lindane and Phenanthrene) and two types of sources were investigated: a constant inflow concentration (infinite source, with Phenanthrene) and a decreasing concentration due to desorption of the compound (finite source, with Lindane). Total mass degraded vs. cumulative flux to the groundwater (the lower model boundary), as well as the maximum breakthrough concentration at the outflow were evaluated. For each source type, combinations of sorption and degradation kinetics were identified, at which the transient simulation showed deviations from the predictions based on the steady-state simulation. Results are plotted as the difference in the observed maximum breakthrough concentration and the difference in cumulative mass fluxes as functions of degradation and sorption kinetics (“Damköhler plots”). Overall results indicate that steady-state flow conditions are appropriate for most field scenarios; only extreme infiltration events lead to higher pollutant concentrations in transient simulations because of the short residence time of seepage water in the unsaturated zone and thus reduced biodegradation and sorption.