Role of degradation concepts for adsorbing contaminants in context of wastewater irrigation

Marginal water, like treated effluent or other types of wastewater, induces contaminant accumulation in different environmental compartments such as soil and groundwater. With a root zone model and daily rainfall data as input, accumulation in the case of two different degradation concepts was modeled: (I) the solute degrades only in the solution phase, and (II) the solute degrades in both the solution and the adsorption phase. For linear adsorption, both degradation concepts not only yield similar results, but adsorption does not affect the long-term concentration. This can be readily explained with the governing equations. For nonlinear adsorption, the degradation concept does affect the concentration and fluxes. When the first concept is valid, the long-term concentration is not affected by adsorption, whereas the short-term and fluctuations are. For the second degradation concept, the long-term average concentration and fluxes are also affected by adsorption. A correction factor was derived to correct the analytical solution of the time-dependent concentration of nonlinearly adsorbing solutes. For all scenarios, reasonable estimates of the long-term concentration could be obtained with analytical models for most parameter combinations, which can be used to identify contaminant species that should be prioritized for further research.