Understanding nitrogen and carbon biogeotransformations and transport dynamics in saturated soil columns

- Advection-dispersion-sorption-biodegradation transport model was considered.
- The retarded transport observed was in the order NH4+ > acetate > NO3−.
- Microbial assimilation and biodegradation were the major nitrogen removal mechanisms.
- The numerical model well predicted the transport of nitrogen compounds.

Nitrogen, originating from fertilizers, wastewater irrigation, livestock, septic tank leakages and other waste disposal sites, poses a major threat to surface and groundwater resources. Both hydrological and biogeochemical processes should be considered for accurate prediction of transport and transformations of nitrogen compounds. In this study, an attempt was made for better prediction of the existing advection-dispersion-reactive transport model by a coupled sorption-biodegradation sink term including advanced biokinetics and inhibition effect for nitrogen movement in saturated soil. The model addresses all the major biogeochemical transformations occurring in the soil similar to the flooded soil environment. The proposed numerical model was validated using a laboratory scale cylindrical soil column with immobile mixed bacteria operated under saturated conditions. Soil column experiments were performed to understand the dispersion, sorption, leaching and biodegradation of nitrogen (100 mg/L of NH4+, NO3−) and carbon (3000 mg/L of acetate) compounds during wastewater infiltration. It was observed that the model simulations matched closely in the case of sorption experiments for NH4+, NO3− and acetate with average model efficiency (E) of 0.986. In the case of sorption with leaching experiments, the model prediction was average for both NH4+ and NO3− at both low and high flow rates. The model predictability was improved by introducing a calibrated parameter λ which accounted for microbial activity in continuous column experiments when compared to the values obtained from the batch system, which was calculated by back fitting. As a result, the numerical model simulated the breakthrough profiles of NH4+ (E = 0.812), NO3− (E = 0.701) and acetate (E = 0.611) well. Further, this study implies that relatively shallow aquifers with sandy soil are vulnerable to NO3− contamination at around 10 days if continuous wastewater irrigation is practiced. Hence, long-term studies under field conditions under different irrigation scenarios in addition to simulation modeling must be carried out to generalize flow and nitrogen compound transport under various soil types.