Numerical assisted assessment of vadose-zone nitrogen transport under a soil moisture controlled wastewater SDI dispersal system in a Vertisol

► Soil moisture control can effective regulate soil nitrogen impact from wastewater. ► Simulation suggested field denitrification rate was higher than 50% CFD. ► Engineering potential is revealed and deserves further investigation. A real-time soil moisture controlled wastewater SDI dispersal system was tested in a Vertisol for approximately one year to regulate soil nitrogen from an artificially prepared septic effluent. The control strategy was to allow wastewater hydraulic disposal only when the drain field moisture level was near field capacity. Due to limited field sampling and resulting uncertainty in rate of field nitrification/denitrification, numerical simulation (HYDRUS 2D) was utilized to assess system control over nitrogen concentrations in the drain field. Using nitrification/denitrification rates estimated from cumulative frequency distributions (CFD), simulated drain field moisture levels were found statistically lower than actual field observations over the entire year, suggesting persistent drain field soil swelling which is theoretically favorable to denitrification. Short- and long-term simulations suggest that denitrification level should be 50% higher than 50% CFD (first-order reaction, 0.042day−1). Although no conclusive results are presented, this study indicates a potential management approach that favors denitrification by regulating nitrogen application using soil moisture. Further study is required to optimize soil moisture to increase wastewater hydraulic disposal while restricting nitrogen concentrations within the managed soil horizon.