Using MACRO to Simulate Liquid Sewage Biosolid Transport to Tile Drains for Several Land Application Methods

Abstract

Selection of appropriate liquid sewage, or liquid municipal biosolid (LMB), land application options can be assisted by modeling LMB transport in the vadose zone. The dual-permeability soil water flow model (MACRO) was used to simulate vadose zone flow and tile drain discharge resulting from LMB application (at 93,500 L ha-1) to agricultural field plots for three seasons using a rolling-tine aeration (AerWay SSD) slurry deposition system (surface apply over aerator-tilled soil), furrow-based slurry injection system, and surface spread (broadcast) on un-tilled soil followed by incorporation methods. Given the total solid content of the LMB (0.02 to 0.05 g mL-1), we assumed that it behaved, for modeling purposes, as water. The AerWay SSD system, which surface applies LMB immediately over aerator-tilled soil, did not cause measurable or simulated changes in tile drain discharge during any study period condition. For the broadcast + incorporation experiment, preferential flow of LMB to tile occurred within 0.5 h post-application. Modeling did not simulate the small increase in application-induced discharge observed. This discrepancy was likely due to the existence of bypass flow in un-tilled soil. For the injection method, we simulated vadose zone flow processes associated discretely with the application furrow. While the furrow modeling approach adequately reflected tile breakthrough times, there were some overestimations of peak discharge (100%). Mass loads of bacteria associated with the different application methods did not show, nevertheless, marked or consistent treatment effects. Overall, it was felt that the physically based modeling approach employed here could be used to inform water quality risks associated with different methods to land apply LMB.