Modeling biochar-soil depth dependency on fecal coliform straining under subsurface drip irrigation

Biochar has several benefits for soil, including improved flow and retention properties that decrease bacterial transport. To improve understanding of the underlying mechanisms and optimal irrigation strategies for contaminated gray water, we conducted a lysimeter study exploring impacts of biochar level (0, 0.5, and 1% w/w) and 30-days irrigation regime (0.003 and 0.0064 cm h⁻¹). Biochar did not change hydraulic properties but at maximum allowable depletion (MAD) of 30, 50, and 70% biochar had a large impact on fecal coliform retention. At 15–25 cm depth where the subsurface dripper was located, 1% biochar-treated soil had 1.5 times greater retention than the controls. A combination of the lower irrigation rate (0.003 vs. 0.0064 cm h⁻¹) and the greater biochar (1% vs. 0.5%) had greater impacts on bacteria inactivation and retention than the MAD levels. Nevertheless, adding 1% biochar resulted in a 2-fold increase of bacteria retention for 30% MAD and low flow rate than greater flow rates and MAD levels (50 and 70%). Our results provided novel data for inverse optimization to explore bacteria retention by differences in bacteria attachment, straining, survival or growth. Close to the irrigation dripper, straining was the pronounced mechanism under the best performing treatment of 1% biochar and 30% MAD. Therefore, we recommend application of biochar at the rate of 1% where wastewater is used for subsurface drip irrigation.