Biochar and flow interruption control spatio-temporal dynamics of fecal coliform retention under subsurface drip irrigation

Applying wastewater in subsurface drip irrigation helps address water shortage in arid and semi-arid environments. Microbial contamination may result, but soil amendments such as biochar could help protect soil and water resources. To improve understanding, this study investigated spatio-temporal dynamics of fecal coliform retention in a biochar-treated soil under subsurface drip irrigation. Two dripper discharges rates of 2 and 4 l hr⁻¹ containing fecal coliforms (3.99 ± 1.4 × 10⁵ CFU ml⁻¹), three maximum allowable depletion levels (MAD; 30, 50, and 70%), and three biochar levels (0, 0.5 and 1%, w/w) were used. The lysimeters were filled with biochar-treated soil and had drippers located at 20 cm depth. Three irrigations were performed for either Q, and soil samples were taken after each irrigation at different times at the lysimeter centre at 5, 15, 25, and 50 cm depths and at the edge at 20 cm depth (labeled 20 L). Soils containing biochar had much greater recovered coliforms from soil than the unamended controls. The greatest retention was at 15 and 25 cm depth (within 5 cm of the dripper) for 1% biochar, with the recovered cells about 70% for 2 l hr⁻¹ discharge and 60% for 4 l hr⁻¹ discharge. The greatest concentration occurred immediately after irrigation, but over 10 days the number of coliforms gradually decreased and inactivated. Therefore, the coliform residence time, soil depth, and biochar rate all influenced coliform retention if the discharge rates and MADs were considered. We recommend slower subsurface dripper rates with high MAD and biochar amendment to minimize fecal coliform contamination from subsurface field wastewater irrigation.