Effects of water managements on transport of E. coli in soil-plant system for drip irrigation applying secondary sewage effluent

The two-year experiments presented in this paper aimed at investigating the effects of water management practices on transport of E. coli in soil and residuals in plants for drip irrigation while applying secondary domestic sewage effluent. In 2014, the experiments were designed with three lateral depth of 0 (S0), 10 (S1) and 20 cm (S2) below the soil surface along with three irrigation levels that were determined by pan coefficient of 0.6 (I1), 0.8 (I2) and 1.0 (I3). In 2015, three irrigation intervals of 4 days (F1), 8 days (F2) and 12 days (F3) along with three lateral depths similar to the 2014 experiments were used. Groundwater control experiments were applied for the treatments with pan coefficient of 0.8 (I2) in the 2014 and the irrigation intervals of 8 days in the 2015. The fate of E. coli in the soil was greatly influenced by lateral depth, and subsurface drip irrigation could avoid pathogen contamination when sewage effluent was applied. Surface drip irrigation more likely induced E. coli contamination on surface soil and the E. coli concentration demonstrated a decreasing trend after irrigation ceased. In general, a more frequent irrigation and a higher containing level of E. coli increased short term E. coli contamination of soil as it increased contacting opportunities between effluent and soil. However, the E. coli declined during an irrigation interval and was hardly detected three days after an irrigation ceased. On harvest, no E. coli was detected in the stems of asparagus lettuce and few counts of E. coli was detected on the leaves of the crop but a weak association between the irrigation management practices and E. coli contamination of leaves was found. Our study recommended that subsurface drip irrigation is a promising method to avoid E. coli contamination when applying sewage effluent.