Organic and inorganic amendments are routinely applied to ameliorate soil conditions in semi-arid regions. This study aimed to examine the unsaturated transport of nalidixic acid-resistance Escherichia coli (E. coli NAR) in soil amended by cow manure (CM) and natural zeolite (NZ). Two levels of NZ (5 and 15% w/w) were added to a loam soil containing 5% w/w CM and incubated for 60 days. Then, the soil mixtures were poured into cylinders (3.6 15 cm) and a pulse (0.1 pore volume) of bacteria suspension (106 Cell mL1) was added on top of the soil columns and leaching experiments were conducted at a steady-state, unsaturated flow condition and filtration coefficient (kf), relative adsorption index (SR), maximum depth of bacteria transport (Zmax), average (Cav) and cumulative cell density (Ccum) were calculated. CM and NZ application significantly increased the peak density, Zmax, Cav, and Ccum of E. coli NAR. The Cav, and Ccum of E. coli NAR in both 5 and 15% NZ containing mixtures were about 1 and 2-fold higher compared to the control. CM and NZ addition to soil decreased the filtration coefficient (kf) and relative adsorption index (SR) compared to the control. According to our results, we should conclude that the presence of NZ and CM, separately or together, can significantly increase the risk of groundwater contamination and it should be used with prudence.
