Microbial transport into groundwater from irrigation: Comparison of two irrigation practices in New Zealand

Rising demand on food is leading to an increase in irrigation worldwide to improve productivity. Irrigation, for pastoral agriculture (beef, dairy and sheep), is the largest consumptive use of water in New Zealand. There is a potential risk of leaching of microbial contaminants from faecal matter through the vadose zone into groundwater. Management of irrigation is vital for protection of groundwater from these microbial contaminants and maintain efficient irrigation practices. Our research investigated flood and spray irrigation, two practices common in New Zealand. The aim was to identify the risk of microbial transport and mitigation practices to reduce or eliminate the risk of microbial transport into groundwater. Cowpats were placed on lysimeters over a typical New Zealand soil (Lismore silt loam) and vadose zone and the leachate collected after irrigation events. Samples of both cowpats and leachate were analysed for the microbial indicator Escherichia coli and pathogen Campylobacter species.

A key driver to the microbial transport derived from the model applied was the volume of leachate collected: doubling the leachate volume more than doubled the total recovery of E. coli. The persistence of E. coli in the cowpats during the experiment is an important factor as well as the initial environmental conditions, which were more favourable for survival and growth of E. coli during the spray irrigation compared with the flood irrigation. The results also suggest a reservoir of E. coli surviving in the soil. Although the same was potentially true for Campylobacter, little difference in the transport rates between irrigation practices could be seen due to the poor survival of Campylobacter during the experiment. Effective irrigation practices include monitoring the irrigation rates to minimise leachate production, delaying irrigation until 14 days post-cowpat deposition and only irrigating when risk of transport to the groundwater is minimal.

Aim: To compare the risk of microbial contamination of groundwater from cowpats using two irrigation practices onto pasture.