The movement of bacteria through soil following the land application of piggery effluent

Abstract

In order to evaluate the potential risk of groundwater contamination following the spray application of piggery effluent to typical Canterbury soils, the leaching of bacteria was investigated.

Concentrations of faecal coli forms or Escherichia coli, and the spore of Bacillus subtilis var niger, added to the waste as a potential microbial tracer, were determined in soils and leachates, following effluent application to natural soil/pasture systems.

Effluent was applied to Lismore stony silt loams and Waimakariri stony sand loams, using standing gun and travelling rotary irrigators, under normal operating conditions. The microorganisms were found to the maximum soil depth tested (500mm). Concentrations of both microorganisms were highest at the soil surface and decreased by a maximum of 1000 times with increasing soil depths. Faecal coli forms were more efficiently retained in the soils, than the spores.

A further study was carried out using undisturbed soil monoliths (lysimeters) of Lismore stony silt loam (500 mm depth) and Templeton silt loam (1200 mm depth). Application of the piggery effluent was made by hand, in a manner intended to represent standing gun irrigation, at the standard rate of 200 Kg N/ha (Fietze,1988) and twice this amount. Autumn application to the Lismore soils allowed the leaching of E.coli, but the deeper Templeton soil did not. Lismore leachates were free of E.coli after 24 days.

Winter leaching studies using lysimeters revealed that both E.coIi and the spore of B.subtilis var niger were leached in significant quantities. The total amounts of the respective microorganisms leached in a sampling period were able to be related to rainfall, using linear regression analysis. Approximately 0.003 % E.coli and 12 % B.subtilis spores were leached from the Lismore lysimeter over the 26 days duration of the experiment. Comparatively, 0.018 % E.coli and 0.29 % B.subtilis spores were leached from the Templeton lysimeter. Only slight reductions in the concentration of E.coli were observed.

The use of repacked soil columns using lismore stony silt loam peds of diameter 2 to 4 mm confirmed the observations from field and lysimeter experiments; the spores were less effectively retained in the soil than E.coli. Soils at field capacity allowed much greater quantities of both E.coli and B.subtilis spores to be leached, than dry soils. The protection of bacteria in dry soils by the process of "macropore bypass", whereby the flow of water through macropores by-passes the contents of intra-ped micropores, was hypothesized, but could not be verified.

The results indicate that the application of piggery effluents to Canterbury soils can probably be recommended in summer and autumn months, but that significant leaching may occur in winter. The spore of B.subtilis var niger has potential as a tracer of long-lived components of land applied effluents.