This issue of Water Resources Update focuses on microbial pathogens. We highlight some surprising research findings and ongoing efforts to reduce faecal contamination of our waterways. – Jim Cooke, Centre Leader.
And you thought the water was dirty?
The sediment in the bottom of a stream can contain up to 1000 times more faecal contamination than the water itself under baseflow conditions. This sediment acts as a reservoir of contamination, which can be released during floods.
NIWA runs telemetered salinity monitoring services to alert shellfish farmers to plumes of freshwater which may be contaminated. Much of the contamination comes from land wash-off, but artificial flood experiments in fine weather (when there is no wash-in from land) show that a high proportion of faecal microbes are released from the stream sediment. The surprising result here, says NIWA scientist Dr Rob Davies-Colley, is the way ‘floods dominate the amount of pollution released. In one intensively-farmed stream catchment, 95% of pollution was delivered in floodflow.’
Faecal microbes, including indicator bacteria such as E. coli, are ‘sticky’ and become incorporated into the biofilms growing over sediment particles on the stream bed during baseflows. Dr Davies-Colley says even small increases in flow release some faecal microbes, but large floods move disproportionately large quantities of contamination.
These findings come from ongoing research funded by the Foundation for Research, Science & Technology and NIWA, mostly on the Toenepi and Topehaehae Streams, Hauraki Plains. ‘The research suggests that farmers’ efforts to fence streams should give significant benefits, especially when the fences are set back to provide a riparian buffer.’