Land-use influences on bacteria levels
Rivers and streams with the highest average levels of faecal pollution are those in towns and cities. Faecal matter from birds, cats and dogs may be carried by stormwater into urban waterways, although there is little evidence that this source on its own results in infectious levels of bacteria. A significant amount of faecal material comes from human waste leaking from sewerage systems.
Rivers and streams in pastoral areas also have high levels of bacteria (relative to waterways in natural catchments). While the levels of bacteria in pastoral waterways are lower than in urban sites, the worst pastoral sites that are monitored have significantly higher levels of bacteria than the worst urban sites that are monitored.
Levels of Escherichia coli (E. coli) bacteria in rivers and streams in catchments with different land uses, 1997–2002
Read table of data of this graph
| Dominant land cover (number of sites in brackets) |
95th percentiles (Average 1997-2002) (E.coli/100 mL) |
||
|---|---|---|---|
| Median | 5th | 95th | |
| Natural (122) | 209 | 7 | 5406 |
| Pasture (338) | 1179 | 105 | 13996 |
| Urban (26) | 2558 | 920 | 7714 |
Notes:
(1) Statistics in the graph are derived from 95th percentile data.
(2) These results include winter sampling results, when faecal-bacteria loads are typically relatively high. This is because there is increased run-off from higher winter rainfall and bacteria tend to live longer in cooler temperatures. However, in general, recreational activities such as swimming do not take place in winter.
(3) mL = millilitres (of water).
Data source: Ministry for the Environment
(1) Statistics in the graph are derived from 95th percentile data.
(2) These results include winter sampling results, when faecal-bacteria loads are typically relatively high. This is because there is increased run-off from higher winter rainfall and bacteria tend to live longer in cooler temperatures. However, in general, recreational activities such as swimming do not take place in winter.
(3) mL = millilitres (of water).
Data source: Ministry for the Environment
Farm stock with access to river and stream beds can contribute high amounts of faecal matter directly to the water. One study has shown that if cows cross a stream on their way to and from milking, they are 50 times more likely to defecate in the water than on adjacent raceways.
Many sites in predominantly natural catchments, where land-use pressures are considered to be lowest, also have high levels of bacteria. These high levels could be caused by faecal matter from birds and other wild animals, such as possums, deer and goats. Predominantly natural catchments may also have small pockets of urban or pastoral land use that deliver significant amounts of faecal material to the water.
This information has come from the latest state of the environment report, Environment New Zealand 2007.
Return to the bacteria in rivers page.
Last updated: December 2007
