Parts of New Zealand are drawing pre-treated drinking water from rivers found to be regularly polluted with harmful nasties like E.Coli and campylobacter.
New research, led by a group of Massey University scientists, raises more warnings about the impacts of agricultural intensification, at a time of heightened worry over the quality of many of New Zealand's drinking water supplies.
The scientists investigated levels of four microbes – the bacteria campylobacter and E.coli, and protozoa cryptosporidium and giardia – at 16 waterways supplying drinking water for 13 towns and cities.
Between 2009 and 2019, with three-month intervals, they collected more than 500 samples from abstraction points at each of the study sites.
The waterways in the study ranged from those draining catchments heavy in agriculture to rivers running through native bush.
To pick apart what role the surrounding landscape played on pollution levels, the study used machine-learning algorithms to determine contributions of land use, geology, topography and vegetation.
Those rivers in highly-stocked catchments – such as the Waikato River and Oroua River in Manawatu – were found to have all four microbes present, and often in high numbers.
At the Waikato River, near Tuakau, 10 of 37 samples taken over the decade carried E.coli levels that were above safe limits, along with 10 containing campylobacter, 32 with giardia and 26 with cryptosporidium.
But at the study sites at the Hutt River and Big Huia Creek in Wellington, the scientists found no pathogenic microbes, or any unsafe levels of E. coli, which is known to cause severe stomach cramps, vomiting and diarrhoea.
"They have catchments entirely of native vegetation," said study co-author and Massey University freshwater ecologist Professor Russell Death.
The two protozoa were found to be more prevalent in waterways with lower water quality and higher numbers of ruminant animals like cows, sheep and deer in the catchment – especially in September and December.
"The protozoa live in the guts of ruminants and the eggs come out in faeces," Death said.
"The eggs are very resistant and so probably get transported to streams quite easily."
The two bacteria, meanwhile, became more abundant with higher rainfall, saturated soils and catchments where more than a third of the land was used for agriculture.
"These bacteria can come from a range of animals including farm livestock," Death said.
"It seems when the bacteria are deposited on grassland, in faeces, it can get into waterways more easily than if it is deposited in a forest.
"But there may also be more because farm animals produce a lot more waste - 25kg per dairy cow per day - versus not much from say, a wild deer or duck."
At Hutt River, near Wellington, scientists found little sign of harmful microbes. Photo / Supplied
Study co-author Professor David Hayman said the survey work went directly to the Ministry of Health, where it helped national drinking water treating standards.
"It does not, of course, tell people about the safety of their particular water supply or deal with rare events," Hayman said.
"But it can identify the drivers of contamination and therefore if we wish to improve drinking water quality without simply relying on increased water processing and the costs with it, so these analyses are really informative."
All of the water tested was what's called raw water, and hadn't been treated in plants.
Although modern treatment protocols usually remove many pathogens from drinking water, Death and his colleagues noted that several recent outbreaks of waterborne disease happened because of treatment failures.
"There is a lot of concern over the quality of drinking water supplies in New Zealand," Death said.
"Many are excellent, but many are not, especially in smaller rural communities.
"The issues in Havelock North a few...