Tanks may be polluting water supply

When you turn on a tap, you expect clean drinking water to arrive. Yet frequent boil notices for E coli contamination are a reminder that this is not always the case. And now researchers are discovering that the source of this contamination may lie close to home, thanks to misuse of septic tanks.

"There's reason to believe that septic tanks could be a contributing factor," says architect Jim O'Donohoe, a researcher with the national centre for freshwater studies at Dundalk IT (Dkit).

As part of a pilot project to help protect drinking water sources, he is measuring groundwater contamination around septic tanks near Milltown Lake in Monaghan. The lake provides water for the local group scheme.

E coli bacteria usually live harmlessly in our guts and pass out in faeces, but if the bug or other coliforms contaminate drinking water intestinal upsets can result.

Septic tanks deal with the problem by percolating out foecal-contaminated water into the surrounding ground where the natural microbial action can deal with the potentially harmful bugs, explains Donohoe.

"The idea is that the ground is actually treating it. And by the time that it gets to anywhere that could cause problems, either into the groundwater or into surface water, it will have been dealt with by the microbes in the ground itself," he says.

But when O'Donohoe bored sampling wells around six septic tanks in the lake's catchment area and tested the groundwater, he found remarkably high levels of E coli and of nutrients like phosphorous and nitrogen, which in excess can trigger an overgrowth of algae in streams and lakes and disrupt the ecosystem.

"In all cases we found levels of E coli and nitrogen and phosphorous that were higher than would be acceptable," he says. "We also found that the water table, the level at which you find water rather than damp ground, was higher than would be acceptable for septic tank use."

And when he inspected around 50 other tanks in the lake's catchment area, he found the majority had the potential to contaminate nearby groundwater. "Just by opening up the normal manhole system and looking in you could see that things weren't working correctly - there were obvious problems in about two-thirds of them," says O'Donohoe, who admits to being surprised by the level of misuse.

One of the biggest problems is that people don't get the tanks emptied frequently enough, he explains, suggesting that cost could be a factor. He hopes the Dkit project will highlight the need to monitor septic tanks around the country to help avoid groundwater contamination.

"We accept that cars have to be tested regularly to ensure that they are roadworthy. The same logic can apply to a septic tank," he says, noting that tanks are often inspected only after a problem has arisen. "That would be the equivalent of doing the NCT only on cars that have crashed."

The Government-funded pilot project is also looking at water quality in Milltown Lake and at indicator species of plants and animals in its feeder streams. O'Donohoe plans to sample groundwater in a wider area around septic tanks to see how far the contamination spreads.

The overall aim is to develop ways of stopping contamination at source, which will ultimately save money and human misery. He adds. "The cleaner the water going into the treatment plant, the less treatment that has to be done. So it's less costly to treat it and there's less likelihood of problems in the delivered water."

• Researchers from the water source protection project presented their work at the Environ 2008 colloquium at Dundalk Institute of Technology