According to Ollard, misconnections are particularly difficult to deal with because identifying the source of the problem – and who is responsible – is often complex.
“It’s hard to track,” she told the Local Democracy Reporting Service. “You can only find out once you see the discharge.”
Thames Water is responsible for investigating leaks and tracing them through drainage networks.
However, once a misconnection is identified and traced to a property, it is a legal requirement for the homeowner to fix it.
The company has said it has investigated more than 20 suspected misconnections in the past three years.
Ollard said many homeowners are unaware their property is misconnected, and that older homes can have historic plumbing errors that only come to light once pollution is detected in a river.
These issues are also not usually identified in surveys, meaning people may buy a home without realising it has an issue.
She said: “Then they will get a letter saying it has been identified and they will then be required to do the work, which costs thousands of pounds.”