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High levels of bacteria from sewage found in River Cam





A study testing water quality along the River Cam has found evidence of high levels of faecal bacteria believed to be caused by treated sewage effluents.

The research was carried out by the Cam Valley Forum, who started the Citizen Science project to gain a better understanding of contamination of the river by bacteria from faeces and to determine the sources of the waste.

Although at an early stage, preliminary results suggest that most of the faecal indicator bacteria - such as E coli - come from sewage water treatment centres, particularly the site at Haslingfield.

Mike Foley, of the Cam Valley Forum on Sheep's Green by the river Cam. Picture: Keith Heppell. (52962582)
Mike Foley, of the Cam Valley Forum on Sheep's Green by the river Cam. Picture: Keith Heppell. (52962582)

However, the scientist behind the research says more batches will need to be tested before firm conclusions can be made about whether the water is safe for swimming.

Mike Foley, of the Cam Valley Forum, said: “We can’t start campaigning loudly until we’ve built up our database and we can only do that by doing more testing.

“Today we’re looking at faecal indicator bacteria, which give us an idea of the amount of faecal contamination traveling down the river. Now in that contamination, there will be organisms that cause disease in humans. And by looking at our indicator bacteria we have a measure of the threats from these pathogens

“It’s really difficult to use our tests to date to say to swimmers, this stretch is safe for swimming and that stretch is less safe. Because as the river conditions change, we may find that a different pattern of faecal contamination emerges.”

The tests reported so far were carried out in June and August when river levels were lower than normal.

However on both sets of tests the researchers noticed high levels of faecal contamination downstream from Haslingfield sewage treatment works.

Mr Foley says: “ I’m sure we have wild sources (of faecal contamination) all along, keeping the levels low, but not zero. But then, as soon as you come to a sewage treatment works, your results go up in the river, and then they start declining again. So it’s pretty clear cut the sewage treatment works are producing most of the bacteria.”

They also carried out tests along Grantchester Meadows where herds of cattle regularly graze but found no spike in faecal indicator bacteria there, suggesting they were not major contributors to the contamination.

Wild swimming in the River Cam has become popular. Picture: Keith Heppell. (53024205)
Wild swimming in the River Cam has become popular. Picture: Keith Heppell. (53024205)

The report by the Cam Valley Forum explains: “As part of our aspirations to safeguard and improve the River Cam, Cam Valley Forum started the project to better understand faecal contamination in the river by evaluating counts of faecal indicator bacteria in both watercourse samples and effluent from sewage treatment works (STWs) according to seasonality, river flows, and agricultural operations.

We were looking for evidence of different types of sources, but especially of point sources which might then be addressed and improved by remedial action. The faecal indicator bacteria monitored were the standard E. coli and intestinal enterococci and total coliforms.”

The sites tested extended from Meldreth, on the Rhee, downstream to the Cam, through Cambridge to Clayhithe (Waterbeach) just north of Cambridge and included tributaries above Cambridge. Included were both the Haslingfield and Cambridge (Milton) sewage treatment works’ effluent.

Earlier this year King's College tried to ban swimming in the River Cam on their land due to safety fears. Picture: Keith Heppell. (53024208)
Earlier this year King's College tried to ban swimming in the River Cam on their land due to safety fears. Picture: Keith Heppell. (53024208)

The report explains that both sewage treatment works’ discharges had “much higher levels of bacteria than any in-river samples: 38,730 E. coli /100ml of pure effluent at Haslingfield STW, and 7380 E. coli /100ml at Milton.”

These were in “treated” effluent, with no evidence of storm overflows of untreated sewage into the river. After taking into account the different dilution rates imposed on the effluent by the river flow, the counts of E. coli converted into river counts were 3495 and 2959 /100ml respectively. Thus, the estimated river count at Cambridge was 85% of Haslingfield’s, on the day of sampling. The lowest river count was 55 E. coli /100ml in a sample from Bourn Brook.

Applying the EA Water Framework Directive standards, the high phosphate levels where samples were collected on the Granta-Cam, Rhee, and Cam conferred a unofficial ‘poor’ status on the river water.

“Mr Foley said: “Finding high levels of bacteria just down from the sewage treatment works did alert me to the possibility that, in other conditions, these bacteria may survive better and travel further down the river and we might get more bacteria coming into the popular swimming areas around around Cambridge such as Grantchester Meadows and Sheeps Green.”

Mr Foley added that he did on occasion swim at Grantchester Meadows but that he was careful to keep his mouth shut and avoid swallowing any water.



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