Officials are investigating a potential link between the Great Morass and Southsea’s water quality

A long-lost Southsea marshland could hold clues to the city’s poor bathing water, as investigators examine historic drainage routes and leaking sewers.

Portsmouth City Council’s sewage discharge scrutiny panel is investigating storm overflows and their impact on water quality. During these meetings the Great Morass has been highlighted as a potential source of pollution.

Southsea East has been classed as “poor” by the Environment Agency since 2023, while Eastney has been rated “good” since the same year. Eastney was downgraded from “excellent” in 2022.

The difference has prompted questions over why two neighbouring bathing water sites have such different classifications.

The potential causes of poor bathing water quality at Southsea include sewage discharges, contaminated groundwater, leaks from foul sewers, broken private sewer drains beneath Southsea Pier, underground pathways and bird fouling.

The Great Morass was a large area of marshland in Southsea, occupying low-lying land around what is now Southsea Common and Canoe Lake. It was gradually drained and reclaimed as Portsmouth expanded during the 19th century.

The potential contribution of the Great Morass was raised by Richard Lee, the council’s assistant director of regulatory services, at a scrutiny panel meeting on July 16.

Meeting minutes from the July 16 panel revealed that a “working group investigated by digging test holes along the beach at low tide. If the holes filled with water at low tide, this would support evidence of groundwater or leachate seepage. Leachate levels may also be expected to respond to tidal conditions.”

The minutes also raised concerns about the condition of the nearby sewer network, which “may have deteriorated or developed defects, potentially allowing leakage into groundwater or historic drainage pathways associated with the Great Morass and subsequent discharge beneath the beach and into the sea”.

A council spokesperson said: “Investigations have not shown that the Great Morass is a single or proven source of pollution. Some of the tests carried out did not find evidence to support the potential pathways being investigated. However, separate testing has occasionally found higher levels of bacteria in groundwater near the Southsea East monitoring point.

“More recently, Southern Water found several leaking sewer pipes in the area above the former Great Morass and is carrying out repairs. Although these leaks may have affected water quality, there is no evidence that they are the main cause of Southsea East’s bathing water classification.

“Ongoing monitoring after the repairs are completed will help us understand whether the leaks were contributing to the issue.”

A Southern Water spokesperson said: “We’re currently relining sections of combined sewer that have been leaking into ground at the southern end of the Great Morass, but we’re unable to confirm the impact of this work until it’s been completed.

“Bathing waters are impacted by a range of factors, and we continue to explore other potential contributors in Southsea East, working closely with partners including Portsmouth City Council and the Environment Agency. We’re investing £160m across the Portsmouth catchment to help protect and improve water quality.”

The Environment Agency said it is also unable to establish whether the former Great Morass is contributing to poor water quality at Southsea East.

A spokesperson added: “We are working in partnership with Southern Water, Portsmouth City Council and Havant Borough Council to investigate various sources of poor water quality at Southsea Beach.

“We urge members of the public to follow UKHSA advice about swimming safely and to always look at Swimfo if they are considering swimming at one of our 464 bathing waters across the country.”

The Environment Agency said it monitors bacteria levels using samples taken from bathing waters, while also carrying out surveys of drains, ditches, rivers and streams which flow into bathing waters.

Additional samples are taken to help identify potential pollution sources and understand how contamination could be reaching the sea.

It said investigations into poor bathing water quality can be a lengthy process, often involving potential sources being ruled out before the cause is identified.