What killed the fish at Groby Pool?

December 2008 The controversy over the causes of the pollution incident at Groby Pool continues. It will take visitors to the Pool during the pollution a long time to forget the unpleasant stench that greeted them and the distressing sight of dead fish in the water. As the Spotlight reported last month the situation is returning to normal, but the threat of a possible recurrence at some time in the future concerns to local residents.

Source of pollution unclear

The Environment Agency believe that the pollution relates to some kind of incident some distance away from the Pool, but cannot pinpoint the source. “The water quality has recovered well, confirming the incident is not an ongoing problem. Frustratingly, despite the time and resources we have put into this investigation, we have not been able to prove the source of the pollution,” said Jonathan Hall, Environment Management Team Leader at the Agency. “ We suspect it was caused by liquid slurry or similar material which entered the Slate Brooksome distance upstreamwest of Groby Pool and the pool at Grey Lodge. Detailed chemical analysis, specialist biological survey work, field observations, weather and local geography all support this conclusion.”

“We regulate the application of slurry to land and are aware that operators were spreading in the Slate Brook catchment,” he continued. “We are also aware that we had heavy rainfall on the 5th of October with other scattered showers on the following days. Although we have visited all the farms in the catchment, we have been unable to find evidence proving a source. As the incident was reported to us on 13th October, (probably at least a week after it happened), and in light of the heavy rainfall we had, it is not a surprise that we had no visual contamination to trace. Liquid slurry would not leave sludge on the bed of a watercourse but would have impacted on the two pools in the Slate Brook catchment in the way we saw. This is why we had to rely on our detective techniques to try and find the source of the pollution.”

Local concerns

But locally there are still concerns that this was not the source of the problem and that further investigation is required. Questions have been asked about the amount of slurry run off that would be needed to contaminate such a large volume of water and that might have left evidence of contamination forseveral days after. It has also been suggested that the strong ammonia like smells reported in the vicinity of Groby Pool could indicate the ingress of a gas, ammonia or similar seeping through the substrata and that such a gas could have removed the oxygen from the waters of the lake, resulting in the death of the fish

With these questions still in the air the Spotlight asked Jonathan Hall to provide a more detailed report of the investigation, and he has produced a day by day account for readers.

First published in the Groby Spotlight December 2008

Diary of a pollution incident

On 13th October we received a report of dead fish in Groby Pool and started our investigation. We immediately tested the water with field kits which revealed low oxygen readings, high ammonia levels and a normal pH value, (it was not too acidic or caustic). When oxygen levels in water are low, fish will struggle to breathe and can die and higher levels of ammonia are harmful to aquatic wildlife. Low oxygen levels often occur when there is a large amount of biological material breaking down and decomposing in the water. It is broken down by bacteria which use up the oxygen and cause the living aquatic wildlife to suffer.

We immediately investigated the Landfill site which has a sewer that connects the site to a local sewage treatment works. We quickly asked for this sewer to be pressure tested because we wanted to make sure it had not burst. We tested the water in the local watercourses and took water samples from them and Groby Pool. We received further information stating that gulls were seen swooping down on Groby Pool to feed on dead fish on 8th October.

By the 14th October we had widened our investigation and checked the surrounding fields and watercourses. Additional water samples were taken from Groby Pool and Slate Brook. Low oxygen and high ammonia levels were found in the Slate Brook and an agricultural smell was noted. A second pool on the Slate Brook, near Grey Lodge, upstream of Groby Pool was also found to be polluted in the same way, showing the pollution had come down this stretch of watercourse. The manager of this pool was advised of the problem. The pressure test on the Landfills sewer had been completed and shown no leaks. Severn Trent Water sewers were checked and no pollution sources identified.

On 15th October we called on the assistance of our Biology specialists to help us trace and detect the source of the problem. They also reported an agricultural odour to the water in the Slate Brook and confirmed the pollution had come down the brook, impacting on both Pools. Over the following days they managed to trace a trail of dead invertebrates, (creepy crawlies), up the Slate Brook to a stretch near the M1 motorway, some distance west of Groby Pool and upstream of the Landfill site. No incidents have been reported on the motorway itself.

17th October, contractors with a boat finished removing the dead fish from the Pool, which was photographed and reported in the Leicester Mercury on 18th October. Our Officers continued in conducted site visits in the catchment.

On 20th October results from the detailed chemical analysis of the water in Groby Pool and the Slate Brook was received, supporting our view that the incident was a result of an organic pollutant and not due to landfill chemicals or other chemical contamination.

Markfield Parish Council was told in May by County Councillor David Sprason that the Environmental Agency had confirmed that rotting organic matter from Slate Brook had fed into Groby Pool, causing the contamination in October 2008.

February 2011 : Poor quality outlook for Groby Pool

February 2011 A few years ago the Spotlight reported on the condition of Groby Pool when it was last assessed by the government department now known as Natural England. As the next assessment was due in 2010 this seemed a good time to enquire about the results. Unfortunately this has not taken place.

“There has been no new assessment of Groby Pool,” explained a spokesman for Natural England. “It is unlikely that the condition of the interest features will have recovered because water quality is still likely to be an issue for the Site of Special Scientific Interest(SSSI). Since the pollution incident there will have been some recovery of the natural habitats (and normality) as well as recovery of the fish and other animal populations. However, the recovery will occur up to a certain point and will favour those species which are more tolerant of nutrient rich aquatic environments. Lowland water bodies like Groby Pool are being influenced by adverse water quality which is generally measured in terms of phosphate concentration. The source of the phosphate is generally the catchment and also the lake sediments which would have originated from the catchment in the past.

In the case of Groby Pool, phosphate levels were relatively high (for a lowland lake as measured by SSSI water quality standards) even before the pollution incident and trying to restore more natural phosphate levels is a really big challenge. This is likely to take a long time since it will require management of the phosphates coming from the catchment and also the phosphates which get released periodically from the sediments.”

Natural England is still some way off from achieving this, and as a result, believes Groby Pool is likely to remain in unfavourable condition for the foreseeable future, the spokesman concluded.