The old ways can still be the best
In these days of sophisticated GIS-assisted monitoring technology, Atkins Environment finds it can be refreshing to know there is still a function for tried and tested basic techniques.
Harecastle Hill, on the Trent and Mersey Canal, provided numerous seams of coal which led to the construction of the two affected canal tunnels and saw narrow boats loaded with worked coal - mined above them - before carrying it onwards. The western tunnel was built between 1766 and 1773 by James Brindley with an eastern counterpart – to provide two way traffic – completed by Thomas Telford in 1827.
The drainage outlets into the canal from the mine workings overhead were initially beneficial to both parties, draining the mines and replacing water lost through locks to the north and south of Harecastle which is at the crest of that stretch of the Trent&Mersey.
But, since the abandonment of mining, oxidisation means the average 3.05million litres of water a day cascading into the canal contains dissolved iron and consequently colours the waterway orange for up to 2.5km .
Hence the need for Atkins Environment, and the initial task – since November 2002 – has been to monitor flows and determine the iron content of the discharges. The analysis is taken care of by a scientist from TES Bretby alongside the Atkins bucket men.
The monitoring effort is complicated by the fact Harecastle is a working tunnel used by around 5000-6000 holiday boats a year. This means the two hour monitoring exercise has to be completed by the time Harecastle is opened to the public at 8am.
“Typically we use a bucket or large plastic container held underneath each discharge,” said environmental scientist Keith O’Brien. “By timing how long it takes to fill the container to a set level we can measure the flow – it’s very sophisticated technology!”
This Heath Robinson approach can be tricky with some of the discharges dumping 20 litres a second of minewater into the canal.
“We often have to take several measurements to get an accurate figure on the heaviest discharges,” added Keith. “One of the discharges is particularly fragmented so we have to use sand bags to channel the flow and get an accurate measurement.”
The Eastern tunnel has 10 definitive discharges with 14 smaller seepages although the number varies seasonally. The Western tunnel is disused after suffering mining related subsidence so the discharge is monitored at both ends.
The monitoring and analysis is providing a robust data set which currently estimates an average iron as Fe (total) loading per day of 62.8kg discharged in an average flow of 3,050 million litres every day.
The pH varies from 3.3 to 8.8 with an average of 6.6.
These figures will allow Atkins Environment consultants to design an appropriate remediation strategy – quite an engineering headache when the tunnel has to remain open to vessels and therefore the water has to stay in the system to avoid the canal draining.
The consultants also have to bear in mind the portals of both tunnels, and the retaining wall connecting them, as these are Grade II listed structures.
The discharges will have to be captured from within the tunnel, pumped to a treatment area and cleaned, probably using passive technology with a combination of settlement ponds and wetlands, before being discharged back into the canal at significantly improved quality.
But before all that there is one final shift on the rescue vessel – on May 14 – the prospect of which left Keith philosophical.
“Life is never dull as an Environmental Scientist,” he said.