Engineering works downstream of Vicars Bridge

Anyone travelling along the Dollar-Muckhart road or on the back road from Vicars Bridge to Blairigone will have noticed the large scale excavations underway by the track that runs down to the river by Mains of Blairingone

Mains of Blairingone - 060316

These works are being undertaken by contractors employed by the Coal Authority and are very much for the benefit and protection of the river. The Devon Angling Association are delighted that these works are being undertaken now, having made a number of representations to the Coal Authority over the years.

So what’s going on?

Well the area between the plantation and valley side has for about the last twenty years has been a passive water treatment facility, helping to clean iron contaminated water from old mine-workings before it reached the Devon. It was installed following a serious outbreak of minewater from an old adit in the vicinity of Mains of Blairingone in 1994 which had resulted in serious pollution in the Devon. Anglers alerted the Forth River Purification Board when up to 2km of the Devon downstream became coated in Ochre (the characteristic orange gunge). The small burn it discharged into was rendered devoid of all life over 800m. What had caused this rapid deterioration could never be established but the finger was pointed at British Coal who at the time were working the Lambhill Opencast Coal Site at Blairingone. As was their policy at the time they denied liability for pollution but worked with the Forth River Purification Board to deal with the problem.

The solution was to capture and pipe the minewater under the Blairingone road were it is allowed to flow down a purpose built cascade, before flowing into the shallow lagoons which were allowed to be populated with reeds, rushes and other wetland plants

Cascade

I will save you a chemistry lesson, but essentially the problem with the minewater begins with Fools Gold (Iron Pyrites) which is commonly found in coal, ironstone and shales. It oxidises in contact with air, and as it does it produces heat which accelerates the oxidation (which in certain circumstances can lead to spontaneous combustion in coal, which resulted in some tragic accidents in collieries).  In oxidising the Iron Pyrites is converted to Iron (Fe2) Sulphate which is solvent in water. When the mines are abandoned and the water table rebounds to its natural level, that water dissolves the Iron Sulphide in the first instance Iron (Fe2) Sulphate and then Iron (Fe3) Sulphate. When the soluble iron remerges and comes into contact with oxygen again, it rapidly oxides forming Iron Hydroxide which is the orange gunk known as Ochre.

The cascades are designed to oxidise the soluble iron, which is then carried by the stream and fed into the “reed beds” which filter the water to collect all the iron before the clean water is discharged into the Devon. The system has been very successful in cleaning the water and returning this section of the Devon into a very healthy stream.

We were becoming concerned that the effectiveness of the reed beds was diminishing as the lagoons filled with sediment and that the bunds were not as substantial as they may have been. It is therefore really pleasing that the Coal Authority are taking a ‘belt and braces’ approach to renovating the treatment system.

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