On the main Environmental Issues page, we discussed how due to past mining activity, Midgelden Brook (our local watercourse) is subject to contamination by ochre water amongst other pollutants.

Water flowing through old mine workings and seeping through the ground becomes loaded with minerals, some of these minerals being chemical compounds of Iron (Fe). We know there is definitely Iron Pyrites (FeS2), sometimes called fool's Gold, present in the land around Woodlands View as we have found nodules of it while looking for fossils. Iron Pyrite is commonly found associated with coal beds and when exposed to Oxygen and water, decomposes into iron oxides and sulphates. Some of these are soluble and get picked up by the water where they may undergo further reactions upon exposure to Oxygen. There are also other forms of Iron present and there may be different mechanisms occurring at the same time and there are even bacteria in the rock and soil that act upon the Iron compounds. An animation of the main process that occurs is shown below.

Very generally it is the following reaction that produces the Iron Sulphate in a soluble form and acidifies the water by producing Sulphuric acid. At this point it is not unusual for the water to look absolutely unnaffected.

2 FeS2 + 7 O2 + 2 H2O → 2 FeSO4 + 2 H2SO4

Upon further exposure to Oxygen, typically where the mine water meets the surface, the Iron Sulphate oxidises to become ferric Iron - a hydrolysis (in the presence of water) reaction which causes the precipitation of Iron III Hydroxide - Fe(OH)3 - these particles are not soluble in water and coat river and stream beds. This is what gives the characteristic Red/Brown staining.

The Iron III Hydroxide for most creatures, is not a poison in itself, but it coats aquatic plants and prevents photosynthesis from taking place which reduces the amount of dissolved Oxygen in the water. It can also block the gills of aquatic invertebrates so they take in less of the Oxygen that is still available. This may not seem so important to a lot of people, but as the invertebrate population is more or less at the bottom of the food chain, the effects can be felt further up, resulting in reduced if not non-existent fish and bird populations.

 

 

In 2013, the Coal Authority spent £3,000,000 on a scheme to treat minewater feeding into Midgelden Brook. They use a combination of chemical teatments and filtration to teat the water. Further details of their minewater treatment can be found here:- About minewater and acid mine drainage

 

Monitoring of water quality is regularly carried out by the Environment Agency, but this is done on a rolling programme and together with reductions in funding from the Government, staff cutbacks and increased demand for flood prevention work, it is possible that sampling may only take place every three years which is not often enough to detect potential problems and act upon them.

 

A proposal was put forward by the Calder & Colne Rivers Trust invertebrate monitoring sub group "for monies to enable the funding to train eight volunteers to undertake a three year programme of invertebrate monitoring of the upper Calder catchment area streams affected by minewater discharges."

 

Of particular interest was the section of Midgelden Brook adjacent to the minewater treatment plant itself. Please note that access to this land is not permitted for the general public. We have permission from the landowner and the Coal Authority to be there for the purpose of invertebrate monitoring and we also have to be specially insured.

 

 Above images courtesy of pmstudios