Diary of an eco-nazi part two: the hands-on approach -
Environmental disaster prevented at Cwmrheidol Mine, Mid Wales, 1992-4


The Welsh Weather & Dyfi Valley landscapes Image-Library - Click

It's been a while since I added an environmental piece so here is one which is of a topical nature since the Cwmrheidol Mines near Aberystwyth have been in the news recently. The story concerns pioneering technology which is proving successful in stripping heavy metals from the pollution-riddled water-discharge from an old system of lead-mines. I was not involved in this particular piece of technical work: however, I was involved in a rather more hands-on manner nearly twenty years ago, when part of a small team contracted to the Environment Agency. I recorded both periods of work with my old Canon A1, thankfully, since the job was one of the most unusual and colourfully messy ones I have ever done. This piece is based on a presentation I did for an event organised by Ceredigion Council and the Environment Agency a couple of years ago.


The story begins hundreds of years ago, when people first discovered the lodes carrying ores of lead, silver, zinc, copper and iron crossing the hills near Ystumtuen, a small upland hamlet a dozen miles east of Aberystwyth. From the 17th Century onwards, mines were sunk deeper and deeper along the lodes, and by the early 19th Century the workings were getting to a depth where better drainage was urgently needed. There were three main mines, from east to west Bwlchgwyn, Penrhiw and Ystumtuen:
The Ystumtuen mines from the air

In 1824, a deep drainage tunnel was started. It was driven into the side of the Rheidol valley at Cwmrheidol, and eventually reached the workings, coming in under Ystumtuen mine. As all three mines were connected underground, the tunnel drained all the water entering the workings from Ystumtuen through to Bwlchgwyn. The map below shows the relative levels:

Ystumtuen mines map

Here is a view of the Ystumtuen mine at surface: tips of waste-rock lie around the old shafts. Some of the waste is grassed-over but some remains bare: this is because it is full of toxic metals and is highly acidic, for reasons that will be explained below.

Ystumtuen mine

Later in the 19th Century a still deeper drainage-tunnel was driven in from Cwmrheidol. The relationship between these tunnels - the No.6 Adit and the lower No.9 Adit, is shown in the sketch-section below.

Ystumtuen mines sketch-section


The Ystumtuen mines were unusual in the Central Wales mining district. In an old mining-area where there are dozens of mines and hundreds of trial-workings, in only a handful was the iron sulphide mineral, marcasite, really abundant. The image below shows a section of the lode exposed underground: the pale grey stuff is all marcasite, well over a metre in width. In places the mineral was actually mined and sold - not for making iron but for making sulphuric acid.

Marcasite in stope pillar, Tynyfron mine

Marcasite is metastable. That basically means it's mostly fine if undisturbed deep in the ground. But expose it to air and water and it starts to decompose rapidly. The equation can be written thus:

Marcasite + oxygen + water = sulphuric acid + iron in solution

In other words, it is a source of acid mine drainage. What makes matters worse is that the acid then readily attacks any other ore minerals present. Thus you end up with an acidic mine discharge carrying all sorts of dissolved metals - all of which are toxic at the levels present.

Ochre outside No.9 Adit, July 1992
The ferrous iron dissolved in the water oxidises by its chemical reaction with air and a lot of it precipitates into an insoluble hydroxide known as ochre.

This photo was taken in July 1992 during a routine site inspection. In the background is the entrance to the No.9 Adit. The dry weather has evaporated the acid mine discharge, leaving the ochre cracked in the sun.

So much ochre occurs here that at one time it was collected and sold for pigment-making. Stir a bit up with water and it stays suspended for ages. Get it on your skin and it stains it for many days.

The water issuing from this tunnel has a pH of between 2 and 3. That's acidic enough to burn sensitive skin.

The result of a lot of this stuff suddenly entering the Rheidol is not difficult to imagine - and in the late 1960s that's exactly what happened.


In the 1960s, mineral exploration companies were looking around Central Wales for further mineable reserves of lead, zinc and copper. At Cwmrheidol a deep borehole was planned, so that an area of the steep hillside needed levelling to make a drilling-platform.

As this work was ongoing, the digger-driver was alarmed to see water squirting out of the ground and went to report it to the site office. By the time they returned, a tremendous outburst of bright orange water was surging down the hill and into the Rheidol. The position of the No.9 Adit had been discovered!

This turned out to be a major environmental disaster. The whole river flowed bright orange and dead sea-trout and salmon were everywhere. The colouration spread to the Harbour at Aberystwyth and the sea around its entrance.

Fast-forward over 20 years and I found myself commissioned to undertake a mineralogical survey of the Ystumtuen mines for an environmental consultancy company, who in turn had been asked to report on the mines by the Powers That Be, with a view to undertaking works to address the pollution. Having done the mines up on the hilltop, a wet late autumn afternoon saw mining consultant Simon Hughes and I walking up the tips towards the No.9 Adit at Cwmrheidol. The river was in dark, peat-brown spate as it had been raining hard the past few days and the trees bent to the gale as dusk gathered. We made our way carefully around the evil-looking pool of ochre and water and peered in past the remains of the gate. This was what we met with:

No. 9 Adit - blocked near entrance

ER, Houston, we have a problem.....

What had happened was that the timber lagging had burst in - allowing the scree to pour into and block the tunnel; the water had then backed up to roof-height. We knew the likely gradient and dimensions of the tunnel and a calculation suggested in excess of half a million gallons of this acidic, metal-laden water were sitting there atop thick ochre, held back by no more than a few tons of fine shaly scree. In a severe rain event, it was possible that a head of water could build up within the mine, pressurising the dam to blowout-point.

The sight was shocking and we each took a few photos and left, deeply troubled as to what to do.

The first thing was to alert the National Rivers Authority (NRA), now the Environment Agency. We met on site and having examined the problem the question was raised, "can this be fixed?": "yes" was the reply, and thus began the filthiest job I have ever done. The battle to save the Rheidol was on.


The acidic water in No.9 Adit would have to be removed before it went on its own en-masse. A controlled pumping-out was the only option, but first we took some major precautions. The dry lagoon below, situated directly below the mine entrance and with a large capacity capable of catching the water should the dam go on its own, was floored with many tonnes of powdered limestone, which would reduce the acidity of the water.

Spreading powdered limestone in the lagoon

The stream from No.6 coming down past No. 9 Adit

Pumps were brought in and a digger very, very carefully dug out the bank until the scree stopped sliding down and uncovered the back of the tunnel inside of the dam. Once enough room had been made, the pumps were started.

The water was drawn from the top to avoid ochre disturbance and it was piped and discharged into the stream coming down from the higher No.6 Adit.

This may sound like a deliberate act of pollution but the choice was this or disaster: in addition, the stream was treated with agricultural lime every 15 minutes on a 24-7 basis, which meant splitting the team into a day and night-shift.

The liming was successful in keeping the pH of the stream in the mid-5s - higher than it usually was.

Once the water had been drawn off, a bigger sludge-pump was brought in to deal with the ochre, which lay feet-deep on the tunnel floor. It was pumped down to the dry lagoon and sandwiched with powdered limestone. Here's a view looking in from the entrance, with the pipe carrying the sludge on the far RHS.

No. 9 adit entrance

Here's a typical view of the site at this messiest stage of the work - the ochre got absolutely everywhere!

Ochre everywhere!

Finally, the levels were down to the floor. The Rheidol had been saved! We removed the dam and laid heavy twin-wall pipes so that should another collapse occur the water would be able to escape beneath it....

The burst lagging of the No.9 Adit

Of course, we simply had to have a look what lay beyond!

This is a few paces past the dug-out dam looking back towards it. Ochre covers the floor, walls and roof, but it is only a thin covering - most was now encapsulated in limestone powder down below. As vile places go, this took some beating!

Inside the No.9 Adit at Cwmrheidol

This is as far as we could safely go. Not because of the dodgy timber but because of the low oxygen content of the air, making breathing laboured. It would have been madness to go any further in.

Inside the No.9 Adit at Cwmrheidol

The gate was fastened and the site cleared. "Bad Air - Keep Out" was painted on the gate shortly after this was taken.

This was regarded as a job well done, but Mother Nature had an ace up her sleeve.....

No.9 Adit repair nearing completion


One Autumn afternoon in 1993 I wandered up to No.9 to see how things looked and was horrified to be met with the following:

Debris swept down hillside to No.9 Adit entrance

Water jetting from No.6 Adit, late 1993
The surge of water and debris had clearly come from above and was nothing to do with the work completed at No.9. I walked up to investigate.

The NRA had previously installed a settling tank and discharge-pipe at the entrance to the No.6 Adit. When I arrived the water was surging out in a jet. It had gullied the course of the stream, washing out large trees and sweeping down tonnes of shale.

So a water-surge had come down No.6 Adit. Where had it entered the workings? The answer would surely be found at one of the Ystumtuen mines up on the top....

And it didn't take long to find. Old workings at Bwlchgwyn mine had opened up right under the Nant Bwlchgwyn and had swallowed the entire stream:

Nant Bwlchgwyn swallowed by the mine!

Culvert carrying Nant Bwlchgwyn over subsided stopes

This was a straightforward job: a half-section of strong Armco pipe was installed to bridge the collapse and the Nant Bwlchgwyn flowed normally again.

The NRA wanted the drainage within No.6 Adit inspecting and sorting out where necessary, and so began the next phase of the project.


No. 6 is a long tunnel cut through hard, dark grey Silurian slaty mudstone. This shot is the view outwards from where we used to stop for breaks:

No. 6 Adit, looking out towards entrance

No. 6 Adit: the fallen ground
Where the tunnel met the lode, old workings had collapsed, blocking it almost to the roof with boulders and shale.

Shortly beyond this point, a T-junction is reached from which tunnels lead off left and right. That on the left led, according to old plans, past a winze - an underground shaft - that led on down to No.9 Adit.

Because the No.6 Adit was blocked and the water behind backed-up, it was able to escape along the left-hand tunnel and down the winze into No.9 Adit.

This, and the risk of more debris damming the tunnel right up, led to the decision to remove this dam too, and put in pipes to maintain a safe and steady flow.

No. 6 Adit: the fallen ground dug through

The job was easier than No.9 because the dam was solid and so we simply removed it block by block, allowing the water level to fall each time.

There was still ochre, but nowhere near as much as in No.9 Adit and in addition, because of the connected workings, there was a good flow of fresh air - it was quite a pleasant place really!

Here's a sketch showing where we were working:

Ystumtuen mines sketch-section

The dam removed and the water-level down, we were able to see the T-junction. The left-hand tunnel had been walled-up, but water was seen to be flowing from its base - in the right direction now! Note the ochre stalactites and the tide-marks left by higher water-levels:

No.6 Adit: crosscut meets drivage east & west along lode

Finally, in went the pipes and the water again flowed clear....

Clear water once again flowing, No. 6 Adit

Old settling tanks at Cwmrheidol

The water continued to flow down into the old settling-tank by the river, although this was regarded as ineffective now. It is full of limestone chips - the problem being that the following chemical reaction takes place:

calcium carbonate + sulphuric acid = calcium sulphate and carbon dioxide

The bad bit of this reaction has nothing to do with CO2 - it is the calcium sulphate - or gypsum, to give its mineral name. Gypsum is only very weakly soluble in water, so it effectively forms, in time, an impenetrable rind on the limestone, severely reducing the reaction.

The new technology now installed up at Cwmrheidol takes a biochemical approach - with a mixture of compost, cockle shells and limestone being used to treat the water. Bacteria that thrive within this mixture fix the heavy metals, so that the water leaving the experimental plant has had some 99% of its metals loading removed.

The pilot-project has proved a major success and the technology has great potential to sort out heavy metal pollution worldwide. More can be seen here on the BBC Wales website:


Reading it brought back vivid memories of the work I did up at these mines - and above all, that ochre that found its way everywhere! Ten years later, when my Series 3 Landrover finally fell apart, it still carried the bright yellow stains where I had brushed against the inside of the door whilst grabbing my sandwiches. But it was a job I have been proud to have been involved with to this day.

The Welsh Weather & Dyfi Valley landscapes Image-Library - Click HERE