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PART 1 - The problem:

Cwmrheidol mine accessed a lead-zinc bearing mineral lode but there was also a great deal of the iron sulphide marcasite present. In time this decayed into sulphuric acid which then attacked other minerals, resulting in a mine drainage that was highly acidic and laden with an unpleasant cocktail of polluting metals including zinc, cadmium and aluminium.

Dried up ochre outside No 9 Adit, Cwmrheidol, 1992

Here is an image of the entrance to the No 9 Adit which was taken in the summer of 1992. Dried-up, cracked ochre (hydrated iron hydroxide) covers the foreground. The timber lagging of the adit where it passes through shaly scree can clearly be seen. The discharge flows into Afon Rheidol via a settling tank put in by the C.E.G.B. in the 1970s. Filled with limestone chips, this worked initially but is now less effective because the reaction in time coats the limestone with insoluble gypsum.

1m rib of marcasite exposed in the neighbouring Tynyfron mine

The mines in this immediate area all worked mineralised fractures associated with the Castell Lode, a major ENE-striking tensional fault system. Immediately to the west of Cwmrheidol the lode could in the early 1990s be seen at the neighbouring small Tynyfron mine, where over 1m of shattered marcasite is exposed in pillars along the small stopes (above). This gives an indication of the mineralisation cut by Cwmrheidol, and why there is such a pollution problem there.

Diagram showing rough layout of Cwmrheidol mine

This is a simplified diagram of the mine. The oldest part is up on the hilltop at Ystumtuen. Here, workings go back into the 1700s. Lewis Morris described the marcasite at Ystumtuen thus, in the mid 18th Century:

"There is a vast quantity of Marcasite in this work, and it shoots into Chrystals of Copperas by the very heat of ye Sun, but it is of very little value here being so far from the Sea. The waste of this mine is worked over and over to profit Every eight or ten years, the Marcasite being dissolved by the Rains and Heat of the Sun Suffers the Ore to be disengaged" - an early example there of heap-leaching!

The No 6 and No 9 levels at Cwmrheidol were driven more recently to make dewatering the mine easier. No 6 was started in 1824 while No 9 was a later 19th Century venture. The main mine drainage exits from No 6 as a moderate stream. This tunnel drains a lot of the Ystumtuen area and in heavy rain the water roars out. However, in the early 1990s water was backed up due to a fall of ground, which allowed it to escape down a winze into No 9 Adit.

Acidic water pours over roof-high shale dam in No 9 adit, Cwmrheidol mine, 1992
In 1992, site investigations were commissioned with a view to solving the pollution problems. I was working with mining engineer, historian and archivist Simon Hughes of Talybont back then. Examination of the No 9 adit entrance identified an immediate cause for concern, with a shale dam that had formed where the tunnel's wooden lagging had failed, allowing the loose scree to run in. A local reported seeing water squirting out upwards after prolonged rain.

What if this dam should have failed? The result would have been a sudden influx of over 500,000 gallons of ochreous acidic water into Afon Rheidol. This actually happened in the late 1960s, when the blocked adit was disturbed by a JCB and burst open. Thousands of fish were killed and the sea off Aberystwyth Harbour turned orange! So, in this case what did we do??

Next: No 9 Adit - The solution to the problem