 PREVENTION OF
ENVIRONMENTAL DISASTER AT CWMRHEIDOL MINE,
CENTRAL WALES, EARLY 1990s
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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.
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. |
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. |
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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.
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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??
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Next: No 9 Adit -
The solution to the problem
EXIT
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