Parys Underground Group / Grwp Tanddaearol Parys

Dewatering project (2003)

Following the discovery that a large volume of water within the mountain was only being retained by a poor quality concrete dam in the Dyffryn Adda adit a Dewatering Project group was set up.
The group consisted of members of PUG, AIHT, Anglesey mining PLC, Welsh Development Agency, Countryside Commission for Wales and the local council.
Following discussion it was decided that the safest course of action was to pump out the water from a shaft which had been identified behind the dam.  The Cementation civil engineering company were contracted to carry out the work.



The water was pumped via an existing drainage and settlement system to the Sea at Amlwch Port.
The quality of the water was monitored every day by engineers working on the project. Samples are also being taken and analysed by the Environmental Agency.
The water was clear, with a pH of about 2.5. As you would expect the water contains both copper and zinc and traces of other metals.
The pumps where capable of removing around 50 litres per second of water from the mountain. In total around 30 million gallons of water was removed.


The line of the adit can be seen by these old air shafts in the field.
Some of these old air shafts where cleared and made safe. This then allowed the engineers to enter the adit and make a proper assessment of the dam once the water had been removed from the other side of it.

Once safe access to the dam was available a number of test holes were drilled to ensure no significant amount of water was held up behind the dam.  A high expansion explosive was then used to destroy the dam.
Here the dam has been breached.  The build up of large amounts of debris behind the dam over 60 years can be seen.