Case study:Carnon River: Abandoned Metal Mines
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Project overview
| Status | In progress |
|---|---|
| Project web site | |
| Themes | Environmental flows and water resources, Habitat and biodiversity, Monitoring, Water quality |
| Country | England |
| Main contact forename | Hugh |
| Main contact surname | Potter |
| Main contact user ID | |
| Contact organisation | Environment Agency |
| Contact organisation web site | |
| Partner organisations | The Coal Authority (UK) |
| Parent multi-site project | |
| This is a parent project encompassing the following projects |
No |
Project summary
The Carnon River catchment is located in a region of Cornwall historically renowned for tin and copper mining activities. It flows through an area described in the 19th century as ‘the richest square mile anywhere on earth’ and is now part of the Cornish Mining World Heritage Site http://www.cornish-mining.org.uk/areas-places-activities/gwennap-kennall-vale-and-perran-foundry). Mining in the catchment started with simple tin streaming to exploit the alluvial tin deposits along the river. By the early 18th century, the area became one of the most heavily mined, with deep workings across the region exploring the rich mineral lodes for tin, copper, arsenic, silver and lead. Some of the workings associated with Wheal Jane and Mount Wellington mines were extended underneath the river itself at a very shallow depth – just a few metres below the surface.
The entire length of the river is impacted by historic mining, as are both major tributaries, with numerous individual sources. The headwaters around Chacewater contain several historic mining and processing sites, notably Wheal Daniel. Further downstream at Twelveheads, the St Day Stream joins, carrying drainage from the Wheal Maid mine and tailings dam as well as other mineworkings in the Poldice Valley. Below Twelveheads, the County Adit discharges into the Carnon. This is not associated with one particular mine, rather it drains a huge heavily mined area to the west of the river. Construction of the Great County Adit started in 1748, and it is made up of a network of tunnels nearly 40 miles in length, draining over 100 individual mines. Recent data suggest County Adit contributes 70-80% of downstream loadings of cadmium, nickel, copper and zinc, and effectively 100% of arsenic and iron. The average annual loads from the adit are: Cd 20 kg; Ni 570 kg; As 1,500 kg; Cu 1,600 kg; Zn 13,700 kg; Fe 80,000 kg.
Further downstream, the biggest tributary, the Hicks Mill Stream, enters the main Carnon. This drains a very heavily mined area on the outskirts of Redruth and contributes 20-25% of loadings of cadmium, copper and zinc. Also here at Bissoe is the Wheal Jane mine site and tailings dam. Wheal Jane was the last operating mine in the area, but when it finally closed in 1991, the dewatering pumps were removed and the workings flooded. In January 1992 a massive uncontrolled release of highly acidic minewater occurred through the Nangiles adit portal. This became one of the most notorious pollution incidents in South West history with a large area of the Fal Estuary stained bright orange by the resultant plume. Although the effect was determined to be short-term, options for long term treatment of the Wheal Jane minewaters needed to be explored. Passive treatment was trialled but ultimately found to be inadequate and since 2000, a full scale treatment plant has operated at the mine site, discharging treated minewater into the Carnon via the Clemmows Stream. This system is managed by the Coal Authority on behalf of Defra at a cost of £1.5m per year.
The whole river length from headwaters to tidal limit fails the environmental quality standards (EQS) for cadmium, nickel, arsenic, copper, zinc and iron, and so fails to achieve good status for the South West River Basin Management Plan. Typical annual average magnitude of failure in the Carnon at the compliance monitoring site at Bissoe are: • Cd = 22x EQS • Ni = 4x EQS • Cu = 153x EQS • Zn = 103x EQS • Fe = 2x EQS
Invertebrate surveys here exhibit ‘poor’ or ‘bad’ status. The river is effectively dead in terms of ecology.
Remediation Treatment and management of the Wheal Jane minewaters will continue under the Coal Authority. We're working with the Coal Authority to explore the feasibility of treating some of the County Adit discharge in the existing Wheal Jane system, as well as other options for improving water quality in the Carnon in the medium to long term. It is possible that because of the extent, number and nature of other sources within the catchment, further measures may be deemed technically and/or financially unfeasible.
Monitoring surveys and results
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