Case study:Carnon River: Abandoned Metal Mines

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Location: 50° 14' 23.41" N, 5° 8' 25.73" W

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Project overview

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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

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Project summary

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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 Daniell. 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.

There are large areas of mine spoil along the river valley sides which are almost certainly causing diffuse impacts in wet weather. For example, monitoring by the Environment Agency showed that the Wheal Maid tailings dam is causing serious environmental impacts and it has therefore been included on the inventory of abandoned mine waste facilities required by the EU Mining Waste Directive (http://apps.environment-agency.gov.uk/wiyby/139297.aspx).

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.

WFD Compliance The whole river length from headwaters to tidal limit is non-compliant with Water Framework Directive Standards to varying degrees for cadmium, nickel, arsenic, copper, zinc and iron. The WFD monitoring site at Bissoe is below all the main impacts and most recent mean levels here are: Cd 1.72ug/l (EQS = 0.09); Ni 35ug/l (EQS = 20); As 80ug/l (EQS = 50); Cu 123ug/l (bioavailable EQS ~2); Zn 1054ug/l (bioavailable EQS ~12); Fe 2578ug/l (EQS = 1,000).

All of these are non-compliant, but copper and zinc concentrations are approximately 80-90 times the standard for good status, whilst cadmium (a Priority Hazardous Substance under the WFD) concentrations are about 20 times the level needed to achieve good status. Invertebrate surveys here have also exhibited ‘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 with funding from Defra. However, because of the extent, number and nature of other abandoned metal mines impacting the catchment, we do not currently know if it will be technically feasible to clean up the river at an acceptable cost.

Monitoring surveys and results

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Image gallery

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Wheal Maid tailings dam, looking east (July 2013)

Wheal Maid tailings dam, looking west (June 2014)

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