Case study:Carnon River: Abandoned Metal Mines

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Location: 50° 14' 23", -5° 8' 26"
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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
The County Adit

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 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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Lessons learnt

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Image gallery


Wheal Maid tailings dam, looking east (July 2013)
Wheal Maid tailings dam, looking west (June 2014)
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Catchment and subcatchment

Catchment

River basin district South West
River basin West Cornwall and the Fal

Subcatchment

River name Upper Carnon River
Area category 10 - 100 km²
Area (km2)
Maximum altitude category 200 - 500 m
Maximum altitude (m) 234
234 m
0.234 km
23,400 cm
Dominant geology Siliceous
Ecoregion Great Britain
Dominant land cover Improved grassland
Waterbody ID GB108048001160



Site

Name
WFD water body codes GB108048001160
WFD (national) typology
WFD water body name Upper Carnon River
Pre-project morphology
Reference morphology
Desired post project morphology
Heavily modified water body No
National/international site designation
Local/regional site designations
Protected species present No
Invasive species present No
Species of interest
Dominant hydrology
Dominant substrate
River corridor land use
Average bankfull channel width category
Average bankfull channel width (m)
Average bankfull channel depth category
Average bankfull channel depth (m)
Mean discharge category
Mean annual discharge (m3/s)
Average channel gradient category
Average channel gradient
Average unit stream power (W/m2)


Project background

Reach length directly affected (m)
Project started 2013/07/01
Works started
Works completed
Project completed
Total cost category
Total cost (k€)
Benefit to cost ratio
Funding sources

Cost for project phases

Phase cost category cost exact (k€) Lead organisation Contact forename Contact surname
Investigation and design
Stakeholder engagement and communication
Works and works supervision
Post-project management and maintenance
Monitoring



Reasons for river restoration

Mitigation of a pressure Pollution incident, Mine drainage metal concentrations
Hydromorphology
Biology
Physico-chemical PH, Nutrient concentrations
Other reasons for the project


Measures

Structural measures
Bank/bed modifications
Floodplain / River corridor
Planform / Channel pattern
Other
Non-structural measures
Management interventions Remediation Treatment
Social measures (incl. engagement)
Other inverte-brate analysis, Improving water quality


Monitoring

Hydromorphological quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

Biological quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

Physico-chemical quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

Any other monitoring, e.g. social, economic

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative


Monitoring documents



Additional documents and videos


Additional links and references

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Supplementary Information

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