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The abandonment of the Wheal Jane mine in Cornwall in 1991, followed by recovery of water levels, caused an uncontrolled release of acidic metal-laden water into Cornwall’s Carnon River and the Fal Estuary in January 1992. The Environ-ment Agency implemented a series of emergency measures to control and treat the minewater discharge and therefore limit the impact of the release on the environment. Emergency pumping and treatment measures have been progressively developed since 1992 to form the existing treatment plant, which consists of a simple form of lime dosing for precipitating the dissolved metals, and relies on the Clemows Valley Tailings Dam for the settlement and storage of the resulting minewater sludge. The dam is expected to become full by 2001, and this storage depletion date has established the future programme for determining and implementing the long-term solution.
An initial study by Knight Pièsold considered a range of active and passive minewater treatment options, concluding that Wheal Jane was the dominant source of pollution within the Carnon catchment and that the existing plant was the most cost-effective method of treatment whilst the tailings dam was available for sludge settlement and storage. Knight PiÈsold were commissioned by the Agency in June 1996 to undertake further studies to evaluate the key chemical and biological features of the Carnon River and Fal Estuary, model water quality and provide a more rigorous cost-benefit appraisal of alternative water qualities in the Carnon River and Fal Estuary.
Knight PiÈsold have acted as lead consultant to a six party consortium comprising Risk Policy Analysts, Plymouth Marine Laboratory, W.S. Atkins, RTZ Consultants and Henry Butcher Smith Vincent.
Catchment modelling studies were performed to provide long-term predictions of the quantity and quality of water discharging from various sources within the Carnon catchment. These predictions were based on short-term investigative surveys and the evaluation and interpretation of flow and water quality data collected by the Agency. The relative contribution of contaminants from each source in the Carnon catchment was determined; the major source was found to be untreated Wheal Jane minewater, with County Adit, an historic mine drainage system, also contributing significant proportions of certain metal contaminants.
The different treatment technologies available for processing Wheal Jane and other minewater sources were examined, focusing in particular on effluent quality, process maturity, residue handling and relative capital and operating costs. Three main alternative routes for the active treatment of Wheal Jane minewater were selected for detailed consideration from a wide range of alternatives:
- Oxidation and Chemical Neutralisation (OCN), using well-established recycled sludge technology based on lime dosing to produce a metal hydroxide sludge for disposal;
- Biochemical Sulphidisation (BCS), using relatively new technology based on sulphate reducing bacteria to produce a metal sulphide sludge requiring anaerobic disposal;
- Ion Exchange (IEX), using synthetic resins to extract the metals followed by a conventional, but small lime dosing system.
Passive treatment
The potential use of passive treatment technology for the treatment of Wheal Jane minewater was thoroughly assessed via the operation of three pilot treatment systems at the site. As a result of this and earlier studies, it was concluded that there is insufficient suitable land area available within the Lower Carnon Valley site to contain a treatment plant capable of achieving any of the required water qualities for the Carnon River and Fal Estuary.
From these studies, it was concluded that the treatment of the volume of Wheal Jane minewater produced by the existing six pumps (up to 330l/s) by an OCN route would result in a lower order set of water qualities being achieved for the Carnon River and Fal Estuary. To achieve a higher order set of water qualities would require the treatment of both the effluent from this OCN plant together with the remaining volume of water in the Carnon River to IEX quality.
Estimates of the capital and operating costs for these two treatment options were combined with an assessment of the economic benefits from treating minewater to achieve these lower and higher water qualities. This cost-benefit analysis demonstrated that the treatment of Wheal Jane minewater by OCN technology to be the more cost-effective solution. The Environment Agency, together with the Department of Environment, Transport and the Regions (DETR), endorsed this recommendation in October 1998. The Agency has evaluated tender submissions, with a view to awarding a contract for the design, construction and operation of an OCN plant at Wheal Jane for commissioning before the end of 1999.
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