wheal jane minewater study environmental appraisal and treatment strategy

446 Pages·1610·19.36 MB·English

WHEAL JANE MINEWATER STUDY
ENVIRONMENTAL APPRAISAL
AND TREATMENT STRATEGY
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WHEAL JANE MINEWATER STUDY
Environmental Appraisal
and Treatment Strategy
NRA
National Rivers Authority
South Western Region
Knight Piesold RPA
Kanthack House Risk & Policy Analysts Ltd
Station Road Warren House, Beccles Road
Ashford, Kent Loddon, Norfolk
TN23 1PP NRJ4 6JL
WHEAL JANE MINEWATER STUDY
ENVIRONMENTAL APPRAISAL AND TREATMENT STRATEGY
Contents
EXECUTIVE SUMMARY
1. INTRODUCTION
2. BACKGROUND
3. THE RELEASE OF MINEWATER FROM WHEAL JANE
4. EXISTING TREATMENT SYSTEM
5. THE CURRENT SITUATION
6. HYDROLOGICAL MODELLING
7. DEVELOPMENT OF WATER QUALITY OBJECTIVES
8. LOCATION OF LONG TERM TREATMENT PLANT
9. PREVENTION & CONTROL OF DISCHARGES
10. PASSIVE TREATMENT TECHNOLOGY
11. ACTIVE TREATMENT TECHNOLOGY
12. SLUDGE DISPOSAL
13. ECONOMIC BENEFITS OF IMPROVEMENTS IN WATER
QUALITY
14. TREATMENT OPTIONS
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Wheal Jane Minewater Study Executive Summary
Environmental Appraisal and Treatment Strategy
EXECUTIVE SUMMARY
INTRODUCTION
Wheal Jane is an abandoned underground tin mine in Cornwall. After mine closure in 1991,
underground pumping ceased, allowing groundwater levels to recover, releasing acidic metal
laden minewater into the Camon River. The result was a highly visible and widely reported
pollution incident extending into the Fal Estuary.
In 1992 the NRA set up a project with the following objectives: -
• Amelioration of the effects of the metal rich minewater from Wheal Jane on the
Camon River and Fal Estuary.
• Development of water quality objectives for the Camon River.
• Research into the most appropriate and cost effective long term treatment strategies
for achieving various water quality objectives.
This report provides the basis for the NRA’s recommendations, to the DoE, on the long-term
options for treating the Wheal Jane minewater.
BACKGROUND
The mines in the Camon Valley have been worked as far back as 2000 BC, although
extensive mining only began in the 17th Century. By the 1850s the mines in the Camon
Valley were the largest group of copper producers in the world.
The effect of this industrial activity on the valley has been:
• The deposition of metal-rich silt in the Camon River, Restronguet Creek and Carrick
Roads.
• Dewatering and lowering of the groundwater table by both pumping and the
construction of drainage tunnels (adits).
• Elevated metal concentrations in the local watercourses and the Fal Estuary.
THE RELEASE OF MINEWATER FROM WHEAL JANE
Following mine closure and the cessation of dewatering, it became apparent that a release
of minewater into the Camon River was inevitable. As minewater rose to the level of
drainage adits, the NRA exercised its statutory powers to instigate an emergency treatment
system. Despite attempts at treatment, there was an uncontrolled release of minewater on
January 13, 1992 after an adit plug failed.
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Wheal Jane Minewater Study Executive Summary
Environmental Appraisal and Treatment Strategy
Water quality in the Camon River and the estuary was greatly affected, with concentrations
of many metals exceeding Environmental Quality Standards (EQS) by up to two orders of
magnitude. The release of high iron loadings also caused short-term discolouration of
Restronguet Creek and the Carrick Roads.
The treatment system, instigated by the NRA, has since been progressively upgraded and
comprises:
• pumping of minewater from underground;
• the addition of hydrated lime to neutralise the acidic water, resulting in the formation
of a metal hydroxide precipitate (metalliferous sludge);
• flocculation, to promote rapid settlement of the metalliferous sludge;
• sedimentation and storage of the resultant metalliferous sludge in the Clemows Valley
Tailings Dam.
The existing treatment system is located on the Wheal Jane mine site and is operated on
behalf of the NRA by South Crofty pic.
The system currently treats on average 155 1/s but now has sufficient capacity to handle up
to 300 1/s. Monitoring has indicated that on average 97.5% of the metals are recovered from
the treated minewater. By the end of 1994, the system had removed some 12 500 t of metal
from the outflowing minewater and has been responsible for minimising the environmental
impact on the Fal Estuary. Monitoring has demonstrated that the effects of the minewater
release on both water quality and discolouration were short-lived. There appears to have
been no major adverse effects from the incident on the biota of the estuary.
THE CURRENT SITUATION
Although the existing treatment strategy has significantly reduced the release of untreated
minewater to the Camon River, an overflow of untreated minewater still occurs during the
winter and spring from Nangiles Adit. Metal concentrations in the Wheal Jane mine have
declined since 1992 and are typically one order of magnitude lower, as can be seen on
Figure EX. 1. Metal concentrations in the Camon River have also declined rapidly and are
now at pre-incident levels.
However, due to other sources of contamination, the concentrations of a number of metals
in both the Camon River and Restronguet Creek still exceed EQS under the European
Community (EC) Dangerous Substances Directive. Zinc levels in part of the Fal Estuary
also exceed the EC Shellfish Waters Directive.
The Wheal Jane complex, together with County Adit, are the major sources of contamination
in the Camon River. However, significant amounts of metals also are contributed from other
tributaries and non-point sources. For this reason there would still be a significant metal
loading in the Camon River even if the two major point sources (Wheal Jane and County
Adit) were treated.
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Environmental Appraisal and Treatment Strategy
Photograph Ex.l Camon River - Catchment above Grenna Bridge
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Wheal Jane Minewater Study Executive Summary
Environmental Appraisal and Treatment Strategy
Figure EX 1 Measured Total Zinc and Iron Concentrations
in No.2 Shaft
Iron Concentration (mg/1)
NRA South Western
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Wheal Jane Minewater Study Executive Summary
Environmental Appraisal and Treatment Strategy
WATER QUALITY OBJECTIVES FOR THE CARNON RIVER
Based on statutory ana other requirements, the following water quality objectives have been
adopted for the Camon River:
• The "No Deterioration" objective : to maintain water quality at 1993-1994 levels.
• The "North Sea Commitments" objective : to achieve a reduction in the zinc loading
in line with UK commitments made at the 1985 North Sea Conference.
• The "EC Directive" objective : to achieve full compliance with the EC Dangerous
Substances Directives.
In addition a "No Treatment" scenario (do nothing option) has been considered.
The maximum permissible metal concentrations allowable for each of water quality objectives
are summarised in Table EX-1, together with the predicted metal concentrations for the "No
Treatment" scenario.
Table EX-1 : Water Quality Objectives for the Carnon River
Predicted Water Quality Objectives
"No Treatment"
Substance No North Sea EC Directive
Water Quality
Deterioration Commitments
Cadmium 95 %ile 11 (T) - -
as /ig Cd/1 AA 21 (T) 6 (T) - 1.0 (T)
Copper 95 %ile 0.9 (T) - -
as mg Cu/1 AA 0.8 (T) 0.6 (T) - 0.028 (D)
Zinc 95 %ile 13 (T) - -
as mg Zn/1 AA 26 (T) 6 (T) 3 (T) 0.5 (T)
Arsenic 95 %ile 0.3 (T) - -
as mg As/1 AA n/a 0.1 (T) - 0.050 (T)
Iron 95 %ile 17 (D - -
as mg Fe/1 AA 52 (T) 8 (T) - 1.0 (D)
Manganese 95 %ile 1.0 (T) - -
as mg Mg/1 AA 1.7 (T) 0.7 (T) - -
Aluminium 95 %ile 4.0 (T) - -
as mg Al/1 AA 6.2 (T) 2.1 (T) - -
PH 95 %ile 4.2 - 6.0
as pH units 5 %ile n/a 7.1 - 9.0
Notes : EC Directive EQS values based on hardness > 250 mg/1 CaCOj/1.
AA ... Annual Average
(T) ... Total metal; (D) ... Dissolved metal.
n/a ... not available
Where a particular objective does not contain targets for every parameter, the values ascribed in the
prcccding obj*xlive have been applied.
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Wheal Jane Minewater Study Executive Summary
Environmental Appraisal and Treatment Strategy
An assessment was made of how much minewater from Wheal Jane would need to be treated
in order for the river to meet the various water quality objectives. The treatment
requirements necessary to achieve the "No Deterioration" and "North Sea Commitments"
water quality objectives are summarised in Table EX-2.
Table EX-2 : Minewater Treatment Capacity to meet each
Water Quality Objective
Annual probability of non- 50% 5%
compliance
Maximum Average Maximum Average
capacity flow capacity flow
(I/s) (I/s) (1/s) (1/s)
No Deterioration
Annual Average 190 160 270 180
95%ile 210 170 300 190
North Sea Commitments
Annual Average 230 175 300 190
EC Directive Unable to meet all EC Directive Requirements
Modelling of water quality in the Camon River has indicated that the EC Directive Objective
cannot be achieved, even if all the waters from both Wheal Jane and County Adit were
treated. This is due to the presence of unidentified diffuse sources of contamination
associated with other abandoned mineworkings.
THE LONG TERM STRATEGY
As part of the development of a long-term treatment strategy, studies have been undertaken
to assess the potential for minimising the environmental impact of the minewater by:
• Passive treatment.
• Active treatment.
• Prevention and control.
PASSIVE TREATMENT TECHNOLOGY
The treatment of acidic metal-rich minewaters using passive systems is well established, with
the design of each passive system dependent upon site specific criteria.
A pilot treatment plant has been constructed in the Camon Valley and has been designed to
treat up to 1.7 1/s of minewater (1% of the average outflow). A series of treatment cells
promote the removal of iron, arsenic and manganese by aerobic processes, and cadmium,
copper and zinc by anaerobic processes. The pilot plant also incorporates alternative
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