South Wheal Francis is an abandoned copper-tin mine situated approximately 3 km SE of Camborne, Cornwall (UK). The site is of considerable archaeological interest in retaining many of the original mine structures and buildings, and has been developed as a heritage site for the tourist industry. Like other copper mines in the area, the site was found to be heavily contaminated with a range of heavy metals (Cu, As, Pb, Ni, Zn) and suffered the additional hazards associated with shafts, stopes and other voids; the buildings also required consolidation to make them safe for public access.

An unexpected contaminant discovered at the site was blue asbestos (the mineral crocidolite, a variety of the amphibole riebeckite). The mine, which operated during the late 19th and early 20th Centuries, was powered by steam produced by a bank of six Lancashire boilers. Blue asbestos was used for thermal insulation of the boilers and, when the mine closed and the boilers were removed, the asbestos was simply discarded. During rehabilitation works to make the site safe for public access, considerable quantities of blue asbestos were identified at or near the surface in and around the boiler house.

This paper describes the site investigation, the work undertaken to remove the asbestos, and the final rehabilitation of the site. Special problems relating to the distribution of asbestos (as discrete asbestos objects rather than as a dispersed contamination), the need to protect the archaeological features of the site, the disposal of material contaminated with both heavy metals and asbestos, the health and safety aspects of the site remediation and long-term monitoring are also discussed.
 
 

INTRODUCTION

This paper describes the investigation, remediation and monitoring of an asbestos-contaminated mine site at South Wheal Francis near Camborne, Cornwall, South West England. The work was commissioned by the site owner, Kerrier District Council, which is also the local authority with environmental responsibility in the area in which the mine is situated, and was undertaken by the Earth Resources Centre, University of Exeter, working with Frank Graham Consulting Engineers (now WSP).

South Wheal Francis is an important heritage site forming part of the Mineral Tramways Project, a major tourism initiative intended to attract many visitors. The site is also very close to the small settlement of Piece, with its primary school and popular public house. The presence of asbestos on site was, therefore, considered to have wide implications for public health and was the subject of detailed investigation leading to extensive remediation works.

SITE DESCRIPTION

South Wheal Francis Mine is an abandoned copper-tin mine situated immediately east of Treskillard, approximately 3 km south east of Camborne, Cornwall (National Grid Reference SW 681394). The site extends to some 8 hectares and features many important mine buildings including several Scheduled Ancient Monuments which make it of high archaeological value (Smith, 1992).

The mine is situated at an elevation of about 170 metres above Ordnance Datum on the northern boundary of the Carnmenellis granite at its contact with the surrounding killas (Devonian slates and sandstones). The area is heavily mineralised. The recorded history of the mine dates from 1824 (1). Originally worked for copper, production had switched to the deeper tin by 1877. The mine became part of Basset Mines Ltd. in 1896 and closed in 1918. Steam to power the mine machinery was generated in the Boiler House using a bank of six Lancashire boilers (2) and, as the main asbestos contamination occurs within and around this building, it appears to be the boiler insulation that is the main source of asbestos on site.

South Wheal Francis is of special importance owing to the wide range of buildings that have survived at the site. These buildings are clustered mainly around Marriott's Shaft, with two additional buildings around Pascoe's Shaft at the western end of the site (Figure 1). Asbestos contamination was first observed at the surface in and around the Boiler House, Pumping Engine House and Winder House during early work to consolidate buildings. The detailed investigation of the nature and extent of this contamination and the subsequent cleanup operation are described in this paper.

Mine sites are characterised by a range of potential hazards, each of which has been assessed and addressed during the development of South Wheal Francis as a heritage site. These hazards include voids (shafts, stopes and other excavations), derelict buildings (some initially in a dangerous condition), heavy metal contamination and, of particular importance at this site, asbestos contamination. Geotechnical and contaminant studies were carried out at South Wheal Francis by Frederick Sherrell Consulting Engineering Geologists of Tavistock, Devon, in 1992/93. These investigations included trial pitting and drilling and an assessment of chemical contamination at the site. Remediation of the hazards associated with voids, derelict buildings and heavy metal contamination are beyond the scope of this paper but the presence of high levels of heavy metals as well as asbestos in contaminated soils has implications for their disposal, and some heavy metal determinations (not reported here) were carried out on asbestos-contaminated material prior to removal for disposal.

SITE INVESTIGATION

The site investigation described here focuses only on the asbestos contamination but, having determined the distribution of this principal hazard, further investigations were carried out to determine the level of heavy metals in the asbestos-contaminated soils, as this affected the cost of disposal. The asbestos survey was carried out in three phases.

Visual inspection

The first phase of site investigation consisted of a walkover survey with visual inspection of material exposed on the ground and on other surfaces. Blue fibrous material was observed at the ground surface in and around the Boiler House and in the Winder House and Pumping Engine House. Samples of this blue fibrous material were collected and analysed at the Earth Resources Centre, University of Exeter, by X-ray diffraction (XRD) techniques. This confirmed that the blue fibrous material exposed on site was the mineral riebeckite (an asbestiform amphibole also known as crocidolite), referred to here as blue asbestos. Having confirmed the presence of this highly hazardous material at the surface, the next stage of investigation entailed a soil survey to determine the distribution of buried asbestos around the site.

Soil surveys

The purpose of the soil survey was to obtain representative soil material in order to determine the possible presence of buried asbestos. Samples were collected by augering to depths of up to 0.60 m, the sampling depth being limited by the stony nature of the ground. Special attention was paid to mounds and other bodies of dumped material.

As the auger survey was essentially of a 'walkover' type, precise sampling locations were not recorded except where samples were taken for analysis. Augering density varied across the site according to the nature of the ground, previous knowledge of asbestos distribution around the buildings, and accessibility. In the areas surrounding Marriott's Shaft (Zone I, Figure 1) and Pascoe's Shaft (Zone II), augering took place at about 5 - 10 m intervals (subject to accessibility). In the landscaped area (Zone III) and the north eastern extension of the site (Zone IV), the sampling interval was approximately 20 - 25 m; in the thickly vegetated Zone V (where access was difficult), the sampling density was lower.

From a total of some 220 auger points, 18 soil samples were selected for analysis. At each auger point, soil material was examined visually and any fibrous material present was noted. Samples were selected for analysis by X-ray diffraction on the basis of this visual examination and to provide a reasonable cover of samples across the site.

Of the 18 auger samples selected for analysis, only one (from the Boiler House floor) was shown positively to contain riebeckite. As the cleanup operation later revealed, auger sampling has its limitations where the asbestos occurs as discrete objects (ropes, bags, sheeting etc.) rather than as a dispersed contamination. There was, therefore, some overlap of the investigation and cleanup phases of the work, as the full extent of the asbestos contamination could not be determined until excavation commenced (as discussed further below).

Surface (deposit) sampling

In addition to the soil sampling, deposit (dust) sampling was also carried out from surfaces in buildings to determine the extent of asbestos contamination in accumulated aeolian material. Sampling of these surface deposits took place under damp conditions with subsequent analysis by X-ray diffraction (XRD). Selected surface material was sampled by hand-picking and scooping with a knife. These samples included fine dust collected from surfaces within and around buildings, representative samples of the widely distributed blue fibrous material, and some white fibrous material collected from the Miner's Dry. A total of 17 surface samples were collected. Riebeckite (blue asbestos) was identified by XRD in six of these samples, all located within the main group of buildings around Marriott's Shaft. In these samples, the riebeckite was found to occur either in the form of small woven mats or pads (up to a few cm in dimensions) or, in one case, as asbestos rope. Blue fibrous material was also identified visually in Pascoe's 80-inch engine house and on the surface of a small dump adjacent to Pascoe's Whim engine house. No asbestos was identified in the fine dust samples collected on ledges and other surfaces. The white fibrous material observed in the Miner's Dry was identified as glassy in structure and appeared to be glass fibre.

The surface contamination appeared to be due largely to the dispersion of asbestos by the wind and, importantly, by nesting birds, notably jackdaws (Corvus monedula). In fact, much of the blue asbestos observed at the surface occurred beneath nesting sites.

RISK ASSESSMENT

The investigation revealed the presence of considerable quantities of blue asbestos at the site, and the subsequent review and assessment of the findings confirmed the need for remediation of the asbestos contamination.

Asbestos is a fibrous silicate mineral which is chemically inert, heat resistant and mechanically strong, and has in the past been used extensively for industrial purposes, including fire protection and thermal insulation. Asbestos materials are prone to break down into fibrous particles or dust, especially when dry, exposed or disturbed. Inhalation by humans and animals by breathing in dust is therefore the greatest risk, and long term harm to health is a consequence in various forms including asbestosis, bronchial carcinoma and mesothelioma, which can be fatal. Ingestion by humans and animals is another possible route for asbestos fibres to enter the body, either by direct ingestion or in drinking water.

Blue asbestos (crocidolite/riebeckite) is considered to be the most harmful form of asbestos to human health. Various control limits and action levels have been set for exposure by humans to asbestos inhalation. The most commonly quoted is the control limit for blue asbestos exposure of 0.6 fibres per millilitre of air over 10 minutes (or 0.2 fibres per millilitre over 4 hours) (3).

Risk assessment methodologies were used to evaluate the risks and the need for remediation. In the existing conditions, there was ample opportunity for direct inhalation by humans or animals when walking or playing on the site, with the potential to transport the particles home attached to clothing or hair, and there was also the likelihood that airborne particles could be windblown off site onto adjoining agricultural land and nearby habitation. Under existing conditions, the risks and consequences of asbestos contamination were considered to be high. During the remedial works, there were similar risks to workers, and also an enhanced level of risk from site disturbance allowing more widespread contamination unless the works were properly controlled and implemented. Following completion of the remediation, which was carried out under proper management and control, the remaining risks from asbestos should be reduced to low and acceptable levels, with all asbestos either safely removed from the site or safely buried and covered.

REMEDIATION PLAN

Before any remediation works are undertaken, it is important that the proposed actions are carefully considered and planned. A Remediation Plan was prepared to identify the methods and procedures, to allocate responsibilities and to obtain approvals. It formed the basis of the employer's requirements from the remediation works contractor. The contractor appointed was required to demonstrate proper accreditation and competence to undertake asbestos removal works, and to produce a detailed method statement describing the implementation and management of the works. Technical competence as well as contract price were, therefore, emphasised as the basis for selection and award.

REMEDIATION WORKS

Having determined the distribution of asbestos contamination at the site, the remediation programme was implemented during the period March - May 1997 by licensed contractors R. I. & H. J. Bartlett Ltd., of Sturminster Newton, Dorset, with the assistance of specialist asbestos contractors Southern Counties Asbestos Removal Ltd. (SCI). During the removal work, air quality was monitored for health and safety purposes by Argus Laboratories Ltd., Brighton. The work included the bulk excavation and removal of the contaminated material, the installation of a protective capping layer and the reinstatement of the surface for safe public access. The excavation work focused on the buildings and their surroundings in Zone I; in other parts of the site, where no buried asbestos had been identified, surface material was carefully collected by hand and removed along with the buried asbestos. The total cost of the work was in excess of £250,000, funding being provided by English Partnerships under the Leasehold Reform, Housing and Urban Development Act 1993.

Excavation and capping (Zone I)

Following careful excavation of the surface soils, with careful damping of the contaminated materials during dry periods, and bulk excavation and removal of all asbestos materials, the reduced ground surface was carefully inspected and checked to be clean and ready to receive a new surface capping layer. The capping layer (shown in Figure 2) comprised:

• a geosynthetic separator membrane - Terram 900 - to act as a physical barrier to prevent possible upward migration of any undiscovered asbestos fibres residual in the underlying soils;
• a galvanised wire mesh (or "rabbit net"), with 20 mm mesh size and 1 mm gauge wire, to act as a physical barrier to rabbits or other burrowing rodents which might otherwise compromise the capping system;
• a granular aggregate surface layer of well-graded crushed rock material derived locally from a client-specified source of mine waste, with a 500 mm minimum depth, to achieve final surface levels.

Installation of barrier system, Boiler House

As the buildings present at the site were of historical importance, their protection was part of the excavation plan and great care was taken to ensure that the fabric of the buildings themselves was not damaged. It was, therefore, necessary to carry out much of the excavation work by hand or with small mechanical excavators. There was also close liaison with the district archaeologist.

Until excavation commenced, the full extent of the asbestos contamination could not be fully determined. Thus there was some overlap between the site investigation and cleanup. This created some difficulty as far as the work schedule was concerned, the cleanup period being unavoidably extended as more asbestos was discovered. Excavation continued until all observed asbestos was removed, even where it extended beyond its originally-identified boundaries. Prior to implementation of the remediation works, the volume of asbestos-contaminated materials to be excavated and removed was estimated to be in the order of 50m³, but approximately double this volume was actually removed.

Forensic walkover (Zones II - V)

In addition to the excavation of buried asbestos in Zone I, surface contamination was removed from Zones II - V through the implementation of a 'forensic walkover' undertaken by a team of workers in which all observed surface contamination was collected by hand during a systematic sweep on foot of the entire site.

Nesting sites

In light of the observed occurrence of asbestos contamination beneath nesting sites, and its presence in fallen nests, the cleanup programme included removal of asbestos from the upper parts of buildings used as nesting sites. This was carried out manually at the end of the nesting season to minimise disruption to the life cycle of the birds themselves, access being gained through the use of a hydraulic lift.

UK LEGISLATION GOVERNING THE HANDLING AND DISPOSAL OF ASBESTOS WASTES

The excavated asbestos-contaminated material was handled, transported and disposed of in accordance with UK regulations governing the management of hazardous wastes. These regulations prescribe the correct procedures that ensure the safe management of hazardous materials.

Waste containing asbestos is classified as 'controlled waste' under the Environmental Protection Act, 1990 (EPA) and as 'special waste' under the Special Waste Regulations 1996. Under these regulations, movements of asbestos waste have to be traced by means of a 'consignment note' system until they reach a disposal site (a landfill) licensed under the Waste Management Licensing Regulations 1994 to accept asbestos waste. Licensed sites operate special procedures to ensure the safe disposal of asbestos. Carriers of asbestos waste must also be licensed and are subject under the EPA to a 'Duty of Care', which places a responsibility on everyone handling or controlling the waste to ensure it is managed safely and transferred only to persons authorised to deal with it.

The transport of asbestos waste must also be in accordance with a number of regulations, Statutory Instruments and codes of practice. The Health and Safety Executive (HSE) also issues guidance on the transport of dangerous materials.

Asbestos waste should be kept separate from other waste, should be double-bagged in heavy-duty polythene bags and clearly marked with the label prescribed for asbestos prior to transport to the disposal site. The asbestos waste should also be carried in a suitable container, such as an enclosed skip, which should also be labelled. Carriers who transport waste must be registered with the appropriate regulatory authority: the Environment Agency (in England and Wales) or the Scottish Environment Protection Agency.

In the case of South Wheal Francis, asbestos was double-bagged, the outer bag red in colour and suitably labelled, and was transported in sealed skips by licensed road carrier to a licensed landfill approximately 10 km from the site. The requirement for such handling of asbestos waste imposes costs on the cleanup of sites like South Wheal Francis, but wastes resulting from the cleanup of historically contaminated land are exempt from the Landfill Tax established under the Finance Act of 1996.

Safe working with asbestos was a central consideration at all aspects of the works, which were undertaken with reference to current UK regulations and guidance as published by HSE and the Construction Industry Research and Information Association (CIRIA), including The Control of Asbestos at Work (Amendment) Regulations 1992 and the accompanying Approved Code of Practice (3) and the Guide to Safe Working Practices for Contaminated Sites (4).

HEALTH & SAFETY ASPECTS

As asbestos is a hazardous material, great care was taken to ensure the health and safety of site workers during all stages of the site investigation and cleanup and a risk assessment was carried out before any work commenced.

Site investigation

During the site investigation, special care was taken during sampling. As the soil environment in Cornwall is almost always damp (and sampling was only carried out under these conditions), there was no asbestos dust hazard associated with soil sampling and no special protective measures were needed. Protective suits, filter masks, goggles and gloves were, however, available at all times in case site conditions changed. Surface sampling was considered to present greater risk requiring some protective measures to be taken. Protective suits (with hoods), filter masks and wellington boots were worn at all times during sampling, though conditions remained generally wet during the sampling period.

Remediation

During the remediation phase, the working area remained fenced off at all times with access permitted only to authorised and properly protected personnel. Within this area, workers wore disposable red protective paper suits with hoods, filter masks or respirators (depending on site conditions), goggles and protective boots. Access to the work area was restricted to a single controlled entrance/exit where, on leaving the work area, protective suits were removed and bagged for disposal and where boot washing facilities were available.

Health and safety monitoring was carried out throughout the cleanup operation to ensure the protection of site workers. The results of this monitoring, carried out by specialist health and safety contractors, Argus Laboratories Ltd., demonstrated that maximum permitted levels of airborne fibres were not exceeded at any time during the asbestos removal operation.

ENVIRONMENTAL MONITORING PROGRAMME

In addition to health and safety monitoring to ensure the safety of site workers, environmental monitoring was also carried out throughout the 8 week period of asbestos removal in order to determine the extent of any asbestos dispersion that might have accompanied disturbance of the buried asbestos. The primary asbestos contamination was confined to the buildings around Marriott's Shaft, notably the Boiler House, Winder House, Pumping Engine House and Compressor House, and the monitoring was focused on asbestos removal from these buildings.

Two approaches to air quality monitoring were adopted:

(A) Passive sampling: 'frisbee' dust samplers were installed at four different points on site, their locations being chosen so that sampling would allow for changing wind direction (Figure 1). Using the 'frisbee' technique, dust collected in the sampler is washed into a collection bottle by rainfall or with water and is later separated by filtration. The filtrate is then analysed for blue asbestos (riebeckite) by X-ray diffraction. Three of the four samples collected using the 'frisbee' samplers did not show any detectable asbestos and there was only a very small trace in the fourth sample (F3, Figure 1). It is worth noting that sample F3 was collected at a site immediately north of the heavily contaminated area to the east of the Boiler House (see Figure 1); this was the scene of extensive excavation works during the cleanup operation and represented one of the major potential sources of airborne contamination. The absence of any serious contamination from sample F3 indicates a good level of environmental control during the works. The results suggest the absence of any widespread airborne asbestos dispersion during the asbestos removal operation.

(B) Active sampling: thirteen active (pumped) samples were collected, each over a period of one working day using a Negretti personal sampler operating at a pumping rate of approximately 2.0 litres per minute, some 960 litres of air being sampled over an eight hour period. Each sample was collected at a height of approximately 1 metre above ground level, at a distance of between 5 and 10 metres from the working area in a downwind direction. Using the personal samplers, dust is collected on a filter which is then examined for asbestos by scanning electron microscopy (SEM). The samples collected by this active sampling did not show any significant asbestos dispersion during the cleanup operation. Some fibrous material was observed in some samples but most of these fibres were shown by scanning electron microscopy with secondary electron analysis to consist of paper (probably from the protective suits worn by site workers) and organic material (from vegetation).

A small number of fibres identified as 'probably asbestos' were observed in four samples but represented very low levels of contamination. This suggested that very little dispersion of asbestos took place during the cleanup. This is consistent with the results of the health and safety monitoring carried out by Argus Laboratories Ltd. which showed that the maximum permitted level of airborne asbestos fibres was never exceeded during the asbestos removal operation.

POST-REMEDIATION INSPECTION

Following completion of the asbestos removal operation, a visual inspection of surfaces, walls and exposed ground was carried out to ensure that no visible asbestos remained on site. Special attention was paid to areas previously seen to be contaminated with asbestos and to areas used for the handling and loading of asbestos-contaminated material. The inspection included both the Marriott's Shaft and Pascoe's Shaft areas. The visual inspection did not reveal any remaining asbestos contamination at the surface, even at the previously contaminated parts of the site. Furthermore, none of 10 additional surface samples collected immediately after asbestos removal showed any trace of blue asbestos contamination, even though many of these samples were collected from locations where dispersed or residual asbestos contamination might have been suspected. The results indicate that no asbestos remained at the surface and that no significant asbestos dispersion occurred during the cleanup operation.

CONCLUSIONS

Blue asbestos (riebeckite, variety crocidolite) was found to contaminate the site of the former South Wheal Francis copper-tin mine. The primary asbestos contamination was confined to the main buildings around Marriot's Shaft, especially the Boiler House where it was used for boiler insulation. Secondary surface contamination was also present due mainly to dispersion by the wind and by nesting birds.

The blue asbestos contamination was investigated and removed successfully and the site made safe for public access. The programme was not entirely straightforward, however, as the nature of the asbestos contamination (as discrete objects such as ropes, bags etc.), made identification of the extent of the contamination problematic (individual objects are difficult to locate in an auger survey). There was, therefore, some overlap between the site investigation and cleanup phases of the work. This had implications for scheduling the cleanup operation, delays being introduced as additional asbestos contamination was discovered as the work progressed.

The results of the passive and active air quality monitoring show that no significant airborne dispersion of asbestos occurred during the asbestos removal operation. This was confirmed by the analysis of surface deposits collected after asbestos removal. This was ensured both by careful damping of soils during their removal and by the generally wet conditions of this Cornish site during the work period.

The results of the post-cleanup inspection show that no visible asbestos remained on site and that the surface environment appeared clear of asbestos. The overall results show that the impact of the asbestos removal on environmental air quality was negligible and that its removal was accomplished successfully without any significant environmental impact.

REFERENCES

1. Dines, H. G. (1956). The metalliferous mining region of South-West England. Memoirs of the Geological Survey of Great Britain. HMSO, London.

2. Smith, J. R. (1992). Kerrier Land Reclamation Scheme. Marriott's and Pascoe's Shafts South Wheal Francis: Archaeological Assessment. Cornwall Archaeological Unit Report.

3. Health and Safety Executive (1992). The Control of Asbestos at Work (Amendment) Regulations 1992 with accompanying Approved Code of Practice.

4. CIRIA (1992). Guide to Safe Working Practices for Contaminated Sites. CIRIA, London.