Introduction
Erionite is a naturally occurring “asbestiform” mineral that belongs to a group of silicates called zeolites. It is listed by the International Agency for Research on Cancer (part of the World Health Organization) as a Group 1 known carcinogen (Pacella et al., 2018). Studies have demonstrated that erionite is more carcinogenic than asbestos (Giordani et al., 2017). Erionite exists in two forms: woolly and crystalline (Figure 1). It was first identified, described in deposits from an opal mine in Oregon and named by Eakle (1898). At the time, it was described as having white, wool-like fibres, and was considered extremely rare. Indeed, it was not reported upon again until 1959 when it was identified in crystalline form in Nevada and Wyoming in the USA (Deffeyes 1959). In these cases, the crystals were discovered in vesicles of basaltic lavas, or as microscopic fibrous crystals in diagenetically altered, silicic tuffs, deposited in lacustrine sediments. Here, although the erionite originated from silica-rich volcanic rocks, it was later dissolved and recrystallized as a zeolite, and found in the local sedimentary rocks.
Figure. 1: (A) Example of “woolly” erionite from Te Henga Road Quarry, Waitakere Ranges (Rod Martin); (B) crystalline erionite (hexagonal crystal and acicular habit) from the Waitemata Group, Hobsonville (sample AU42046).
Crystalline erionite, found in sedimentary rocks, crystallizes as needle-like (acicular) fibres of nanometer to micrometer in width, and the disturbance of rocks containing erionite can generate airborne fibres that are similar in dimension to asbestos fibres (Figure 1). This form of erionite has been identified in several parts of New Zealand, but in particular, in the Auckland Region, in rocks of the Waitakere and Waitemata Groups (e.g., Davidson and Black, 1994). It can be identified from rock samples using several different laboratory approaches, including X-ray diffraction (XRD), as well as Raman spectroscopy.
Asbestiforms and Cancer
Asbestos-related malignant mesothelioma (MM) is of worldwide concern but particularly in New Zealand (Sjoeberg and van Zandwijk, 2015). This is cancer of the thin tissue (mesothelium) that lines the lung, chest wall, and abdomen, and is often caused by asbestos exposure. The highest MM incidence in New Zealand is found in the construction and building trades (Gluckman, 2015). Asbestos-induced MM for both men and women is of increasing concern, in part due to the discovery of the potential transfer of asbestos from the workplace to the home, and the death of relatives of workers who had been exposed to asbestos in the workplace (Glass and Clayson, 2017).
Erionite is highly pathogenic, similar in appearance and properties to asbestos, but from tests on humans and rodents, its toxicity and carcinogenic potential largely exceeds that of asbestos (Matassa et al., 2015). This may be due to erionite being able to accumulate iron on its surface, despite its very small content of this element (Bertino et al., 2007), as well as its in vivo durability, respirable size, and very high surface area. Thus, the inhalation of erionite has been shown to be correlated with diseases similar to those known to result from exposure to asbestos.
In the 1970s, it was realised that residents of three villages in the Cappadocia Region of Turkey had epidemic-level rates of MM caused by erionite exposure; and in one of the villages, the mortality was 52%. The cause of the mesotheliomas in the area was determined to be from environmental exposure to erionite in the homes and living areas of residents. Residents were found to have inhaled the erionite fibres during excavations for housing, as well as from the erionite fibres present in the region’s unpaved roads, soils and building stones (Van Gosen et al., 2013).
More recently, in the USA, several clusters of MM have been identified in North Dakota and Nevada, and have been unequivocally caused by erionite exposure (Pacella et al., 2018). In this region, erionite deposits encompass large areas, and subsequent urban development has resulted in exposure of the local population to the deposits. In addition, 450 km of roads were covered in erionite-containing gravel, which has the potential to generate high levels of airborne erionite when disturbed. Thus, occupational exposure to erionite from road maintenance is thought to be a significant health risk. However, a further particular concern in North Dakota was the public health risk; elevated air concentrations of erionite were found alongside roads at school bus stops, and inside buses, even with the windows closed (Carbone et al., 2011). Nevertheless, safe occupational exposure limits (OELs) and public health exposure limits have yet to be developed for erionite in the USA or elsewhere in the world.
Erionite in New Zealand
Despite the recognised impact of erionite exposure on mortality rates overseas, the extent of the contribution of erionite to occupational or public health in New Zealand has yet to be identified.
A range of zeolites have been identified from several parts of the country (Brathwaite, 2017), and, in some cases, extracted for various industrial purposes (Figure 2). In New Zealand, zeolites are found in three main geological settings:
1. Hydrothermally-altered rhyolitic pyroclastic rocks of the Quaternary age in the Taupo Volcanic Zone;
2. Marine tuffs and volcaniclastic sandstones of Miocene age in Northland and in Auckland;
3. Weakly metamorphosed marine tuffs and volcaniclastic sandstones of Triassic–Jurassic age in Southland and in southwest Auckland (Brathwaite, 2017).
Figure. 2: Map of sedimentary zeolite deposits in New Zealand (based on data in Brathwaite, 2017).
The presence of erionite in the Auckland Region has been known for several decades (Davidson and Black, 1994), and in 2015, New Zealand’s then Chief Scientist, Sir Peter Gluckman, acknowledged that erionite was a more potent carcinogen than asbestos (Gluckman, 2015). However, while his report correctly described the presence of erionite in volcanics in New Zealand, it neglected to outline that in addition, erionite is present in sedimentary rocks, including in the Miocene Waitemata Group of the Auckland Region. This is in addition to the presence of erionite in the Waitakere Group volcanics, which have been quarried for aggregates over many years.
Auckland is New Zealand’s fastest growing region, with large infrastructure projects that often include, and have included, tunnelling. The Waterview Connection saw twin tunnels driven through weathered and unweathered Waitemata Group rocks, generating c. 800,000 m3 of spoil. The material was excavated from tunnels, and transferred via conveyor, before being trucked to Wiri Quarry. The City Rail Link (CRL) and Central Interceptor excavations will also generate significant volumes of spoil. Moreover, there are other open excavations being carried out in the city, suburbs and region leaving Waitemata Group and Waitakere Group rocks exposed. Thus, there is the potential for the exposure of Auckland’s population to erionite-bearing dust, not just in occupational settings, but also in residential and central urban areas, where the material is disturbed, transported, and disposed of.
Implications and concluding remarks
Although it has been almost two decades since the ICE/DETR report “Managing Geotechnical Risk” (Clayton, 2001) was published, the main recommendations and essential principles are still pertinent. These include recognising that:
- The ground is a common cause of significant delay and cost increase in construction projects.
- There is a need for systematic risk management to be carried out, starting early with both a desk study and a walk-over survey, the expert identification of geotechnical hazards and risks and communication.
- There are implications of conditions of contract for risk sharing and methods of dealing with unforeseen ground conditions.
- The importance of design in minimising risk.
Given the evidence from overseas, it seems that erionite should be listed in at least some of the project geotechnical risk registers in New Zealand. The inclusion of erionite in geotechnical risk registers should be required for works involving excavations of rock that is known to contain erionite. The degree of risk (R) is determined by combining an assessment of the probability (P) of the hazard occurring with an assessment of the Impact (I) the hazard and the associated mitigation that it will cause if it were to occur (R = P x I). Then, an appropriate hierarchy of controls would be able to be applied to the hazard, in order to develop mitigation options.
Finally, erionite in Auckland, and other areas presents significant source-to-sink problems. Some challenging questions remain, and there are at least five key areas for further study and the analysis of erionite (and other zeolitic asbestiforms) in New Zealand. Future study is necessary to:
1. Develop techniques to identify erionite in the field in fresh excavations, soils and other environments;
2. Develop geological models of the distribution and concentration of erionite in key regions (in particular, in Auckland);
3. Monitor the airborne transport and dispersal of erionite;
4. Undertake research into the epidemiology of erionite exposure (e.g. malignant mesothelioma clusters) among quarry and construction workers, and the public, in particular in areas where erionite-bearing rock has been excavated or disposed of; and
5. Develop, with international partners, safe occupational exposure limits (OELs) for erionite.
Acknowledgements
Rod Martin is thanked for detailed discussions and permission granted to use images.
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