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Serpentine, the state rock of California, was once considered a valuable component
in building materials. It is now recognized as a common source of potentially
hazardous, naturally occurring asbestos minerals, or NOAMs. NOAMs are contained
in Earth materials, but can be disturbed by construction or mining. Once airborne,
these fibers could cause harm. Geologists have a key role to play in determining
where NOAMs might occur so that projects can be planned around them.
Serpentine and other mafic and ultramafic source rocks are present in one form
or another in at least 50 of the 58 counties in California and occur in at least
27 of our 50 U.S. states. Nor are NOAMS unique to the United States, having
been reported as potential health hazards in South Africa, Turkey, Greece, Australia,
Russia, Canada, Finland and China.
Asbestos manufacturing and use in construction materials began nearly 200 years
ago. In the mid-1970s, asbestos was banned in the United States with the discovery
of a relationship between exposure to asbestos fibers and respiratory diseases
including asbestosis, lung cancer and mesothelioma (cancer of the pleural lining).
Epidemiological studies continue to explore this relationship, but in the case
of mesothelioma, are severely hampered by the low frequency of less than 400
new cases per year in the United States. But one concept remains relatively
uncontroversial: Few people want to wait for more case studies and most want
to implement effective, practical measures to reduce the potential exposure
risks today. However, in the absence of widely accepted threshold air concentrations
for asbestos fibers (not unlike the absence of threshold values for hydrocarbons
in the early 1980s), the extent of airborne fibers that represent a health hazard
cannot be quantified.
As a result, it is even more difficult to understand the relationship between
source concentrations and the level of soil and rock disturbance necessary to
generate harmful doses of NOAM-bearing dust. Until widely accepted risk models
become available, assessment of the risk depends on the qualitative identification
(i.e. presence or absence) of NOAMs on a particular site, thereby placing this
responsibility on those broad shouldered-geologists willing to accept the challenge.
Two schools of thought represent the dichotomy of concern over NOAMs: The less
conservative "never been a problem" group who feel nothing needs to
be done, and the more conservative "panic and epidemic" group who
feel dealing with and building on NOAMs cannot be done safely, no matter what
measures are taken. The application of practical, sound science seems to represent
the only reasonable middle ground and a rare opportunity for geologists to contribute
to the development and implementation of appropriate public process and policy.
The most affected regions are those where NOAMs occur geologically and urban
growth results in disturbance. Geologists from Fairfax County, Virginia, Libby,
Montana, and several counties in California continue to exchange their experience
with and new methods for assessment and mitigation. The result is a collaborative,
effective process of assessment and management discussed herein.
Geologic assessment of NOAMs requires an experience-based understanding of the metamorphic environment of occurrence of NOAMs alongside knowledge of proposed disturbance scenarios for a given project. In order to perform an effective assessment, the specific NOAMs, or target constituents needs to be clear.
Asbestiform minerals and asbestiform crystal habits occur primarily, but not exclusively in mafic and ultramafic rock formations, and faults or shear zones within or near these mafic rock units. In both cases, NOAMs typically occur as very thin veins, fracture coatings, or finely disseminated mineral grains. In many cases, this mineralization is not visible in a hand specimen and therefore represents the proverbial "needle" in a haystack.
Without disturbance, there is no exposure. Understanding the exposure pathways between potential disturbance sources and human inhalation opportunities, or receptors is essential to the assessment process. The pathway between the disturbance source and receptor site is referred to as the exposure pathway and, when clearly defined for a specific project, can be assessed, monitored, and mitigated.
The geologic assessment of NOAMs is a classic application of engineering geology. Once the disturbance scenarios for the project, and the potential sources of NOAMs are recognized, the assessment process should focus on evaluation of the presence of NOAMs in the areas where the NOAMs are likely to occur and be disturbed. Based on the initial site reconnaissance, one can arrive at three possible alternatives. First, the observed conditions are not conducive to the formation of NOAMs, and work can progress without modification. Second, the outcome of the site reconnaissance is that NOAMs are either found or assumed to be present, in which case mitigation and monitoring are warranted. Third, on questionable sites where the presence of NOAMs cannot be ruled out or confirmed, a simple cost benefit analysis is appropriate to guide further actions.
The additional investigation step is analogous with the Phase II Environmental
Assessment process and requires field investigation involving additional surface
and/or subsurface sampling (depending upon the depth of proposed site excavations)
and petrographic analyses. The appropriate analytical procedures for the assessment
of NOAMs are still quite controversial; largely because the test methods that
have been accepted for a long period of time for asbestos construction materials
and the application of the data or, in many cases, their applicability to field
conditions and disturbance scenarios are poorly understood. Clearly, one of
the most important considerations in selecting an applicable laboratory testing
methods is the value of the resulting data as input parameters for risk assessment
modeling.
The NOAM assessment hierarchy described above requires the direct input of a
professional geologist, as applicable for each state. In addition, the responsible
geologic professional should have local geologic knowledge and experience regarding
the occurrence of NOAMs in the project area. It is also important to remember
that assessment continues through the life of a construction project.
The application of engineering controls to reduce exposure to disturbed NOAMs is now very well established. Earthwork requires the application of appropriate OSHA regulations. These can all be used to minimize worker exposure and to eliminate exposure to residents neighboring construction areas and to properly communicate information indicating that the property is underlain by mitigated naturally occurring asbestos. The practice of asbestos removal from buildings has demonstrated the importance of water in dust suppression. The judicial use of water is important in protecting workers. Other exposure pathways, such as dust blowing off of storage piles, require management and mitigation tools such as covering by tarps or soil during periods of construction inactivity (when constant wetting may be impractical) such as weekends or holidays.
Engineering controls need to be designed with the potential receptor in mind.
The placement of a thick cover of clean soil over asbestos containing soil provides
an effective barrier against exposure. Future site improvement activities, such
as utility installation, swimming pool installation, or massive landscaping,
need to consider where NOAMs have been or may be exposed in the subsurface.
Ultimately, the final site conditions need to be disclosed to end users of the
property. Improvements that require a permit should trigger additional scrutiny
by regulatory agencies to determine if more asbestos management and mitigation
may be necessary.
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