COMMISSION

assessment investigation. As in April 2010, the consultant investigation team of motorcycle
trail riders equipped with air sampling gear rode trails within the ACEC in November and
December 2013 and January, February, and March 2014. Coupled with the April 2010 data,
this created a consecutive-month dataset that spans the traditionally popular fall, winter, and
spring CCMA visitation months. Additionally, the dataset contains more than twice as many
motorcycle-trail-riding air filter samples as the Region 9 dataset.
The results of the expanded study are presented in an updated report dated July 2016.
The report presents airborne fiber concentrations that were determined based on the
consultant’s air filter analyses and the duration of air filtration for each air filter sample.
Those concentrations were then extrapolated to determine exposure to an individual riding
a motorcycle on trails within the ACEC of CCMA for eight hours per day, five days per
year, which is very similar to visitation scenarios now allowed by BLM for other activities at
CCMA. Risk for contracting asbestos-related cancer for that activity and exposure was then
calculated according to Environmental Protection Agency methodology. As presented in
Serpentinite
Serpentinite rock originated as oceanic crust. For
the New Idria Serpentinite, previous plate tectonic
dynamics pushed the crust toward the west
coast, where over millions of years, it slid beneath
the continent via a process called subduction.
While most of the crust slid to the earth’s mantle,
some of the oceanic rock smeared onto the
leading edge and underside of the continental
crustal rock and so was only partially subducted.
Nonetheless, these ocean floor remnants were
subjected to considerable heat and pressure, causing the rock and its chemistry to change,
or metamorphose, into serpentinite. Generally, the changed chemistry makes serpentinite
less dense than overlying rock. This allowed the New Idria Serpentinite rock body to push
overlying rock slowly upward and aside, and eventually the serpentinite was exposed by
erosion.
The name serpentinite is used because the metamorphic rock is mostly composed of the
serpentine group of minerals—a collection of about 20 minerals with similar chemistry but
varying microcrystalline structures. Most of these minerals have a platy structure, which
makes the rock feel smooth and waxy. A few of the minerals, such as chrysotile, appear
fibrous because their internal structure is elongated and flexible. The fibrous serpentine
minerals often appear as veins within serpentinite rock. This small subset of minerals is
generically called asbestiform or asbestos.
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California State Parks, Off-Highway Motor Vehicle Recreation Commission