Today’s blog post is intended to provide some
further commentary on the “toxic benzene plume” from my previous blog post: Questions
about the City of Vancouver May 27th Trans-Mountain Expansion Proposal Summary
of Evidence. As readers of my blog know, the Trans Mountain Pipeline
Expansion Proposal (TMEP) Summary of Evidence (SoE) presented to the Vancouver
City Council on 27 May 2015 (ref)
included the results of a modelling exercise which suggested that:
a
major oil spill from Kinder Morgan's Trans Mountain pipeline expansion project
would expose up to 1 million Metro Vancouver residents to unsafe levels of
toxic vapours, and as many as 31,000 could suffer "irreversible or other serious
health effects,"(ref).
Needless to say this conclusion garnered a lot of
headlines. I saw stories from the Georgia
Strait to the Globe
and Mail. The problem is that, as I described in my last post, this
conclusion fails the smell test. The modelling exercise incorrectly compared the
toxicological characteristics for benzene to a “pseudo-component surrogate” that
was made up of a mixture in which benzene was a very minor constituent. This resulted
in a wildly overstated risk to the public which, I will admit, made for some pretty nifty
headlines. This post is intended as a follow-up to my previous post to explain the
“surrogate thing” as well as to relate some surprising additional information I
have uncovered since my last post.
As discussed, the biggest question from my last post
was “what is the deal with the surrogate”? Well the chemical definition of a
surrogate is:
a
pure compound different from, but similar enough to, the analyte that, when
added at a known concentration to the sample prior to processing, provides a
measure of the overall efficiency of the method (recovery). Surrogates have
chemical characteristics that are similar to that of the analyte and must
provide an analytical response that is distinct from that of the analyte (ref).
While the Levelton
Consultants Ltd report (the Levelton
report served as the basis for that portion of the SoE) uses the term “surrogate”
in a chemical context they did not use the term under its chemical definition.
Rather they used the non-technical definition of the term: “one that takes the
place of another; a substitute” (ref). As I pointed out
previously, modelling is hard and to simplify the modelling Levelton took the
theoretical oil in the spill and broke it into 15 “pseudo-components” each of
which was then assigned a surrogate for use in subsequent toxicological
calculations. One “cut” of the dilbit was assigned the surrogate “benzene”. As
I described previously, this resulted in badly skewed risk results because benzene
is by far the most toxic component in the “cut” of dilbit for which it was used
as a surrogate and appears in the dilbit in much lower concentrations than used
in the model. As an analogy, imagine you were tasked with compiling a survey of
the animal population of Vancouver. To simplify the survey you didn’t ask your
surveyors to try to identify the dogs by species instead asking them to group
the dogs by size. For a subsequent risk analysis you then assigned the pit bull
as a “surrogate” to describe the behaviour of all dogs smaller than 2 feet tall
identified in your survey. Would you then feel comfortable with the outcome of
that risk analysis knowing that the analysis treated every Chihuahua it counted
as if it were a pit bull for risk purposes? If someone subsequently warned you to stay off the street for fear of being attacked by "surrogate pit bulls", based on this analysis, would you stay off the street? Well that is what they did in this
report with benzene.
As a follow-up to my last post I also did a bit of
digging into the documents referred to in the Levelton Report. Specifically, I
located the Intrinsik
and Tetra
Tech EBA reports (caution both are large files that take a while to
download) used to rationalize the use and choice of surrogates in the modelling
exercise. The TetraTech EBA report does indeed use “pseudo-components” as
surrogates; however, in their analysis “benzene” is used as a “surrogate” only for the benzene component in an oil spill (confusingly it is thus used as a surrogate for itself
only). As such instead of representing around 1% - 2% of the total spill mass (best guess on the number used by Levelton) it was determined to represent 0.088% of the spill mass (a fraction
based on the Tetra Tech EBA analysis of the future pipeline composition). In the
Intrinsik report, “benzene” is also restricted to the actual benzene component
of the dilbit and for toxicological calculations only benzene is compared
against the acute inhalation exposure limits for benzene. So when the Levelton
report claims to follow an approach that “is
consistent with the approach taken with the Human Health Risk Assessment (HHRA)”
that consistency does not extend to how they approached the critical component
described in the SoE and the one that garnered all the headlines: benzene.
The biggest surprise in my follow-up reading was to
discover that this was not the first modelling exercise to examine benzene
vapour concentrations derived from a theoretical oil spill in Vancouver Harbour. As I described in my
previous post, what made my chemist’s antenna go haywire was the
model output which said that in the case of a spill thousands of City of Vancouver residents
would be exposed to benzene concentrations over 800 parts per million (ppm). As I pointed out, previous in situ
studies (actual studies in the field) of oil spills of comparable API gravity
crudes had measured benzene
concentration ranging from 7 ppm to less than the detection limit (ref).
A study of a lighter crude (with more volatile components than dilbit) (ref)
and sampled from a mere 2.5 cm above the oil surface measured instantaneous benzene
concentrations ranging from 80.4 ppm to 3.5 ppm. Finally, most everyone in the
modelling community must have heard of the modelling study of the Exxon Valdez spill (ref).
It, too, got a very similar result to the in
situ experiments. Thus, when I read the Levelton report I was more than a
bit surprised by the output from their model. Well imagine my surprise to discover
that the Tetra Tech EBA report, used as a reference by the Levelton authors,
actually included a modelling exercise almost identical to the one carried out
by Levelton. The difference was that the Tetra Tech EBA modelling used benzene
proportionate to its concentration in dilbit. Unsurprisingly, the resulting
outputs were entirely consistent with the academic literature. The maximum
1-hour average ground concentration for benzene was less than 100 ppm over the
small portion of Vancouver affected by the spill. Certainly not headline
worthy, I will admit, but entirely consistent with the rest of the science out
there. Nowhere in the Levelton report, which otherwise references the Tetra
Tech EBA report, do they contrast their results to those generated by Tetra
Tech EBA. It is almost as if they didn’t want anyone to know that the previous modelling
exercise had been carried out and had generated such non-threatening (boring? non-headline worthy?) results.
In the academic community there is a simple rule: if a
new study runs contrary to a body of research then it is incumbent on the
authors of the study to explain the discrepancy. Sometimes the new study is a paradigm
changer, but most of the time it represents an outlier of dubious use in
decision-making. Unfortunately, the Levelton report does not explain why its
results differ so dramatically from the scientific consensus. More troublingly, it does not even acknowledge the existence of the body of research out there, including an almost identical modelling study, that came to such startlingly differing conclusions. I’m sure the
Vancouver City Council, and the local media, would be as interested as myself in finding
out why such an outlier result was trumpeted on May 27th?
Gasoline has a higher benzene content than a heavy blend.
ReplyDeleteHere's a keystone pipeline plume study, it's a bit short and incomplete
http://keystonepipeline-xl.state.gov/documents/organization/221246.pdf
I think the objections to a pipeline based on an onshore spill are pure baloney. The risk is in the tankers. And to reduce the risk they simply have to specify which type of crude they want. Long term, Canada benefits if it sells an upgraded syncrude, sulfur free, made to fit the USA west coast and Japan/South Korea markets (the Chinese don't seem to mind air pollution and will buy anything). Such a syncrude can be made in Alberta using upgraders which deliver a 30 to 40 API asphalt free crude. Such a crude is much more biodegradable. One upgrader can deliver 200,000 bopd of really nice high quality syncrude.