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CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering health,
safety and environmental improvements with cross-sector benefits.
Supplement to Guideline – ‘Environmental Risk Tolerability for COMAH Establishments’
Complex Site Example v0.0
Page 29 of 35
Figure 11
–
Simulated dissolved phase concentrations in underlying groundwater from a crude release into secondary containment.
The 50
th
percentile plume indicated that the toxicity based threshold was not exceeded at the receptor and therefore an
adjustment factor greater than 0.5 was likely to be applicable. At the 99
th
percentile the toxicity threshold was simulated to
extend close to the surface water receptor and therefore a maximum adjustment factor of 0.01 was adopted. When applied to
the initiating frequency and taking into account the already reduced risk of a significant subsurface source being generated,
the overall adjustment along this pathway was reduced by a factor of 200 (i.e. the risk of a significant environmental impact
at the surface water receptor from that event within that compartment was estimated to be 200 times lower than indicated by
the unmitigated risk frequency).
In addition to the numerical calculations based on contaminant fate and transport, the risks following a release to secondary
containment also considered the site setting and likelihood that even if product reached groundwater there would still be time
(based on groundwater velocities) to attempt to recover/remediate the resultant plume of product. In this case it was assumed
that a remedial approach would have a 50 per cent chance of being effective in minimising the subsequent risk of a major
accident to the environment.
Overall reduction factors for this single major accident scenario at the site ranged from just 2 where a tank was close to a
receptor and contained a gasoline component which only enabled an emergency response factor to be considered through to
more than 1 x 10
5
for a remote crude tank where the chance of ground penetration and subsequent simulation of migration of
dissolved phase constituents was assessed as being unlikely to result in detectable contamination at the sensitive down
gradient receptor.
In addition to the numerical assessment there was also consideration of the bunding type, presence of tertiary containment,
product toxicity/mobility, etc as part of the mitigation approach. Those bunds which were already constructed to good