INVESTIGATING NAPL BEHAVIOUR IN A COMPLEX FRACTURED BEDROCK ENVIRONMENT FOR RISKASSESSMENT & REMEDIATION DESIGN
Dr. Tom Henman – Director, RSK Environment Ltd.
Contents
1. NAPL and coal tars
2. The Site and project
3. Lines of evidence approach to NAPL assessment
4. Refining the CSM
5. Conclusions
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Tar tank - © CL:AIRE
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1. Introduction to NAPL
What is NAPL?
NAPL = Non-Aqueous Phase Liquid
Does not mix with water and remains as a separate phase
Petroleum hydrocarbons and chlorinated solvents
LNAPL = NAPL that is less dense than water
Gasoline, diesel fuel, jet fuel, and crude oil
Multi-component mixtures
DNAPL = NAPL that is more dense than water
Chlorinated solvents – PCE, TCE, TCA
Single-component products
Coal tar – low density and high viscosity, leads to migration as DNAPL. Lighter fractions can also create LNAPL
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2. The Site and Project
Major former gasworks
Previous remediation of shallow/ ‘accessible’
contamination sources
Deeper, less well understood, coal tar NAPL
impacts in structures, made ground and
fractured bedrock – igneous and sedimentary
Key questions the project sought to answer:
How much NAPL is present within each
stratum?
How bedrock and NAPL behaviour affects
migration risks, and
How to remove NAPL and the most
cost‐effective techniques for its removal
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Project scope
Supplementary SI – percussive and rotary
drilling
Groundwater/ NAPL monitoring
NAPL chemical and physico-chemical testing
Remedial Options Appraisal (ROA)
In situ remediation pilot trials
Updated ROA and remediation cost
estimates
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3. Lines of evidence approach
to NAPL assessment
SI Phase
Percussive drilling through made ground
and rotary coring (water flush)
Supplementary monitoring boreholes
targeted to NAPL source areas and to
address data gaps/ uncertainties
Well arrays installed at same time
targeting made ground or bedrock NAPL
sources
Key site constraints – live IP gas mains,
below ground structures/ obstructions,
other contaminant sources e.g. spent
oxide, asbestos
Approach modified to site conditions
encountered
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Extraction well
(110mm)
Monitoring well 1
(50mm)
Existing
monitoring
well
Steam injection
well (10mm)
Monitoring well 2
(50mm)
Data collection
Detailed rock core logging – inc.
examination under visible/UV light and
PID testing
Downhole geophysics to map fracture
density and orientation
Specialist NAPL physical testing as well
as standard geochemical analysis
Product baildown testing
Remediation pilot trials
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Assessment of NAPL extent
Observations of NAPL from drilling/ rock
core logging and well development
Measurable NAPL in groundwater
monitoring
Soil and groundwater results from
current and previous investigations were
assessed using effective solubility
calculations (Raoult's Law)
Defined indicative source areas for total
NAPL in made ground and each bedrock
type
Refined through further work, especially
in situ trials
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4. Refining the CSM
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Development of
updated CSM
NAPL extent and behaviour observed in igneous vs. sedimentary bedrock
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Refinement of conceptual site model
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5. ConclusionsUse of lines of evidence approach to establish NAPL extent and properties
Test assumptions in CSM and refine throughout process, including being
reactive on site
Variability of NAPL behaviour depending on source and stratum
Importance of considering total vs. recoverable NAPL for risk assessment
and remediation design purposes
Value of undertaking in situ remediation trials alongside SI - this enabled
the NAPL source areas to be refined and the NAPL recoverability to be
evaluated. With the level of uncertainty significantly reduced, cost estimates
and design considerations were developed for full-scale site remediation.
Application to other sites – presence of residual NAPL in bedrock following
previous source remediation works is an emerging theme on many
gasworks sites
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Thank you for your time....
Any questions?