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?