What Happens When LNAPL
Migrates Below the Water Table?
Implications to Site Investigation
and Remedial Efforts
BCEIA’s Third Annual BEST Conference
May 27, 2016, Whistler, BC
Elyse Sandl and Stephen Munzar
Introduction
Traditional understanding of LNAPL
movement: “floats on the water”
Less understood:

LNAPL can migrate below the water table
Significant implications for site
investigation and remediation
Terminology
Conceptual Site Model (CSM)


3D understanding of a site and what
occurs in the subsurface
Includes geology, hydrogeology,
site history, contaminant sources
and contaminant transport
pathways
Light Non-Aqueous Phase Liquid (LNAPL)

e.g., pure petroleum hydrocarbons such as gasoline
Traditional CSM for LNAPL at
Contaminated Sites
 “Pancake Model”
 LNAPL floats on the water table
and capillary fringe and spreads
laterally
Therefore…
 A common site investigation approach is to target the water table,
focus on lateral delineation, and identify horizontal groundwater flow
Mercer, J.W., and R.M. Cohen, 1990. A review of immiscible fluids in the subsurface: Properties,
models, characterization, and remediation, J. Contam. Hydrol., 6: 107-163.
Vertical Equilibrium Model
 “Iceberg Model”
 LNAPL and water coexist in
pore spaces
 LNAPL penetrates below
the water table like an
iceberg
 Saturation decreases with
depth below the water
table, relative to the pore
size and LNAPL head
Gessert, E., Garg, S., and Hers, 2016. An Improved Understanding of LNAPL Behavior in the
Subsurface. Interstate Technology & Regulatory Council
Exception to the Rule
In certain environments, the geology
and hydrogeology can give rise to
migration of LNAPL below the water
table
 Fractured bedrock
 Secondary porosity in fine grained
soils (Case Study)
 Aided by downward vertical
hydraulic gradients
Munzar, S. LNAPL Deep Below the Water Table – How Did It Get There? The Importance of Proper
Site Characterization and Implications for Remediation
Implications
Application of the incorrect CSM or failure to identify
the vertical flow and transport pathways may result
in:
Incomplete characterization of the site
contamination
Erroneous groundwater pathway assessment
during completion of a risk assessment
Underestimation of cost and remedial efforts
during site remediation
Methods and Strategies
 Technical Guidance 8 and its companion document Groundwater
Investigation and Site Assessment
 Plan the site investigation to assess:



Geology (secondary porosity in fine grained soil)
Physical Hydrogeology (vertical and horizontal hydraulic gradients,
hydraulic conductivity)
Contaminant Hydrogeology (source history, vertical and horizontal
contaminant distribution)
 Refine the CSM as you go
Case Study
Background
 Historical automotive service operation site in the Lower
Mainland
 Underground storage tanks leaked over several decades
 LNAPL in the sub-surface
 Gasoline and diesel related contaminants in soil and
groundwater (BTEX, VPH, LEPH, HEPH and PAHs)
 Glacial sediments of interbedded sands and silts
 Sloped topography
 Down gradient aquatic receptor
Previous Detailed Site
Investigation
 Soil contamination up to 6 mbgs
 Water table approx. 6 mbgs
 Estimated excavation volume =
8,000 m3
Data Gap Investigation
 Installed 6 monitoring wells
using sonic drill method
 LNAPL encountered in a
monitoring well screened 10
mbgs
 Water table approx. 6 mbgs
Remedial Excavation
 Final excavation volume > 20,000 m3
 Removed 9,000 m3 of hazardous waste
classified soils
 Logistical restrictions to increasing
excavation size any further (e.g.,
buildings, utilities, slopes)
Remedial Excavation
Remedial Excavation
 LNAPL encountered up to 12
mbgs
 26/130 confirmatory samples
contaminated = not all
contamination removed
Post Remedial Investigation
 Drilled 26 boreholes
using sonic drill
method
 Refined the CSM
 Sampled soil and
groundwater at the
water table and at
depth
Post Remedial Investigation
 Soil contamination at 15 m
depth, more than 10 m below
the water table
 Higher concentrations of
contaminants in the dense
silt units than in the sand
units
 Remaining soil
contamination = 4,500 m3
Post Remedial Investigation
Our CSM
 Stratigraphy is dipping at 14°, parallel to the
sloping topography
 Interbedded stiff silts and sands
Our CSM
 Groundwater flow is towards the aquatic
receiving environment
~300m to Aquatic
Environment
Our CSM
 Localized upward and downward
hydraulic gradients
 Net vertical gradient is downwards
Our CSM
 Soil contamination from historical presence
of LNAPL, indicating that LNAPL migrated
>10 m below the water table
Our CSM
Adamski, M., Kremesec, V., Kolhatkar, R., Pearson, C, Rowan, B. 2005. LNAPL in fine-grained soils:
conceptualization of saturation, distribution, recovery, and their modeling. National Ground Water
Association.
Our CSM
 LNAPL migrated below the water table through
macropores (fractures or sand stringers)
 Hydrocarbons bled out into the over and
underlying sand units
Consequences
 Size of the remedial excavation increased from anticipated
8,000 m3 to 22,000 m3
 Cost of remedial excavation increased from estimated $2.5M
to $4M
 Additional site characterization and risk assessment work was
required at additional cost
 Contamination was delineated on site
 Risk assessment concluded that contamination would not
reach the aquatic receiving environment, nor was it a risk to
human health or the environment
Summary & Key Take Aways
 During site investigation, fully characterize the site geology
and hydrogeology
 Application of the incorrect CSM may result in:



Incomplete characterization of the site contamination
Erroneous groundwater pathway assessment during
completion of a risk assessment
Underestimation of cost and remedial efforts during site
remediation
 Refine the CSM as you go
Thank You!
Questions?
Presenter Contact Information:
Elyse Sandl, GIT
Hydrogeologist
Core6 Environmental Ltd.,
[email protected]
Stephen Munzar, MSc/PGeo
Senior Hydrogeologist / Partner
Core6 Environmental Ltd.,
[email protected]