LNAPL and the MCP

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LNAPL and the MCP

• Stephen S. Boynton, P.E., LSP
• Subsurface Environmental Solutions, LLC
• sboynton_at_subenviro.com

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PRESENTATION OUTLINE

• Definitions
• LNAPL and the MCP Part I - Overview
• Overview of Part II
• LNAPL Characterization Framework
• Mobility Evaluation Methods
• Cost Considerations

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DEFINITIONS

• SATURATION The percentage of the soil pore
  space occupied by any given fluid.
• LNAPL Saturation
• Water Saturation
• Air Saturation

AIR
LNAPL
n
WATER
1
SOLIDS
1-n
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DEFINITION OF NAPL

• CURRENT DEFINITION
• Nonaqueous Phase Liquid and NAPL each means oil
  and/or hazardous material that is present in the
  environment as a continuous separate phase as
  measured in a groundwater monitoring well or
  otherwise observed in the environment. (310 CMR
  40.0006).
• PROPOSED DEFINITION
• Nonaqueous Phase Liquid and NAPL each means oil
  and/or hazardous material, or a mixture thereof,
  that is present in the environment as a separate
  non-gaseous phase liquid, that is relatively
  immiscible with water.

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LNAPL and the MCP

• Part I
• April 2005

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PROBLEMS WITH CURRENT MODEL

• Tank and Pancake model does not approximate the
  observed distribution of LNAPL in the
  environment. LNAPL does not fully saturate the
  pore space.
• LNAPL does not float in a monolithic layer on the
  groundwater table.
• Product thickness is a poor measure of LNAPL due
  to temporal and well diameter influences.

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LNAPL AT THE PORE SCALE VIEW

• Water and LNAPL and air co-exist in the
  pore-space
• Peak LNAPL saturation is typically observed at
  10 to 50 saturation
• Water is the wetting fluid in an LNAPL-water
  saturated system

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THE VERTICAL PROFILE
TPH (mg/kg)
TPH (mg/kg)
1000
10,000
100,000
1000
10,000
100,000
0
Depth (ft-bfs)
20
1
10
100
1
10
100
Saturation ()
Saturation ()
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LNAPL TIME DEPENDENCY
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LNAPL PART II
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LNAPL AND THE MCP PART II

• Eliminate the ½ Public Welfare UCL
• Maintain Concentration Based UCLs
• Evaluate current risks (health, eco, safety)
• Evaluate intermedia transfer via LNAPL Mobility
  Evaluation
• Develop a consistent framework for characterizing
  and understanding LNAPL sites, including using
  consistent nomenclature
• Improve assessment techniques (field techniques,
  laboratory tests, and LNAPL mobility analysis)

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AN LNAPL CHARACTERIZATION FRAMEWORK
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LNAPL CHARACTERIZATION FRAMEWORK
NOTE This is an example only, and should not be
used for site specific decision making.
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SOIL SATURATION LIMIT

• Definition of the soil saturation limit (from
  Broust et. al., API June 2000
• For a pure chemical, NAPL will not be present
  at concentrations below the soil saturation
  limit
• Csat, soil, i Si Qw Koc,i foc ?s
  Hi Qa
• ?s

Pore Water
Sorbed to organic carbon
Vapor Phase
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SOIL SATURATION LIMIT FOR SOME COMMON NAPLs
Source Broust, 2000 Soil Groundwater Research
Bulletin No. 9
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EXTENT OF NAPL BASED ON Csat
PEAK CONCENTRATION IN SOIL (mg/kg)
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LNAPL CHARACTERIZATION FRAMEWORK
NOTE This is an example only, and should not be
used for site specific decision making.
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CONCENTRATION AT 100 NAPL SATURATION

• What is the NAPL Concentration at 100
  Saturation?
• Easy to calculate
• Cmax n Gnapl
• (1-n)Gsolids

1
n
gw
1-n
gw
Where Gnapl specific gravity of NAPL and
Gsolids specific gravity of solids
gw
unit weight of water
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CONCENTRATION AT 100 NAPL SATURATION

• Calculation assumes the following
• Gnapl 0.81
• Gsolids 2.7

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LNAPL CHARACTERIZATION FRAMEWORK
NOTE This is an example only, and should not be
used for site specific decision making.
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RESIDUAL SATURATION

• A function primarily of LNAPL type and soil type

Increasing Viscosity
Data from Broust, 2000
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RESIDUAL SATURATION

• Estimate from published values (use with caution)
• Single point measurement using centrifuge test
  (conservative)
• Measure using permeability-type laboratory test

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MOBILITY EVALUATION
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JUST WHAT IS MOBILITY?
DARCYS LAW FOR SINGLE-PHASE FLUID FLOW IN SOIL v
kh i
NOTE Valid only for LNAPL that forms a
continuous phase.
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Darcys Law cannot describe all of the factors
controlling LNAPL mobility and applied alone can
produce misleading results suggesting LNAPL
velocity potential where the plume is in fact
stable.
API Interactive Guide
Mobility represents the potential for LNAPL
migration, independent of gradient. Therefore,
by proposing regulatory limits on mobility, we
are proposing limits on the potential for LNAPL
migration.
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MOBILITY EVALUATION APROACHES

• Weight of Evidence
• Direct Comparison to Residual Saturation
• Site Specific Mobility Evaluation
• Baildown Test Approach

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APPROACH 1 - WEIGHT OF EVIDENCE

• Release Volume
• Documented release date (or termination)
• LNAPL Type (Viscosity, solubility, volatility)
• Soil type (hydraulic conductivity)
• Depth of contamination (accessibility)
• De minimis in-well thickness???

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APPROACH 2 DIRECT COMPARISON TO RESIDUAL
SATURATION

• Compare measured site saturations to site
  specific residual saturation
• Assume no mobility if less than residual
  saturation
• Applicable when remediation has been done (or to
  set remediation goals)

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APPLICATION OF APPROACH 2
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APPROACH 3 SITE SPECIFIC MOBILITY CALCULATION

• Calculate Mo, and compare to a Practical Limit of
  Mobility (PLM)

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APPROACH 4 USE BAILDOWN TEST TO MEASURE
INHERENT MOBILITY

• Perform baildown test
• Calculate inherent mobility
• Compare to practical limit of mobility (PLM)

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COST CONSIDERATIONS

• Wont this be much more expensive than the
  current approach?

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COST CONSIDERATIONS

• Keep in mind that the tank and pancake model is
  scientifically invalid. Therefore, it isnt
  appropriate to compare costs of a scientifically
  defensible approach to an invalid approach.
• Assessment costs may be more or less, but not
  significantly more if planned properly. Field
  screening techniques are encouraged.
• Time is money! Repeated LNAPL thickness
  measurements take time/money.
• If the ½ UCL is preventing closure, money is
  likely being wasted.

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COST COMPARISON FOR ONE DAY SOURCE AREA ASSESSMENT
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CURRENT TECHNOLOGIES
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REAL WORLD VERTICAL PROFILES
SPLIT SPOON SAMPLES AND UVF FIELD SCREENING
ROST LIF SCREENING
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LABORATORY PROCEDURES

• Saturation (Dean-Stark Extraction)
• Relative Oil Permeability
• Residual Saturation via Centrifuge, followed by
  Saturation Test

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WHITE LIGHT AND UV PHOTOGRAPHY
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FIELD SCREENING
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DATA VISUALIZATION
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DATA VISUALIZATION
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DATA VISUALIZATION
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DATA VISUALIZATION
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PRODUCT THICKNESS
No measurable LNAPL in 12 on-site groundwater
monitoring wells. But residual LNAPL estimated
at over 6,000 gallons!