Washingtons Model Toxics Control Act MTCA

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• Washingtons Model Toxics Control Act (MTCA)
• Soil to Ground Water Exposure Pathway
• Chapter 173-340-747 WAC

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Soil to Ground Water Exposure Pathway
Old MTCA 100 x G.W. Standard

• Example
• Benzene MCL 5 ppb
• 100 x 5 500 ppb
• Protective Soil Concentration 500 ppb

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Soil to Ground Water Exposure Pathway

• 3-Phase Partitioning Model
• 4-Phase Partitioning Model
• Fate / Transport Models
• Leach Tests
• Empirical Demonstration
• Residual Saturation

New MTCA Section 747
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Section 747 Criteria747(2)
Soil concentration considered protective if

• Section 720 ground water cleanup level has not
  been exceeded, and
• Non-aqueous phase liquids (NAPL) are not present
  on or in ground water.

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Deriving MTCA Cleanup LevelsConceptual Site Model

• Conceptual Site Model
• The conceptual site model is a tool that is
  used to derive cleanup levels. A conceptual site
  model (CSM) is a summary that (1) describes all
  of the known or suspected sources of
  contamination, (2) considers how and where the
  contaminants are likely to move, and (3)
  identifies who is likely to be affected by them.
  The relationship between the types of information
  needed for the conceptual site model is often
  illustrated as follows

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Conceptual Site Model Soil to Ground Water
Exposure Pathway
Human Exposure 1-2 Liters of Water per Day
Hazardous Substance Release
Drinking Water Well
Contaminated Soil
Ground Water
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Soil to G.W. Exposure PathwayRecent Examples

• Civil Action Woburn, Massachusetts - city water
  supply impacted by release of TCE from W.R. Grace
  Co. and Beatrice Foods.
• Erin Brockovich Hinkley, CA - hexavalent
  chromium migrates from soil into ground water.

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Soil to G.W. PathwayWhy is it so Important?
Worldwide Water Distribution
Source C.W. Fetter, 1988
United States 52 of the Population Depends on
Ground Water for Drinking Water! Source National
Ground Water Association (NGWA)
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Chemical Partitioning Models Soil to Ground
Water Exposure Pathway
3-Phase Partitioning Model Partitioning of
chemical mass among three phases soil, air and
water.
C

Soil concentration (mg/kg)
s
C
Groundwater cleanup level established under
WAC 173-340-720 (ug/l)
w
UCF Unit conversion factor ( 1 mg / 1,000 ug)
Dilution factor (dimensionless 20 for
unsaturated zone soil, 1 for saturated zone soil)
DF

K
Distribution coefficient (L/kg)
d
Water-filled soil porosity (0.3 ml water/ ml soil
for unsaturated zone soil 0.43 ml soil/ml
water for saturated zone soil)
Air-filled soil porosity (0.13 ml air/ml soil
for unsaturated zone soil zero for saturated zone
soil)
H

Henrys law constant (Dimensionless)
cc
Dry soil bulk density (1.5 kg/L)
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Chemical Partitioning Models Soil to Ground
Water Exposure Pathway
4-Phase Partitioning Model Partitioning of
chemical mass among four phases soil, air, water
and NAPL.
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3/4-Phase Model Input Parameters Mass Fraction
of Natural Soil Organic Carbon (foc)

• Definition
• Most soils have a small amount or fraction of
  natural organic carbon.
• Soil foc is typically higher near the ground
  surface and decreases with depth. Common range is
  ? 0.1 - 1.
• Units g soil organic carbon / g soil.
• Commonly expressed as foc, i.e. 0.1 foc
  0.001 g organic carbon per g of soil.

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3/4-Phase Model Input ParametersMass Fraction
of Natural Soil Organic Carbon (foc)

• Use and Importance
• Soil foc is used to calculate the distribution
  coefficient (Kd), which is used to estimate
  retardation or how fast a chemical will be
  transported through an aquifer.

Soil foc is an extremely important fate /
transport model input parameter!
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3/4-Phase Model Input Parameters Distribution
Coefficient (Kd)

• Definition and Use
• Kd is a measure of the chemical mass that
  partitions to both the solid and liquid phases.
  It is used to predict chemical partitioning and
  to estimate retardation.

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3/4-Phase Model Input Parameters Soil Organic
Carbon / Water Partitioning Coefficient (Koc)

• Definition
• Koc is Kd normalized to soil foc. It is the
  ratio of (1) the of the amount of chemical
  adsorbed in the soil per unit weight of organic
  carbon per (2) equilibrium chemical concentration
  in solution.
• Koc ug adsorbed / g organic carbon per ug/mL
  solution ml/g or L/kg.

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3/4-Phase Model Input Parameters Soil Organic
Carbon-Water Partitioning Coefficient (Koc)

• Nonionic Hydrophobic Organics
• Koc may be used in conjunction with soil foc to
  predict the distribution coefficient (Kd)
• Kd Koc foc

Example calculate benzene Kd for soil with 0.1
foc. Benzene Koc 62 ml/g. Kd 62 ml/g
0.001 g carbon / g soil 0.062 ml/g
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3/4-Phase Model Input Parameters Soil Organic
Carbon-Water Partitioning Coefficient (Koc)

• Ionizing Organics
• Koc for the ionized and neutral forms of certain
  substances (e.g. pentachlorophenol) will vary
  according to pH.

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3/4-Phase Model Input Parameters Metals Kd

• Metals Kd
• The relationship between soil organic carbon and
  sorption is not as dominant as it is for nonionic
  organics.
• Soil texture (grain size), pH, iron oxide content
  etc. also control metals sorption.

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3/4-Phase Model Input Parameters Soil Porosity,
Bulk Density, Volumetric Water / Air Content

• Soil porosity void volume / total soil volume
  (dimensionless). MTCA default 0.43
• Dry soil bulk density mass (kg) per volume (L).
  MTCA default 1.5 kg/L
• Volumetric water content ml of water / ml of
  soil pore space. MTCA default 0.3 ml / ml
• Volumetric air content ml of air / ml of soil
  pore space. MTCA default 0.13 ml / ml

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3/4-Phase Model Input Parameters Henrys Law
Constant (HLC)

• Definition
• Henrys law constant (HLC) is the air-water
  partition coefficient. It is the ratio of the
  aqueous solubility of a substance (mg/L) at a
  given temperature to the saturated vapor phase
  concentration (mg/L).

• Example
• HLC Vapor Pressure Molecular Weight /
  Solubility
• Benzene 0.125 atm 78 g/mol / 1,780 mg/L
  5.47 E-03 atm-m3/mol
• Dimensionless (H) (HLC atm-m3/mol x 41) (_at_
  25? C)
• H for benzene 0.22

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3/4-Phase Model Input Parameters Dilution Factor
(DF)

• Definition
• Accounts for the dilution that occurs when soil
  pore water mixes with ground water.
• MTCA Default DF 20 (Dimensionless).
• MTCA default DF is based on EPA Modeling
  Science Advisory Board (SAB) review.

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Qa Ground Water Flow (m3/yr) Qp Infiltration
(m3/yr) C1 Background Concentration (mg/L) C2
Concentration after Mixing (mg/L)
DILUTION FACTOR
Qp,Cw
SOURCE AREA
VADOSE ZONE
AQUIFER
MIXED G.W.
MIXING ZONE
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Leaching Tests

• Section 747 (7)
• EPA Method 1311 (TCLP) EPA Method 1312 (SPLP)
  leach tests may be used to derive protective soil
  concentrations for the following metals arsenic,
  cadmium, total / hexavalent chromium, copper,
  lead, mercury, nickel, selenium and zinc.
• Other leach tests may be used on a site-specific
  basis for all other hazardous substances (e.g.
  petroleum), per Ecology approval.

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Alternative Fate / Transport Models

• Section 747 (8)
• Mathematical algorithms that simulate the fate
  and transport of hazardous substances in soil and
  ground water may be used.
• Finite source assumption and biodegradation may
  be used.
• Model assumptions, input parameters and values
  subject to the criteria specified in Section 702
  (New Scientific Information).

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Empirical Demonstration

• Section 747 (9)
• Empirical relying upon or derived from
  observation or experiment, i.e. site-specific
  data.
• One or any combination of the methods described
  in Section 747 may be used.
• Criteria you must demonstrate that the soil /
  ground water system has achieved steady-state or
  equilibrium conditions. Methods used must also
  comply with the criteria specified in Section 702
  (New Scientific Information).

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EMPIRICAL DEMONSTRATION
UST
SOIL
GROUND WATER
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Residual Saturation (RS)

• Section 747 (10)
• RS is a term that is used to describe the
  phenomena that occurs when oil gets trapped in
  soil.
• RS is primarily a function of soil grain size
  moisture content, as well as the viscosity of the
  oil itself.
• RS may be used as part of an empirical
  demonstration.

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Residual Saturation
NAPL Non Aqueous Phase Liquid
Soil Matrix
Free NAPL
Trapped NAPL
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Fuel Migration is Retarded by Capillary Forces
and Molecular Attraction.