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Where is it going?

Laurence Libelo

U.S. EPA

OCSPP/OPPT/RAD

September 10, 2014
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 After a substance is released into the 

environment, what determines its 

distribution in the environment? 

 How should this information be used to 

determine which environmental 

medium/organism should be tested?
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“Difficult to Test”

 Poorly water soluble

 Volatile

 Sorbed compounds

 Toxicity as a function of solubility

 Unstable compounds

 Ionizable compounds

 Mixtures
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Tox (and fate) testing should be designed based 

on understanding where the chemical may go 

after release to environment

 What environmental compartment will the chemical 

primarily occur in?

 Based on where it may be and what species or niche is 

likely to be most affected?

 Test exposure based on environmental exposure routes

» Air 

» Water

» Food

» Dermal

 Parent or degradation products?
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Environmental Distribution

 Depends on: 

 Location and amount of release

 Persistence in different media

 Long range transport potential

 Inherent chemical properties

 Movement within and between environmental media

» Air

» Water

» Solids (sediment/soil)

» Biota
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Where is it going?

 Environmental fate and transport is a 

complicated function of many interrelated 

processes acting at different rates

 Never have all the information required

 Need to make many assumptions
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Making testing decisions with limited 

data

 Base tox and fate the decision on 

appropriate level of fate and transport 

evaluation

 Highly complex environmental models not 

needed (at least initially)

“Our life is frittered away by detail... simplify, simplify.” Henry 

David Thoreau

Or – what assumptions can we make that allow us to deal 

with the question?
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Start with a simple model of the world

Air

Sediment/soil Water
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Makes the world into a three phase 

system

 Allows estimation of likely distribution into 

compartments based on physical/chemical 

properties of chemical

 Does not account for environmental 

processes, only inherent chemical 

properties 
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Tier 1 models

 Assume equilibrium

 No dissipation/loss reactions

 Instantaneous partitioning

 Often sufficient to direct testing
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Limited information needed:

Based on P-chem and fate properties

 Use experimental data if available

 Read Across from data on structural 

analogs

 Estimate

 EPISUITE
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Minimum partitioning and exposure 

pathways information

 P-Chem Properties

 Melting Point (MP)

 Boiling Point (BP)

 Water Solubility (WSol)

 Vapor Pressure (VP)

 Dissociation constant (pKA or pKB)

 Partitioning properties

 Henry’s Law Constant (HLC)

 Water/solid partition coefficent (Kd or Koc)

 Octanol/water partition coefficient (Kow or P)

 Bioconcentration factors (BCF, BAF)

 Environmental degradation (parent/products?)

 Biodegradation, Photolysis, Hydrolysis
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Water Solubility

 Water Solubility (WSol or WS)

 How the chemical is likely to distribute between 

environmental compartments (air, soil, or 

water/sediment)

 Informs potential for exposure in aquatic 

compartment

 Used to estimate other paramaters
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Vapor Pressure

Air

Sediment/soil Water

Tendency for a chemical 

to transfer to and from 

the gas phase
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Vapor Pressure

 VP > 10- 4(atm) exists mostly in the vapor (gas) phase

 VP 10-5- 10-7 exists in the vapor and particulate phase

 VP < 10-8 exists mostly in the solid phase
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Minimum partitioning and exposure 

pathways information

 P-Chem Properties

 Melting Point (MP)

 Boiling Point (BP)

 Water Solubility (WSol)

 Vapor Pressure (VP)

 Dissociation constant (pKA or pKB)

 Partitioning properties

 Henry’s Law Constant (HLC)

 Water/solid partition coefficent (Kd or Koc)

 Octanol/water partition coefficient (Kow or P)

 Bioconcentration factors (BCF, BAF)

 Environmental degradation (parent/products?)

 Biodegradation, Photolysis, Hydrolysis
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Henrys Law Constant

Air

Sediment/soil Water

The ratio of a chemical's 

equilibrium concentration in 

the gas phase to that in the 

aqueous phase
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Henry’s Law Constant (HLC)

 Provides information on 

 Chemical’s volatility from water

 Environmental partitioning between water 

and air

 Air as an environmental exposure route

 Informs removal by stripping in WWTPs
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Solid/Water distribution ratio (Kd)

Air

Sediment/soil Water

The ratio of a chemical's 

equilibrium concentration 

between aqueous and solid 

phases
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Solid Adsorption Coefficient (Kd)

 Provides

 A measure of the ability of a chemical to associate with 

soil and sediment

 Describes partitioning of the chemical between water 

and suspended solids and sediment

 Indicates the potential for the chemical to leach through 

soil and be introduced into ground water

 Strong adsorption will impact other fate properties
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Organic Carbon normalized solid 

adsorption coefficient (Koc)

 Kd = partitioning between the solid-phase 

(soil or sediment) and water at equilibrium

 Most organic chemicals tend to associate 

with organic portion of solids

 Kd is often normalized with respect to the 

organic carbon content of the sediment or 

soil

Koc = Kd/ %OC X 100
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General Interpretation of Kd or Koc

 Low Koc chemicals will tend to be in aqueous phase

 Limited retention and leaching through soil to groundwater

 Exposure through water column

 Advective transport in water 

 High Koc will tend to be sorbed onto sediment and particulate 

matter

 Exposure through contact with or ingestion of solids and pore 

water

 Transport/exposure in air on particulates and house dust

 May reduce rate of degradation by limiting availability
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Air

Sediment/soil Water

Biota

Kow

Koa

Kow/Kd

Uptake by biota
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Octanol/Water Partition Coefficient 

(Kow or P)

Water

Octanol

CONC octanol

CONCwater

Benzene’s Kow is 135

Log Kow is 2.13

Kow=
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Octanol-water partition coefficient 

(Kow)

 Inversely proportion to the water solubility 

 Indicates how partitioning between water and 

organic phases

 Informs absorption into biota and bioaccumulation
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Conclusions

 P-chem, partitioning and other fate transport 

parameters may be used to estimate 

environmental distribution

 Environmental toxicity testing should be based on 

where the chemical goes (i.e., medium/organisms 

to test)

 Testing can be based on simple parameters or 

models

 More complex models may be needed in some 

cases – Next talk