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# Where is it going?

Laurence Libelo U.S. EPA OCSPP/OPPT/RAD September 10, 20142  

## 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?3 

## “Difficult to Test”  

## Poorly water soluble  

## Volatile  

## Sorbed compounds  

## Toxicity as a function of solubility  

## Unstable compounds  

## Ionizable compounds  

## Mixtures4 

### 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?5 

## 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) » Biota6



## 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 assumptions7 

## 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?8 

## Start with a simple model of the world
Air

Sediment/soil Water9 

## 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 properties10 

## Tier 1 models  

## Assume equilibrium  

## No dissipation/loss reactions  

## Instantaneous partitioning

 

## Often sufficient to direct testing11 

## Limited information needed: Based on P-chem and fate properties  

## Use experimental data if available  

## Read Across from data on structural 

## analogs  

## Estimate  

### EPISUITE12 

## 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 pK B )  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, Hydrolysis13 

## 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 paramaters14



## Vapor Pressure Air

Sediment/soil Water Tendency for a chemical to transfer to and from the gas phase15 

## 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 phase16 

## 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 pK B )  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, Hydrolysis17



## Henrys Law Constant

Air

Sediment/soil Water The ratio of a chemical's equilibrium concentration in the gas phase to that in the aqueous phase18 18 

## 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 WWTPs19 

## Solid/Water distribution ratio (K d 

## )
Air

Sediment/soil Water

The ratio of a chemical's equilibrium concentration between aqueous and solid phases20 20 

# Solid Adsorption Coefficient (K d 

# )
 

### 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 properties21 

# 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 10022 

# General Interpretation of K d 

# or K

oc  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 availability23 Air

Sediment/soil Water Biota Kow Koa Kow/Kd 

## Uptake by biota24 

# Octanol/Water Partition Coefficient (Kow or P)

Water Octanol CONC octanol CONCwater Benzene’s Kow is 135 Log Kow is 2.13 Kow=25 

## Octanol - water partition coefficient (Kow)  

### Inversely proportion to the water solubility  

### Indicates how partitioning between water and organic phases  

### Informs absorption into biota and bioaccumulation26 

## 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