Water-Sediment Studies

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Chapter 10 Water-Sediment Studies Jeremy
Dyson Basel, Switzerland
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Outline

• Defining, Estimating Using Endpoints
• Parent Kinetics
• Similarities/differences to other test systems
• Models and flowcharts
• Statistics and examples
• Metabolite Kinetics
• Similarities to other test systems
• When are metabolite kinetics not required?
• Models and flowcharts
• Concluding Remarks

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Defining, Estimating Using Endpoints
Application of Parent or Metabolite
Volatilisation
Water Column Well-mixed Aerobic Waterparticulates

Metabolism Formation Degradation
Transfer Processes
Water-Sediment Interface
Metabolism Formation Degradation
Sediment Slow-mixing Oxic to anoxic
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Defining, Using Estimating Endpoints

• Persistence Endpoints
• To determine whether various aquatic ecotoxicolgy
  studies are triggered, e.g. fish accumulations
  studies
• Modelling Endpoints
• To use in calculating PEC values as part of an
  aquatic risk assessment, e.g. FOCUS surface water
  scenarios
• Further Aspects of these Endpoints
• For Parent or Metabolites
• For Degradation or Dissipation
• For Whole System, Water Column or Sediment

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Defining, Using Estimating Endpoints

• The Water-Sediment System Definitions
• Behaviour can be more complex than in other
  systems
• Straightforward definitions e.g. dissipation from
  compartments
• Non-straightforward definitions, e.g. degradation
  in compartments
• Study Guidelines and Use
• Not always clear if dissipation or degradation
  required
• Decisions about endpoints used made on a
  case-by-case basis
• Difficulties of Estimation
• Main problem over degradation-transfer
  correlations
• No simple, robust reliable constraints
  procedures
• Default worst-case approach if lack of
  degradation in one compartment, implausible
  transfer rates (Fsed test), or generally
  inconsistent with other environmental fate studies

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Defining, Using Estimating Endpoints

• Kinetic Persistence/Modelling
  Disappearance
• Level Endpoints
  Endpoints
• Level I System (Parent Metabs)
  Degradation
• (1 comp.) Water column (Both)
  Dissipation
• Sediment (Both)
  Dissipation
• Level II Water column (Parent)
  Degradation
• (2 comp.) Sediment (Parent)
  Degradation

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Parent Kinetics

• Similarities to Other Test Systems
• Data entry and exclusion
• Selection of fitting routine
• Standard constraints, underlying kinetics etc.
• Methods of making kinetic decisions
• Differences to Other Test Systems
• Day zero data put all in water column
• Data in terms of mass or equivalent, e.g. AR
• Do not use concentration data
• Operation of the worst-case default approach at
  Level P-II

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Models and Flowcharts Level P-I
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Models and Flowcharts Level P-I

• SFO Kinetics
• Default first choice
• Required for modelling endpoints
• FOMC Kinetics
• Evaluate if data depart appreciably from SFO
  kinetics
• DFOP Kinetics
• Offers more flexibility than FOMC with extra
  parameter
• Hockey Stick Kinetics
• Data sometimes appear to have some breakpoint
  in rate

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Models and Flowcharts Level P-I

• System Degradation/Compartment Dissipation
• Persistence Endpoints
• Tier 1 Check if SFO is an appropriate model
• Tier 2 Identify best-fit model if required
• Modelling Endpoints
• Tier 1 Check if SFO is an acceptable model
• Tier 2 Correction procedures if SFO not an
  acceptable model

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Models and Flowcharts Level P-II
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Models and Flowcharts Level P-II

• Empirical Transfer Pattern
• Able to approximate quite closely
• Simple Transfer Kinetics
• No assumptions about sediment concentration
  gradients
• Appropriate if gradients are complex and not
  measured
• Appropriate to consider before more complex
  alternatives
• First-Order Transfer Kinetics
• Relatively easy to implement in software packages

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Models and Flowcharts Level P-II
Example of Transfer Pattern without Degradation
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Models and Flowcharts Level P-II

• The Fsed Test
• Definition
• Fraction in sediment at equilibrium in absence of
  degradation
• Modelled Fsed Values
• Calculated from fitted transfer parameters of
  Level P-II model
• Fsed rw-s / (rw-s rs-w)
• Theoretical Fsed Values
• Based on system/pesticide properties diffusion
  assumptions
• Fsed (Kd ?b?) / (Zw /ZD)(Kd ?b?)

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Models and Flowcharts Level P-II

• Persistence/Modelling Degradation Endpoints
• SFO Fit (Criteria to be met even if fit
  acceptable)
• Consistent with environmental fate data
• Degradation rates kw and ksgt0 as demonstrated by
  t-test
• The Fsed test needs to be passed

Use estimates as required against triggers/ in
modelling
Use 1 of 3 default approaches tested to ensure
they lead to worst-case PEC values
Yes
No
Criteria met?
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Models and Flowcharts Level P-II

• Persistence/Modelling Degradation Endpoints
• Default approach 1
• Passes Fsed test but one degradation rate is zero
  or fails t-test

Set degradation rate to overall system
half-life in degrading compartment Set
degradation rate to 1 000 day half-life in
non-degrading compartment
Use default as required in modelling
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Models and Flowcharts Level P-II

• Persistence/Modelling Degradation Endpoints
• Default approach 2
• Fails Fsed test due to zero transfer rate from
  sediment to water

Set water column degradation rate to overall
system half-life Set sediment degradation rate
to 1 000 day half-life
Yes
Fitted degradation faster in water column than in
sediment?
Use default as required in modelling
Set water column degradation rate to estimated
half-life Set sediment degradation rate to
overall system half-life
No
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Models and Flowcharts Level P-II

• Persistence/Modelling Degradation Endpoints
• Default approach 3
• Fails Fsed test or inconsistent with E Fate data
  (degradation)

Determine and use default that results in
worst-case PEC values Water column
degradation half-lifeoverall system Sediment
half-life 1 000 days, or vice versa
Use default as required in modelling
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Models and Flowcharts Level P-II

• Persistence/Modelling Degradation Endpoints
• Default approach 3

Strongly sorbing compound no degration in water
column
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Models and Flowcharts Level P-II

• Persistence/Modelling Degradation Endpoints
• Default approach 3

Weakly sorbing compound no degration in water
column
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Models and Flowcharts Level P-II

• What If the Default Options Need Refining?
• Fit a diffusion-based model to water-sediment
  data
• A TOXSWA example for such refinement is in
  Appendix 12
• Development needed for a user-friendly
  implementation of TOXSWA, or a diffusion-based
  model specific to water-sediment systems

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Statistics and Examples

• Assessing Goodness of Fit
• Visual Assessment
• Main tool for assessment
• Plots of model fits residuals
• ?2 Test
• Performed for each compartment, even at Level
  P-II
• Supplements visual assessment model comparison
• Only a guidance value of 15 error value to pass
  test
• t-Test
• Reliability of individual dissipation/degradation
  rates
• Total df with a significance level of 10 to pass
  test

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Statistics and Examples Level P-I
Compound 6
wc
Compound 6
wc

sed
Compound 6
sed
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Statistics and Examples Level P-I
Compartment Modification DegT50/DT50 in days
(?2)
SFO FOMC HS
wc sed Remove outlier
20.1 (3.6) 20.1 (3.6) 19.8 (3.0)
wc Remove outlier 19.1
(2.8) 18.6 (2.7) 18.7 (1.9) sed
Remove outlier 21.1 (9.4)
15.2 (6.5) 17.7 (7.7)
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Statistics and Examples Level P-II
Compound 6
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Statistics and Examples Level P-II
Compartment Modification DegT1/2
Fsed ()
(?2value) Modelled
Theoretical wc
Fix Mo ? (3.1) sed

2.16 (9.0) 44 27 - 57
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Metabolite Kinetics

• Similarities to Other Test Systems
• Data entry and exclusion
• Selection of fitting routine
• Standard constraints, data exclusion, underlying
  kinetics etc.
• Methods of making kinetic decisions
• When Are Metabolite Kinetics Not Required?
• Sometimes not required for minor metabolites
• If risks implicitly assessed via higher tier
  studies
• Sometimes not if also applied as a parent
  substance
• Sometimes not if can add metabolite residues to
  parent

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Models and Flow Charts Level M-I

• Defining Persistence/Modelling Endpoints

Type of Endpoint Compartment Kinetic
Model Dissipation
System Decline from
peak
Water Column

Sediment
Degradation
System Formation
degradation
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Models and Flowcharts Level M-I

• SFO Kinetics
• Default first choice
• Required for modelling endpoints
• FOMC Kinetics
• Evaluate if data depart appreciably from SFO
  kinetics
• DFOP Kinetics
• Offers more flexibility than FOMC with extra
  parameter
• Hockey Stick Kinetics
• Not used

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Models and Flowcharts Level M-I

• System/Compartment Dissipation/Degradation
• Persistence Endpoints
• Tier 1 Check if SFO is an appropriate model
• Tier 2 Identify best-fit model if required
• Modelling Endpoints
• Tier 1 Check if SFO is an acceptable model
• Tier 2 Correction procedures if SFO not an
  acceptable model

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Models and Flowcharts Dissipation Level M-I
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Models and Flowcharts Degradation Level M-I
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Models and Flowcharts Level M-II

• General Recommendations for Development
• Data/Parameter Requirements
• Minimise, e.g. do not use sink data as a first
  step
• Kinetics
• Use first-order kinetics for transfer
  degradation processes
• Formation Fraction
• Option to use same fraction for water column
  sediment
• Option to use a default fraction, i.e. that
  estimated at Level M-I

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Concluding Remarks

• General Remarks
• Complex area of kinetics, but the workgroup has
  increased understanding of strengths
  limitations of approaches, bringing greater
  transparancy consistency
• Parent Kinetics
• Resolved endpoint definition, use and estimation
• In a framework and developed degradation
  refinement process
• Metabolite Kinetics
• Resolving endpoint definition, use and estimation
• Kinetics still need actively developing for Level
  M-II