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Chapter 10
Water-Sediment Studies
Jeremy Dyson
Basel, Switzerland
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
Defining, Estimating & Using Endpoints
Application of Parent or Metabolite
Volatilisation
Water Column
Well-mixed
Aerobic
Water+particulates
Metabolism: Formation & Degradation
Transfer
Processes
Water-Sediment Interface
Sediment
Slow-mixing
Oxic to anoxic
Metabolism: Formation & Degradation
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
• What are the Components of these Endpoints?
– Chemical (Parent or Metabolite)
– Compartment (System, Water Column or Sediment)
– Disappearance (Degradation or Dissipation)
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
Defining, Using & Estimating Endpoints
Kinetic
Level
Level I
(1 comp.)
Persistence/Modelling
Endpoints
System (Metabolites)
System (Both)
Water column (Both)
Sediment (Both)
Disappearance
Endpoints
Dissipation
Degradation
Dissipation
Dissipation
Level II
(2 comp.)
Water column (Parent)
Sediment (Parent)
Degradation
Degradation
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
Models and Flowcharts: Level P-I
Kinetic Concept
Compartment
Initial Level
Mo
Generic Equation
M = Mo F(t)
wc + sed
or
wc
or
sed
Data for wc
Disappearance Graphs
Data for wc + sed
Disappearance Times
DT50/90wc+ sed – calculate directly from the fit
DT50/90wc – calculate directly from the fit
DT50/90sed – calculate directly from the fit
Data for sed
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
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
Models and Flowcharts: Level P-II
Kinetic Concept
Application
Mo
Compartment
Water Column
Mw
rw-s
rs-w
Sediment
k-w
Ms
ks
Generic Equations
dMw = -rw-s Mw + rs-w Ms – kw Mw
dt
dMs = -rs-w Ms + rw-s Mw – ks Ms
dt
Disappearance Graph
Data for wc
Data for water column
Data for sediment
Disappearance Times
DegT50/90w – calculate directly from the fit
DegT50/90s – calculate directly from the fit
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
Models and Flowcharts: Level P-II
Example of Transfer Pattern without Degradation
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+)]
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 ks>0 as demonstrated by t-test
Transfer rate from sediment to water rs-w>0
The Fsed test needs to be passed
Use 1 of 3 default
approaches tested to
ensure they lead to
worst-case PEC values
No
Criteria
met?
Yes
Use estimates
as required
against triggers/
in modelling
Models and Flowcharts: Level P-II
•
Persistence/Modelling Degradation Endpoints
Default approach 1
Passes Fsed test but fails t-test on zero degradation rate
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
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
Yes
Fitted
degradation faster
Set water column degradation
rate to overall system half-life
Set sediment degradation rate to
1 000 day half-life
in water column than
Set water column degradation
rate to estimated half-life
in sediment?
No
Set sediment degradation rate
to overall system half-life
Use default
as required
in modelling
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-life=overall system;
Sediment half-life=
1 000 days, or vice versa
Use default
as required
in modelling
Models and Flowcharts: Level P-II
•
Persistence/Modelling Degradation Endpoints
Default approach 3
Compound 2 Default 3A
Compound 2 SFO fit
Compound 2 Default 3B
Models and Flowcharts: Level P-II
•
Persistence/Modelling Degradation Endpoints
Default approach 3
Compound 6 Default 3A
Compound 6 SFO Fit
Compound 6 Default 3B
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
User-friendly implementation needs developing
Or diffusion-based model specific to water-sediment systems
needs to be developed
Statistics and Examples
•
Assessing Goodness of Fit
Visual Assessment
–
–
•
2 Test
–
–
–
•
Main tool for assessment
Plots of model fits & residuals
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
Statistics and Examples: Level P-I
Compound 6
wc
Compound 6
wc + sed
Compound 6
sed
Statistics and Examples : Level P-I
Compartment Modification
DegT50/DT50 in days (2)
SFO
FOMC
HS
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)
wc + sed
Statistics and Examples : Level P-II
Compound 6
Statistics and Examples : Level P-II
Compartment Modification
DegT1/2
Fsed (%)
(2value) Modelled Theoretical
wc
sed
Fix Mo
(3.1)
2.16 (9.0)
44
27 - 57
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
Models and Flow Charts: Level M-I
Defining Persistence/Modelling Endpoints
Type of Endpoint
Compartment
Kinetic Model
Dissipation
System
Water Column
Sediment
Decline from peak
„
„
„
„
„
„
Degradation
System
Formation & degradation
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
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
Models and Flowcharts: Dissipation Level M-I
Kinetic Concept
Compartment
wc + sed
or
wc
or
sed
Initial Level
Mo
Generic Equation
M = Mo F(t)
Data for wc
Disappearance Graphs
Data for wc + sed
Data for sed
Disappearance Times
DT50/90wc+sed – calculate directly from the fit
DT50/90wc – calculate directly from the fit
DT50/90sed – calculate directly from the fit
Models and Flowcharts: Degradation Level M-I
Kinetic Concept
Application
Mo
Compartment
Parent (wc+sed)
Mp
1-fm
fm
Metabolite (wc+sed)
Mm
Generic Equations
MP = Mo FP(t)
t
Mm(t) = - fm Mo dFP(ti) / dti Fm(t – ti) dti
0
Disappearance Graph
Parent (wc+sed)
Metabolite (wc+sed)
Disappearance Times
DegT50/90wc+sed – calculate directly from the fit
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
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