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
• Further Aspects of these Endpoints
– For Parent or Metabolites
– For Degradation or Dissipation
– For Whole System, Water Column or Sediment
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 (Parent & Metabs)
Water column (Both)
Sediment (Both)
Level II
(2 comp.)
Water column (Parent)
Sediment (Parent)
Disappearance
Endpoints
Degradation
Dissipation
Dissipation
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
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 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
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
Strongly sorbing compound
no degration in water column
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
Weakly sorbing compound
no degration in water column
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 watersediment systems
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