Understanding Water-Sediment System Endpoints: Kinetics and Models

1. Chapter 10 Water-Sediment Studies Jeremy Dyson Basel, Switzerland

2. 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 Outline

3. 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 Metabolism: Formation & Degradation Sediment Slow-mixing Oxic to anoxic

4. 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

5. 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

6. 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 Defining, Using & Estimating Endpoints

7. 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 Parent Kinetics

8. Models and Flowcharts: Level P-I

9. 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

10. 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-I

11. Models and Flowcharts: Level P-II

12. 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

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

14. 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

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

16. Persistence/Modelling Degradation Endpoints Default approach 1 Passes Fsed test but one degradation rate is zero or fails t-test Models and Flowcharts: Level P-II 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

17. Persistence/Modelling Degradation Endpoints Default approach 2 Fails Fsed test due to zero transfer rate from sediment to water Models and Flowcharts: Level P-II 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

18. Persistence/Modelling Degradation Endpoints Default approach 3 Fails Fsed test or inconsistent with E Fate data (degradation) Models and Flowcharts: Level P-II 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

19. Persistence/Modelling Degradation Endpoints Default approach 3 Models and Flowcharts: Level P-II Strongly sorbing compound no degration in water column

20. Persistence/Modelling Degradation Endpoints Default approach 3 Models and Flowcharts: Level P-II Weakly sorbing compound no degration in water column

21. 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 Models and Flowcharts: Level P-II

22. 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 Statistics and Examples

23. Statistics and Examples: Level P-I Compound 6 wc Compound 6 wc + sed Compound 6 sed

24. 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)

25. Statistics and Examples : Level P-II Compound 6

26. 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

27. 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 Metabolite Kinetics

28. Defining Persistence/Modelling Endpoints Models and Flow Charts: Level M-I Type of Endpoint Compartment Kinetic Model Dissipation System Decline from peak Water Column „ „ „ Sediment „ „ „ Degradation System Formation & degradation

29. 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

30. 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

31. Models and Flowcharts: Dissipation Level M-I

32. Models and Flowcharts: Degradation Level M-I

33. 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

34. 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