Guidance proposal for stepped approach for

DegT50 in Dutch surface water

Jos Boesten

partially based on report with co-authors:

Paulien Adriaanse Mechteld ter Horst Ton van der Linden Aaldrik Tiktak

Outline

Problem

Proposed stepped approach

Hydrolysis

Degradation in fresh surface water in dark

Outdoor studies

Conclusions

Problem

Dutch workgroup developed 90th percentile ditch scenario for exposure of aquatic organisms

● Shallow ditch

Geomean DegT50 in surface water needed as input for TOXSWA

Problem

Example scenario calculation - Herbicide Kom = 36 L/kg - Slow water flow in spring/summer - Accumulation of repeated applications Ecotoxicologists: constant levels lead to problems for the chronic effect assessment, so DegT50 may be important (not relevant for FOCUS ditches: quick refreshment of water also in spring)

This calculation with conservative DegT50 = 1000 d Decline in spring and summer due to sorption to sediment

Problem

Example of dossier information

What to use ?

Problem

Alterra mesocosm studies with some ten different substances available

picture on domination of degradation processes based on this limited information:

¼ hydrolysis (e.g. lambdacyhalothrin)

¼ microbial or algae-induced (e.g. prosulfocarb)

½ photolysis (e.g. metribuzin, imidacloprid)

so assessments needed for DegT50s driven by all these processes

Proposed stepped approach

direct

photolysis only

also indirect

photolysis

Hydrolysis

Depends often on pH

Dutch database on pH in surface water: 42000 measurements from 3500 locations

2% of cases pH < 7

2% of cases pH > 9.5

90th percentile: 4% do not matter

Proposed guidance: take longest hydrolysis DegT50 from dossier in pH range from 7 to 9

Aerobic fresh surface water in the dark

(OECD 309)

OECD-309: aerobic incubation (>2/3 of volume is air)

Dutch database on O2: 37000 measurements from 3500 locations

>99% of cases O2 > 0.5 mg/L

so aerobic incubation is OK

Aerobic fresh surface water in the dark

(OECD 309)

OECD-309: two options: only water or with suspended sediment (10-1000 mg/L)

Dutch database on suspended matter: 11000 measurements from 1500 locations: median about 10 mg/L

Proposed guidance: perform OECD-309 with 10 mg/L and 7 < pH < 9.5 for number of relevant water samples

Proposed stepped approach: studies

including light

difficult to extrapolate results from lab studies to field studies so only outdoor studies: closer to reality

Outdoor systems with algae and

macrophytes

Algae and macrophytes may increase degradation rates in surface water

Two branches: photolabile and photostable substances

Photostable substances:

studies needed with different plant species or different plant communities or at least two studies with algae

relatively low densities of plants or algae

higher tier of OECD-309 (aerobic fresh surface water)

Outdoor systems with algae and

macrophytes: photolabile substances

daily global radiation quite variable in Netherlands so useful to include in scenario

Outdoor systems with algae and

macrophytes: photolabile substances

DisT50 (h) of imidacloprid in water

submerged emergent no plants

Römkens, P.F.A.M., W.H.J. Beltman, J.E. Groenenberg, Arts, G.H.P. ,

2012. Potentials of aquatic macrophytes to reduce contaminant levels

in surface waters. Alterra report (in prep.)

floating mesocosm studies with different macrophytes water gently stirred after imidacloprid application

Conclusion: slow degradation under duckweed cover

Metribuzin mesocosms Alterra, applied in May

DegT50 of 7 d in polycarbonate cylinders and 1.6 d in whole

ditch

calculated back to same reference global radiation for period

of 2 × DegT50: cylinders 2.6 times slower than ditch

polycarbonate walls keep out part of the UV light

mesocosms with such walls probably generate conservative

DegT50 (if we forget about the duckweed)

Outdoor systems with algae and

macrophytes: photolabile substances

Brock 2004 Arts 2006

Outdoor systems with algae and

macrophytes: photolabile substances

Comparison of DegT50 values in Alterra mesocosms with labstudies on photolysis

Conclusion: large gap between basic data and mesocosm

Outdoor systems with algae and

macrophytes: photolabile substances

Modelling approaches for photolysis usually complex

● EXAMS, PestSurf

Simple approach preferable for regulatory assessment

Currently Frank & Klöppfer (ABIWAS) is used at EU level

Only direct photolysis, so should be conservative

Input: extinction and quantum yield as a function of wavelength

Output: half-life min-max-mean based on range in global radiation

ABIWAS still to be evaluated: yet unknown whether ABIWAS can

bridge the gap shown on previous slide

Outdoor systems with algae and

macrophytes: photolabile substances

ABIWAS (Frank & Klöppfer, 1989) - follows global radiation pattern (based on number of photons)

ABIWAS calculation with arbitrary substance: mean value

Outdoor systems with algae and

macrophytes: photolabile substances

Watanabe (2006): rate proportional to daily global radiation

(transferred to UV-B region: 280-320 nm, 0.1% of global radiation)

● tested for photostable substances only in paddy water

● exponential decrease of concentration in autoclaved paddy water

outdoors as a function of cumulative UV-B radiation

Conclusions

Need to evaluate predictive value of ABIWAS

● if conservative then first step in tiered approach followed by mesocosm measurements

Describe photolysis rate coefficient in TOXSWA as a function of daily global radiation

Use photolysis DegT50 from mesocosms or ABIWAS calculated back to some reference global radiation as input to TOXSWA

Conclusions

But before introducing this procedure, analyse data on duckweed surface coverage of Dutch ditches as a function of time

● 90th percentile case (shallow ditch) with or without duckweed ?

● with duckweed, then develop duckweed compartment in

TOXSWA and measure photolysis on duckweed

End