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Relationship between leaching in lysimeter studies and ... - pfmodels

Relationship between leaching in

lysimeter studies and in FOCUS

Hamburg groundwater scenario

Jos Boesten


Outline

• Introduction

• Effect of lysimeter boundary condition

• Effect of experimental design of lysimeter study

• Effect of uncertainty in soil/pesticide properties of lysimeter

soil

• Conclusions


Introduction

• FOCUS Groundwater Workgroup:

• Tiered approaches for EU and MS registration

• Tier 1: standard scenario

• Tier 2: lysimeter study (German guideline)

• Discussion on relationship between tier 1 and 2

• My role: scientist presenting his personal view

• appreciating your feedback as input to further discussion in FOCUS

GW

(next meeting 20 December)


Introduction

• Regulatory assumption: protection goal for

groundwater is 90 th percentile in space and

time of annual average concentration in

groundwater for agricultural use area of

pesticide

• Not lysimeter is protection goal but field


Introduction

Comparison of endpoints:

• Tier 1: FOCUS leaching concentration (80 th percentile of 20 periods)

• Tier 2: lysimeter: maximum annual average leaching concentration

• Consensus to correct linearly for pesticide dose (assuming same application period)

• FOCUS leaching concentration closer to protection goal than lysimeter leaching

concentration

• Most simple approach: assume that both endpoints can be directly compared

• Implicit assumptions of direct comparison:

lysimeter boundary condition has no significant influence = H1

• experimental design of lysimeter has no significant influence = H2

• uncertainty in system parameters has no significant influence = H3


Introduction

German lysimeter guideline:

• Use soil with low organic matter

• Ensure annual average rainfall of 800 mm (via irrigation)

• Vulnerability concept based on chromatographic flow

(otherwise emphasis on events and pore structure)

• My procedure consistent with guideline assumptions:

• simulations with chromatographic model that handles both

lysimeter and field boundary conditions

• PEARL (MACRO and HYDRUS would be possible as well)


Effect of lysimeter boundary condition

• Effect on water flow

• Effect on pesticide leaching


Effect of lysimeter boundary condition

PEARL simulations:

• FOCUS Hamburg scenario

• Lysimeter system: 1-m

deep lysimeter

• Field system: 4.5-m deep

soil profile with

groundwater at average

depth of 1.8 m (range 0.7-

3 m)

• Same soil, crop and

weather

lysimeter

field


Effect of lysimeter boundary condition

Scenario characteristics:

• FOCUS runs for applications every year or other year

• Summer wheat (often used in lysimeters)

• Application of 1 kg/ha one day before emergence

• Endpoints: FOCUS leaching concentration for both

systems

• A few model compounds (using PEARL default

parameters for long-term sorption kinetics)


Effect of lysimeter boundary condition on water flow

Field system

Water flow rate at 1 m

depth (mm/d)

Groundwater level

(m below soil surface)


Effect of lysimeter boundary condition on water flow

Water flow rate at 1 m depth

Field system

Lysimeter system

Water flow kinetics differ


Effect of lysimeter boundary condition on water flow

Volume fraction of water at 30 cm depth

Field system

Lysimeter system

Differences in top soil limited


Effect of lysimeter boundary condition on water flow

Volume fraction of water at 1 m depth

Field system

Lysimeter system

At 1 m depth the lysimeter is wetter


Effect of lysimeter boundary condition on leaching

1. field BC gives higher leaching concentration than lysimeter

BC

2. indication: effect larger for mobile, rapidly degrading

pesticides


Effect of lysimeter boundary condition on leaching

P1

P2

P3

Results in terms of numbers


Effect of lysimeter boundary condition on leaching

Possible explanation for lower lysimeter concentrations:

- Lysimeter boundary leads to higher volume fraction of

water in bottom part of lysimeter

- So longer residence time in lysimeter so more time for

pesticide degradation

Consequence: boundary condition has effect so lysimeter

endpoint cannot be used directly in the risk assessment


Effect of experimental design of lysimeter

• Typical design of lysimeter experiment:

• 2 years with 800 mm annual average rainfall/irrigation

• Clean soil profile at start

• Endpoint: maximum annual average leaching

concentration

• Design of FOCUS scenarios:

• Run 20-40-60 y after 6 y warm-up


Effect of experimental design of lysimeter

Procedure: simulation of lysimeter experiment

• 1-m deep lysimeter with Hamburg soil and weather

• Period: 31 March 1913 – 31 March 1915

• Summer wheat emerging 1 April

• Pesticide dose of 1 kg/ha at 31 March 1913

• Annual rainfall was 800 and 826 mm in 2 years after application

• Analysis restricted to 2 years and one application

• Comparison with FOCUS Hamburg scenario assuming

lysimeter boundary condition

• not with field boundary condition to keep comparison as simple as

possible

• application every year (20 y) or every two years (40 y)


Effect of experimental design of lysimeter

P-1

P-2

P-3

lysimeter leaching concentration may be much

lower than FOCUS leaching concentration


Effect of experimental design of lysimeter

FOCUS/lysimeter

P-1 : zero sorption so leaches in first winter

P-3 : moderate sorption: 2 y lysimeter is too short for maximum


Effect of experimental design of lysimeter

P-3: simulated leaching from lysimeter as a function of

time

Two years with

800 mm rainfall

not enough for

moderately

sorbing pesticide

lysimeter period

(K OM = 90 L/kg)


Effect of experimental design of lysimeter

Effect of experimental design depends strongly on

pesticide properties:

for mobile pesticides lysimeter higher concentrations

for sorbing pesticides FOCUS scenario higher

concentratoins

- difference in endpoints may be a factor 100

- also experimental design has effect, so lysimeter

endpoint cannot be directly used in risk assessment


Effect of uncertainty in soil/pesticide

properties of lysimeter soil

Introduction

Assumptions in tier 1:

• Hamburg soil has dispersion length of 5 cm for PEARL

• Average DT50 or K OM

If lysimeter would have been conducted with soil with

• shorter dispersion length

• shorter DT50 or higher K OM

then lysimeter endpoint lower by coincidence: undesirable for higher tier.


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

Example of DT50/K OM

• Mean DT50 of 30 d based on four values: 22, 27, 35, 36

• Mean K OM of 35 L/kg based on four values: 28, 34, 38, 40

• Lysimeter endpoint (PEARL)

• mean values: 0.38 μg/L

• most favorable combination (DT50=22 d, K OM =40 L/kg): 0.03 μg/L

• Uncertainty in DT50 and K OM can lead to wrong risk

assessment conclusion


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

• What is probability of obtaining lysimeter endpoint

concentration of below 0.1 μg/L by coincidence ?

• Important parameters that may be uncertain:

• DT50 and K OM

• Freundlich exponent (curvature of sorption isotherm)

• Dispersion length

• Exploratory calculations on this probability via Monte

Carlo


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

• Stochastic variables in Monte Carlo simulations:

• Normally distributed DT50 and K OM with CV of 25%

• Uniformly distributed Freundlich exponent ranging from 0.8 to 1.0

• Lognormally distributed dispersion length


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

• Justification of probability density functions used in

Monte Carlo simulations


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

- studies with 18 UK soils (>15% clay)

Allen & Walker (1987) + Walker & Thompson (1977)

assumption: variation coefficient of 25% for DT50 and K OM


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

- studies with 18 UK soils (> 15% clay) Allen & Walker (1987)

assumption: Freundlich exponent uniformly distributed

between 0.8 and 1.0


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

dispersion length: lognormal distribution with median of 5 cm and

st.dev. of 0.87 ln(cm) ;

based on pooled distribution of column and field by Jan

Vanderborght [median 6.2 cm and st.dev. of 0.87 ln(cm) ]


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

• Background of output quantity


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

Example distribution

with low % below 0.1

μg/L

Most relevant output for risk assessment:

probability that lysimeter gives less than 0.1 μg/L by

coincidence, so % of runs below 0.1 μg/L.


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

Example distribution

with high % below

0.1 μg/L


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

• Procedural aspects


Effect of uncertainty in soil/pesticide properties of lysimeter soil

Procedure: runs

along two lines in

the DT50-K OM

plane

constant K OM of 35 L/kg and

variable DT50


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

constant DT50 of 20 d and variable K OM


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

DT50 = 75 d

K OM = 35 L/kg

500 Monte

Carlo runs is

enough

different seeds

in all runs


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

• Results


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

- % plotted against concentration for median parameter values

-0.1 μg/L no unique function of concentration level for median values

- smaller effect of uncertainty for low K OM values


Effect of uncertainty in soil/pesticide properties of lysimeter soil

- at 2 to 5 μg/L still 10% of runs below 0.1 μg/L so very wide

distributions of calculated concentrations

- effect of uncertainty in these properties may be very large


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

0.5% below 0.1 μg/L 9% below 0.1 g/L

Examples of simulated probability density functions


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

DT50 = 35 d

K OM

= 35 L/kg

• Large effect of dispersion length on leaching:

• 0.7 μg/L for 5 cm

• 0.2 μg/L for 2 cm


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

How large is effect of

dispersion length

and how large effect of

sorption/degr. parameters ?


Effect of uncertainty in soil/pesticide properties of lysimeter

soil

disp.var.

allvar.

sorp.deg.var.

Preliminary

conclusions:

sorption and

degradation larger

effect than dispersion

effect more than

additive; cause

unknown


Conclusions

• Use of lysimeter endpoint directly for risk assessment

unacceptable because significant effects of

• Lysimeter boundary condition

• Lysimeter experimental procedure

• Uncertainty in soil or pesticide properties

• Recommendation: interpret results of lysimeter studies with

methods that account for such effects

• e.g. Verschoor et al. (2001) presented at 1 st EU Modelling Workshop


End

© Wageningen UR

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