Simulated long term accumulation of plant protection products in the soil of FOCUSSW run-off scenarios

July 19, 2019

David Patterson, Principal Environmental Fate Consultant, TSG Consulting, recently presented the results of a study simulating the long-term accumulation of plant protection products in the soil of FOCUS surface water run-off scenarios at the 29th annual SETAC Europe Meeting (26-30 May 2019) in Helsinki, Finland. Download the paper below.

ABSTRACT: For EU regulatory purposes, predicted environmental concentrations (PEC) of plant protection products (PPP) in surface water (PECSW) and sediment (PECSED) as a result of run-off is currently modelled using four scenarios according to the FOCUS surface water report. Some run-off scenarios have longer model warm-up periods than others (ranging from 0 – 14 years) whilst all scenarios employ a 1-year assessment period.

However, a recent draft scientific report of the European Food Safety Authority proposes that simulations should in future include a 6-year warm-up period for all scenarios followed by a 20-year assessment period. In order to assess the suitability of a 6-year warm-up period and the impact of the proposed changes on PPP accumulation in the soil of run-off scenarios, a range of hypothetical compounds were considered (n = 25, DT 50 100 – 1000 days, K OC 100 – 10000 ml/g). These were applied at emergence to crop types with significantly different run-off potentials (winter cereals, maize, leafy vegetables) across all run-off scenarios. The required warm-up period to achieve a steady state concentration (aqueous + absorbed PPP) in the upper 2 cm of soil during a 26-year simulation was calculated. In all cases the time required to achieve steady state was < 6 years (range 0 – 6 years).

The duration of the warm-up period in FOCUS PRZM is currently determined by the year selected for the assessment period from a 20-year chronology. This varies due to application season and scenario from 0 years (R3 summer) to 14 years (R4 winter). Therefore, in existing modelling some scenarios are at steady state whilst others are not. As a result, 26-year modelling will affect different scenarios and application timings to a greater or lesser extent. This heterogeneity was quantified and for some compound concentrations of PPPs in the soil available for run-off can be expected to increase by a factor of up to 2.7.

This study demonstrates that the proposed 6-year warm-up period is sufficient to achieve steady state in all scenarios for the compounds and crop types tested. Furthermore, the effect of longer duration simulations on PPP accumulation in soil available for run-off will vary across scenario and application season as a result of the different warm-up periods currently used in FOCUS surface water modelling.