Scientific guidance on soil phototransformation products in groundwater – consideration, parameterisation and simulation in the exposure assessment of plant protection products

This Guidance Document gives recommendations how to consider transformation products from soil photolysis (‘phototransformation products’) when modelling the predicted environmental concentrations in groundwater. It describes possible parameterisations of the photolytic pathway (i.e. the photolytic half‐life and the corresponding reference irradiation) in a tiered approach using the FOCUS‐PELMO model. Following the recommendations of the EFSA guidance (2014), separate half‐lives can be derived for the surface processes (k(fast)) and the biodegradation in the soil matrix (k(slow)) from field dissipation studies by using biphasic models. From field dissipation studies evaluated with biphasic kinetics, the k(fast) value can be considered representative for the photolytic degradation on soil surface for non‐volatile substances. This value should be determined after a time‐step normalisation according to irradiance data for the location and period of the field trial, to a reference irradiance of 100 W m(−2). In case irradiance values are not available in the study, these values can be derived from databases such as AGRI4CAST or NASA, freely available in the Internet.