Edge-of-field surface waters and their sediment dwelling organisms

Environmental fate and behavior studies in water-sediment systems and the chronic toxicity tests on aquatic organisms represent the starting point to carry out an environmental risk assessment (ERA) concerning the sediment compartment. As EFSA proposed in its scientific opinion, if in the standard sediment-water degradation studies, the substance concentration in the sediment after 14 days from its application is higher than 10% of the radio-labelled testing material and the ecotoxicological endpoints (from chronic tests on relevant aquatic species) are lower than 0.1 mg/l then, a specific ERA on sediment has to be performed. The key element to carry out this evaluation is the comparison of the predicted environmental concentration in sediment (PECsed), typically derived by FOCUS calculations, and the regulatory acceptable concentration (RAC) linked to the relevant endpoints.

The exposure evaluation has to take into account the different components which contribute to the bioavailability of the plant protection products to benthic organisms. Pesticide molecules can be adsorbed on the sediment surface or be trapped within sediment macropores, or even, stand as free fraction in the pore water. The latest one represents the main exposure route  for benthic algae, rooted macrophytes and microorganisms, while the fraction of pesticide absorbed or trapped in the sediments, is the relevant exposure to deposit feeders.

As screening step, the effect assessment assumes a simple relationship between chronic toxicity data on surface water pelagic organisms and the sediment endpoints to be predicted using the adsorption constant along with an extrapolation factor. The following Tier I evaluation consists in carrying out tests on standard

benthic organisms and additional ones, more specific, for the substance mode of action. When the threshold value is exceeded by PECsed to RAC ratio, an higher Tier evaluation is required where further chronic toxicity data on sediment are used to reduce the extrapolation factor or in a species sensitivity distribution (SSD) approach.

Settembre 2015