March 18, 2022 by Dean Leverett

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A few weeks ago Adam and Graham had a manuscript on the assessment of secondary poisoning by the pharmaceutical diclofenac accepted for publication by the Journal Environmental Sciences Europe. The paper considers the derivation of the EQS under the Water Framework Directive by the European Commission and specifically the potential for accumulation of diclofenac in aquatic organisms and uptake via the food chain that could potentially cause toxicity in higher predators.

Diclofenac is a nonsteroidal anti-inflammatory human and veterinary medicine widely detected in European surface waters, especially downstream from Wastewater Treatment Plants, so wastewater is the key Europe-wide exposure route for wildlife that may be exposed via the aquatic environment. Diclofenac is an ionisable compound and does not follow the usual bioaccumulation pathways that would be expected for general industrial chemicals. Rather than being accumulated in lipids, as non-polar organic chemicals are, it is bound to proteins in the blood and its uptake and removal occurs relatively readily in the wide variety of organisms that have been tested for its bioaccumulation potential. A consequence of this unusual behaviour compared to industrial chemicals is that it does not appear to exhibit any potential for biomagnification in higher predators.

In the paper we summarised information on avian toxicity plus laboratory and field evidence on diclofenac bioaccumulation and bioconcentration in avian food items. A safe diclofenac threshold value for birds of 3 μg kg^-1 wet weight in their food was previously derived by the European Medicines Agency and we recommend that this should be adopted as an EQS under the WFD to maintain consistency across European regulations. This value is also consistent with values of 1.16 – 3.99 µg kg^-1 in food proposed by the EC under the WFD.

A water-based EQS of 5.4 or 230 ng L^-1 in freshwater are derived from these dietary standards, respectively, by the EC and by ourselves, with the large difference caused primarily by use of different values for bioaccumulation. A simple assessment of potential water-based EQS compliance was performed for both of these possible EQS values against reported concentrations of diclofenac in samples collected from European freshwaters.

Overall, this shows that exceedances of the EC-derived EQS would be very widespread throughout Europe while exceedances of the EQS derived by us are confined to a relatively limited proportion of sites in some Member States. Since there is no evidence for any declines in European waterbird populations associated with diclofenac exposure we recommend use of conservative EQS of 3 µg kg^-1 in the diet, or 230 ng L^-1 in water, to protect birds from diclofenac secondary poisoning through the food chain. Monitoring of diclofenac concentrations in prey organisms would provide the most reliable means of assessing any potential risks.