Moorland grouse - Flubendazole use for parasitic worm control: preliminary environmental assessment

Appendix C – Predicted Environmental Concentrations

1) PEC Soil

The PEC soil calculations are based on the VICH guidelines (EMEA, 2016). There are no exposure scenarios that exactly reflect excretion of flubendazole by grouse on moorland. The closest is excretion by livestock on pasture. The calculations associated with this scenario have been used to calculate a PEC_soil for flubendazole excretion by grouse.

The first calculation is to calculate PEC_soilinitial which is the initial predicted environmental concentration of flubendazole in soil and is derived using:

𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙 𝑖𝑛𝑖𝑡𝑖𝑎𝑙} = (𝐷 × 𝐴𝑑 × 𝐵𝑊 × 𝑆𝐷 × 𝐹ℎ ÷ 1500 × 10000 × 0.05) × 1000

Where, 𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙 𝑖𝑛𝑖𝑡𝑖𝑎𝑙} is the initial predicted environmental concentration of flubendazole in soil (µg/kg), D is the daily dose of the active ingredient (mg/kg_bw per day) which has been calculated as 0.3735 mg/kg_bw per day based on an assumption of 500g of grit dosed with 50 mg of flubendazole treating two birds per season at a mean grouse weight of 600g. 𝐴𝑑 is the number of days of treatment (d) assumed to be 100 days based on GWCT (2020), 𝐵𝑊 is the animal body weight (kg_bw per animal) assumed to be 0.6 (Wildlife Trust, 2020), 𝑆𝐷 is the stocking density (animal/ha) set as 1.43 grouse per hectare based on mean grouse density data presented in Werrity et al (2019), 𝐹ℎ is the fraction of the animals treated, it is set as 1 assuming all grouse receive an equal dose. The numbers 1500, 10000 and 0.05 relate to the bulk density of dry soil (kg/m³), area of 1 hectare (m²) and depth of soil penetrated (m) respectively. The number 1000 is a conversion factor so that the final units are in µg/kg.

The 𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙 𝑖𝑛𝑖𝑡𝑖𝑎𝑙} is therefore calculated as:

_{𝑃𝐸𝐶𝑠𝑜𝑖𝑙 𝑖𝑛𝑖𝑡𝑖𝑎𝑙} = (0.3735 × 100 × 0.6 × 1.43 × 1 ÷ 1500 × 10000 × 0.05) × 1000
_{𝑃𝐸𝐶𝑠𝑜𝑖𝑙 𝑖𝑛𝑖𝑡𝑖𝑎𝑙} = 0.0427 μg/kg

This calculation does not account for year on year accumulation of flubendazole in soil. This is a relevant consideration as flubendazole has a DT₅₀ in clay soil of 174 days (Kreuzig et al., 2007). The PEC_soil can be further refined to account for this using the following equation:

𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙 1 𝑦𝑒𝑎𝑟} = 𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙 𝑖𝑛𝑖𝑡𝑖𝑎𝑙} × 𝑒 ((− ln 2×365) ÷ 𝐷𝑇₅₀)

Where 𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙 1 𝑦𝑒𝑎𝑟} is the PEC in the soil 1 year after the start of treatment, and DT₅₀ is the soil half life (days). 𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙 1 𝑦𝑒𝑎𝑟} is therefore calculated as:

𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙 1 𝑦𝑒𝑎𝑟} = 0.0427 × 𝑒 ((− ln 2×365) ÷ 174)

𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙 1 𝑦𝑒𝑎𝑟} = 0.0411 μg/kg

The fraction of flubendazole degraded one year after application is calculated as:

𝐹𝑠 = (𝑃𝐸𝐶𝑠𝑜𝑖𝑙 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 − 𝑃𝐸𝐶𝑠𝑜𝑖𝑙 1 𝑦𝑒𝑎𝑟 ) ÷ 𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙 𝑖𝑛𝑖𝑡𝑖𝑎𝑙}
𝐹𝑠 = 0.0374

The 𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙 𝑝𝑙𝑎𝑡𝑒𝑎𝑢} is the PEC in soil at plateau (μg/kg) and is calculated as:

𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙 𝑝𝑙𝑎𝑡𝑒𝑎𝑢} = 𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙 𝑖𝑛𝑖𝑡𝑖𝑎𝑙} ÷ 𝐹𝑠
𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙 𝑝𝑙𝑎𝑡𝑒𝑎𝑢} = 1.14 μg/kg

The plateau 𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙}(ie approximates to steady state) estimated in the assessment is therefore 1.14 μg/kg.

2) PEC surface water

In the VICH guidelines PEC aquatic is calculated by determining the concentration of the substance in soil pore water and assuming a dilution by the receiving water body. This approach does not account for losses of the substance attached to particulates in runoff. To do this a separate assessment is required.

To calculate PEC aquatic the initial PEC soil is converted to a wet weight PEC and to a soil depth of 20cm. For this the 𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙 𝑝𝑙𝑎𝑡𝑒𝑎𝑢} will be used:

𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙𝑝𝑙𝑎𝑡𝑒𝑎𝑢_𝑝𝑤_𝑑𝑤} = 𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙 𝑝𝑙𝑎𝑡𝑒𝑎𝑢} ÷ 4
𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙𝑝𝑙𝑎𝑡𝑒𝑎𝑢_𝑝𝑤_𝑤𝑤} = 𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙𝑝𝑙𝑒𝑎𝑡𝑒𝑎𝑢_𝑝𝑤_𝑑𝑤} ÷ 𝐶𝑂𝑁𝑉_{𝑠𝑜𝑖𝑙}

Where, 𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙𝑝𝑙𝑎𝑡𝑒𝑎𝑢_𝑝𝑤_𝑤𝑤} is the PEC soil corrected to wet weight and soil depth of 20 cm (µg/kg) and 𝐶𝑂𝑁𝑉_{𝑠𝑜𝑖𝑙} is the dry to weight wet conversion factors set as 1.13 kg_dw/kg_ww. 𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙𝑝𝑙𝑎𝑡𝑒𝑎𝑢_𝑝𝑤_𝑤𝑤} is therefore 0.252 µg/kg.

PEC_{porewater is} calculated using the following equation:

𝑃𝐸𝐶_{𝑝𝑜𝑟𝑒𝑤𝑎𝑡𝑒𝑟} = 𝑃𝐸𝐶_{𝑠𝑜𝑖𝑙𝑝𝑙𝑎𝑡𝑒𝑎𝑢_𝑝𝑤_𝑤𝑤} × 𝑅𝐻𝑂_{𝑠𝑜𝑖𝑙} ÷ 𝐾_{𝑠𝑜𝑖𝑙−𝑤𝑎𝑡𝑒𝑟} × 1000

Where, 𝑅𝐻𝑂_{𝑠𝑜𝑖𝑙} is the bulk density of fresh soil set as 1700 kg/m³ and 𝐾_{𝑠𝑜𝑖𝑙−𝑤𝑎𝑡𝑒𝑟} is the partition coefficient between solids and water in soil which was calculated as 300.2 m³/m³ in appendix B.

𝑃𝐸𝐶_{𝑝𝑜𝑟𝑒𝑤𝑎𝑡𝑒𝑟} = 0.252 × 1700 ÷ 300.2 × 1000

𝑃𝐸𝐶_{𝑝𝑜𝑟𝑒𝑤𝑎𝑡𝑒𝑟} = 0.00142 μ𝑔/𝑙

To derive PEC surface water, the PEC porewater is simply divided by 3 to account for dilution in the receiving water body. PEC surface water is therefore 0.000475 µg/l.

However, this does not account for flubendazole sorbed to solid particles in runoff.

Release to the water environment is also possible in wet conditions direct from medicated grit that has spilt from grit trays or been left in open piles when they have been positioned near to watercourses. No modelling scenario was found for this exposure route and owing to the high number of site-specific variables, attempts to estimate release rates and a PEC_water for this scenario have not been successful. To give some context to the potential release based on each grit tray containing 500g of grit dosed with 50mg flubendazole at the start of the gritting season, and assuming around 10% of the grit is spilt and around 28% of the dose leaches over the course of a year’s weathering, over that period 1.4mg of flubendazole could be released per tray. For open grit piles quantities leached would be higher. As a worst case, if a grit pile remained unused for some reason, up to 14mg flubendazole could leach from it over the gritting season. As one grit tray (or pile) is supplied for every 1.4 ha (based on mean grouse density data and 1 pair of birds using one tray/pile as presented in Werrity et al 2019) and moor sizes can vary between 200 and 10,000 ha, this scenario could be important and requires further investigation either through modelling or monitoring.

3) PEC sediment

PEC sediment is calculated using the VICH guidelines. However, like the PEC surface water calculation it only considers losses in solution that then partition into sediment once in the receiving water and not losses of flubendazole attached to particulate runoff that then settle as fresh sediment. It is derived using the following equation:

𝑃𝐸𝐶𝑠𝑒𝑑𝑖𝑚𝑒𝑛𝑡 = 𝐾_{𝑠𝑒𝑑−𝑤𝑎𝑡𝑒𝑟} ÷ 𝑅𝐻𝑂_𝑠𝑒𝑑 × 𝑃𝐸𝐶_{𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑤𝑎𝑡𝑒𝑟} × 1000 × 𝐶𝑂𝑁𝑉_𝑠𝑒𝑑

Where, 𝐾_{𝑠𝑒𝑑−𝑤𝑎𝑡𝑒𝑟} is the sediment-water partition coefficient which was calculated as 250.8 m³/m³ in appendix B, 𝑅𝐻𝑂_𝑠𝑒𝑑 is the bulk density of sediment set at 1300 kg_wwt/m³ and 𝐶𝑂𝑁𝑉_𝑠𝑒𝑑 is the conversion factor for sediment from wet weight ti dry weight, set at 2.6.

𝑃𝐸𝐶_{𝑠𝑒𝑑𝑖𝑚𝑒𝑛𝑡} = 250.8 ÷ 1300 × 0.000475 × 1000 × 2.6
𝑃𝐸𝐶_{𝑠𝑒𝑑𝑖𝑚𝑒𝑛𝑡} = 0.238 μ𝑔/𝑘𝑔