This summary information about hazard comes from fass.se. The risk is from the report by Goodpoint.
Persistence. Etonogestrel is slowly degraded in the environment.
Bioaccumulation. Etonogestrel has low potential for bioaccumulation.
Toxicity. Etonogestrel has very high chronic toxicity.
Risk. There is a risk associated with the use of etonogestrel/desogestrel and that these substances appear additive with other androgenic gestagens in the environment.
Fass environmental information for Nexplanon (etonogestrel) (downloaded 2021-09-21).
Persistence: "Sediment Transformation (OECD 308) (Ref. XII): DT50 in water: 10–11 days; DT50 in sediment: 35–70 days; DT50 in total system: 9.2–50 days. [...] Etonogestrel has a DT50 for the total system of ≤120 days. Thus, the phrase “Etonogestrel is slowly degraded in the environment” is chosen."
Bioaccumulation: "Log Kow = 3.5. Bioaccumulation (OECD 305): Flow through study with Bluegill sunfish (Lepomis macrochirus) Lipid normalized kinetic BCF = 128."
Chronic toxicity: There is NOEC for 3 trophic levels, lowest NOEC 183 days for fish (Oryzias latipes) 2.7 nanog/L.
PEC/PNEC is based on sales data in Sweden in year 2018. PEC/PNEC = 2.8 which gives the risk moderate.
Etonogestrel has been tested but has been below the quantification limit (LOQ). Etonogestrel is the active metabolite of desogestrel.
Etonogestrel has approximately the same ability to bind to the androgen receptor in fish such as levonorgestrel. A substance's ability to activate the androgen receptor in fish is believed to contribute to increased risk of environmental impact. Etonogestrel also binds to SHBG (Sex Hormone Binding Globulin). Binding to SHBG is believed to contribute to increased bioconcentration (and thus potency) in fish. Based on this, it can be assumed that there is a risk associated with the use of etonogestrel/desogestrel and that these substances appear additive with other androgenic gestagens in the environment.
Author: Health and Medical Care Administration, Region Stockholm