“In England, pesticides are the biggest reason surface drinking water protected areas are rated “at risk” of not meeting their objectives (for 122 out of total 486 drinking water areas); metaldehyde, found in molluscicide used in farming, is the primary cause of exceedances (EA, 2015a). Metaldehyde is a highly soluble, organic compound commonly used in pellets against slugs and snails.17 Although believed to be harmless to humans at the levels currently detected, it is very difficult to remove at water treatment works and meeting the EU targets is not possible using conventional water treatment technology (Castle et al., 2017). The maximum concentration stipulated by the European Union’s Drinking Water Directive (DWD) for metaldehyde is 0.1 microgram per litre (or parts per billion) in treated water. This is the equivalent of one drop in an Olympic-sized swimming pool.
https://www.nature.org/content/dam/tnc/nature/en/documents/Investing_in_Nature_for_European_Water_Security.pdf