This can be registered as a measure of the ecological health of the water, determined by the level of indicator species in relation to a predetermined critical threshold, derived from toxological data; the biochemical oxygen demand can also be used. Land cover change and the expansion of modern agricultural practices has significantly increased leaching of chemicals to the surface waters, leading to the degradation of aquatic ecosystems worldwide (Donner (2003) Glob. Ecol. & Biol. 12, 4). See Beasley and Kneale (2002) PPG 26, 2 on pollution from urban surface run-off.