Standard water quality of Mole and its tributaries has improved markedly since the 1990s. In 1990 the Environment Agency assessed 23% of the watercourses as Grade B (good) or better. In 2002 this figure was 60%. Investment in the Sewage Treatment Works in the catchment area has improved the quality of the discharges into the river, and modifications to the runway and apron drainage systems at Gatwick Airport mean that surface water is diverted to aerated pollution control lagoons and balancing ponds for treatments, including acid/alkali neutralisation, before release into the river.