Although tidal current plays an important role in bringing pollutant to the beach from the ambient environment (e.g. effluents from submarine outfalls of Pillar Point and San Wai sewage treatment works), beach water quality at Butterfly is more correlated with salinity and rainfall. There are two freshwater sources located near the beach, discharging considerable pollutants to the beach. Significant deterioration in water quality at Butterfly beach is usually observed after rainfall.

Monthly distribution of beach gradings at Butterfly beach (2010-2014)