Groundwater in some parts of Calcutta has revealed traces of arsenic, according to a recently published research paper. Scientists from the School of Environmental Studies of Jadavpur University, Calcutta, have traced the source of arsenic to a pesticide factory that discharges paris green (copper acetoarsenite), a chemical used in pesticides, in the open land outside the factory. This has caused the chemicals to percolate and mix with the groundwater reserve. In the southern parts of Calcutta, the level of in the groundwater is about 7.5 milligram per litre (mg/l). The standard set by World Health Organisation and Indian Council of Medical Research is one mg/l. About 150,000 people living in the area have been affected. Skin lesions have become a common problem. During the monsoon, the seepage of paris green increases. The study points out that water from shallow tubewells with a depth of 40 metres, and deep tubewells of the Calcutta Municipal Corporation have also been contaminated with arsenic ( Current Science , Vol 6, No 4).