At least nine of the 16 districts of the state of West Bengal, India, have arsenic concentrations in groundwater > 0.05 mg/l, the current USEPA maximum contaminant level (MCL) for drinking water. Ingestion of contaminated groundwater has affected about 10 million inhabitants of the area and caused several hundred deaths. Contamination has been attributed to natural causes, but human activities may play a role. The extent of arsenic contamination in groundwater was assessed in Purbasthali-I, a severely affected block of the Burdwan district containing 13 mouzas (villages). The aquifer was found to be contaminated from a depth of 15.24 m to 82.31 m. The pH of groundwater ranged from 7 to 7.5. Arsenic concentrations greater than the MCL were found in 11 of the 13 villages and all 13 had concentrations > 0.01 mg/l, the standard for arsenic toxicity set by the WHO. The most affected village, Mandra, had an average arsenic concentration of 0.3 mg/l. Arsenic concentrations in groundwater do not appear to be correlated with either specific conductance or total iron concentrations. Hydrogeological mapping and comparison with previous records indicate that the spatial extent of the contaminated water is rapidly increasing both vertically and horizontally. About 20,000 inhabitants of the block suffer from prolonged weakness, conjunctival congestion, hyperpigmentation, keratosis, respiratory system disorder, hepatomegaly, portal hypertension, polyneuropathy, malignant neoplasm and edema of limbs. Suggested remedies include restricting groundwater use, abandoning affected aquifers, and microbially induced in-situ bioremediation.