Abstract The discovery of significantly elevated levels of arsenic (As) in the sediments of Spy Pond motivated us to identify the principal As source and to investigate the extent of contamination within the pond. Spy Pond is located in Arlington (MA, USA), a residential and commercial town with no known history of As use by industry or agriculture. Spy Pond is a kettle-hole pond composed of two basins (north and south) separated by a shallow sill. Sediment cores from the two basins were analyzed for As content by instrumental neutron activation analysis and dated by measuring 210Pb activity. The As concentration profiles for the north and south basins had maxima that dated to ∼1962 and ∼1956, respectively. These dates are consistent with records of arsenical herbicide use, which indicate that between 1960–1968 sodium arsenite (NaAsO 2) and arsenic oxide (As 2O 3) were applied to the pond to control aquatic macrophytes. Estimates of As loadings to the two basins—∼410 kg (∼32 kg ha −1) to the north and ∼5800 kg (∼580 kg ha −1) to the south—are consistent with the range of application rates reported for other lakes treated with arsenical herbicides. To determine the extent of As contamination in the pond, 68 surface sediment samples were analyzed by energy dispersive X-ray fluorescence. Arsenic levels ranged from 1 to 2600 ppm in the north basin ( n=49) and from 120 to 1100 ppm in the south basin ( n=19). Background sediment-As levels for the area are 10–40 ppm. The highest concentrations of As in the sediments of Spy Pond are comparable to levels measured in lakes contaminated with chemical manufacturing and mining wastes. These results are discussed with respect to As remobilization and sediment treatment measures planned for the pond.