The waters and bottom sediments in the watershed of Union Lake in southern New Jersey are highly contaminated with substantial quantities of arsenic. Total arsenic contents were as high as 2780. μg/l in the water phase, whereas 2290. μg/l was the highest observed in bottom sediments. Pore water from this bottom sediment was 12,500. μg/l. Arsenic contents in the bottom sediments were 4240 times those found in overlying waters. In sediments, the order of predominance of the four arsenic species was: As+5> As+3> MMAA > DMAA. In four of the sediments, the inorganic arsenate was 72.8, 81., 84., and 88% of the total arsenical species. In water, the order of predominance was MMAA > As+3, As+5> DMAA. Difficulties with the analytical procedure for separation of the four arsenical species are discussed.