Oyster, farmed inland on the west coast of Taiwan, can provide a good indication of pollution levels in aqua environments, which may be affected by upland watershed, industrial and municipal waste water discharge, precipitation and local runoff. Using a continuous flow-hydride generation-atomic absorption technique, this study measured arsenic concentrations in Pacific oysters (Crassostrea gigas) collected from several aquaculture sites along the west coast of Taiwan over different seasons. The arsenic concentrations ranged between 8.83 and 19.51 mg/kg dry wt. (mean 13.7 ± 2.2 mg/kg). Excluding Hsianshan, a suspected source of toxin, it was possible to create an exponential decay model between the arsenic content in oyster and accumulated precipitation. We found the lifetime cancer risk associated with the inorganic arsenic in oysters to be 4.2 × 10-6, which is 4 times higher than what is considered acceptable by the U.S. Environmental Agency.