Routine monitoring in early 1975 revealed higher than expected concentrations of arsenic in Sinclair Creek of Kootenay National Park, British Columbia. Follow-up surveys found that the Radium Hot Springs contained about 0.5 mg/1 arsenic.Upon the request of Parks Canada, the Water Quality Branch undertook a ten month study to investigate the prevalence and abundance of arsenic in the waters of Sinclair Creek Basin. Arsenic was determined to be ubiquitous in the waters of the basin with maximum levels (up to 0.8 mg/1) recorded in diffused groundwater sources and in the hot springs serving the Radium Hot Springs Aquacourt (up to 0.3 mg/1). John McKay Creek which is the drinking water supply for the townsite of Radium and the Aquacourt, contained levels of arsenic well below the acceptable limit of 0.01 mg/1 of the Canadian Drinking water Standards. The ‘drinking water for Canyon Campground, on the other hand, was found to contain arsenic at 1eve1s in excess of 0.025 mg/1. The high levels of arsenic in the hot and cold pools at the Aquacourt are not considered to present a health concern.