From 1949-1999, Giant Mine in Yellowknife, Northwest Territories, Canada, produced approximately 257,000 tonnes of arsenic trioxide (As2O3) waste through roasting of arsenopyrite ores. Uncontrolled emissions in the first 10 years of operation led to atmospheric release and regional distribution of 20,000 tonnes as aerosolized dust. An environmental footprint persists in this region 60 years later, and concentrations of arsenic (As) in soils, lake waters, and sediments exceed national guidelines and pose ongoing environmental challenges, especially with shifting climate conditions affecting As mobility. This project investigates the status of As contamination in the heavily mine-impacted Pocket Lake catchment to discover the mechanisms and processes that control As mobility amongst different terrain types. Surface runoff, soil porewaters, and lake profile samples were collected throughout the spring season. V-notch weirs were used to determine surface flow for each terrain type in the catchment. A full catchment soil survey was conducted to estimate soil As pool. Preliminary data suggests surface runoff, primarily from snowmelt, continues to be a significant source of As to lakes. Bedrock landscapes contribute less As to lakes compared to peatlands and soil-filled valleys, likely due to their reduced soil reservoirs. Biomethylation of As, either in snowmelt or shallow layer soils, was observed in peat-dominated landscapes, resulting in production of less toxic organic As species. Furthermore, observations to date suggest that As in snowmelt runoff is associated with other elements (iron, aluminum, and manganese) and dissolved organic matter, implicating these associations as critical mechanisms in mobility of As in this region.