RELATIVE TRANSPORT AND PERSISTENCE OF ACESULFAME-K AND PHARMACEUTICAL COMPOUNDS IN GROUNDWATER AT A SEPTIC SYSTEM SITE, LONG POINT, ONTARIO

Water samples collected from the septic tank and a network of piezometers installed within and downgradient of a 15-year old high-capacity septic system were analyzed to evaluate the relative transport of different emerging organic contaminants. Concentrations of acesulfame-K (an artificial sweetener), five pharmaceutical compounds (carbamazepine, caffeine, sulfamethoxazole, ibuprofen and gemfibrozil), and major wastewater constituents were determined at each sampling location. In the septic tank, the mean concentrations were 46 µg L^-1 for acesulfame-K and < 10 ng L^-1 to 16 µg L^-1 for the pharmaceutical compounds. In the groundwater zone below the tile bed, the acesulfame-K concentration typically was 30-45 µg L^-1, and the pharmaceutical compounds ranged from <10 ng L^-1 to 1 µg L^-1. Concentrations of acesulfame-K and pharmaceutical compounds remained elevated in zones of the plume characterized by reducing conditions, low dissolved oxygen concentrations (< 1 mg L^-1), and elevated NH₄⁺ and Mn²⁺ concentrations. At 200 m downgradient of the tile bed, the maximum concentration of acesulfame-K was 19 µg L^‑1, and the pharmaceutical compounds were below the analytical detection limits (maximum 10 ng L^-1). Acesulfame-K migrated farther from the tile bed than the pharmaceutical compounds investigated and had the same distribution as Na⁺ which is present in wastewater, suggesting conservative transport. Sodium can be derived from a variety of sources, including wastewater plumes, road salt and natural sources. Monitoring the concentrations of acesulfame-K, together with concentrations of pharmaceutical compounds, in the septic tank and downstream wastewater plumes may provide a unique tracer for constraining mass balance calculations.