Geospatial analysis of total mercury concentrations in stream and lake sediments across Canada
University of New Brunswick
This study focused on geospatially analyzing and mapping total mercury concentrations (THg) in stream and lake sediments across Canada, as compiled by the sediment surveys of the Geological Survey of Canada, Quebec, and Nova Scotia (total number of samples = 254,133). The objective was to quantify how sediment THg varies by atmospheric Hg deposition, climate, geology, topography, stream and lake morphology, vegetation/land cover type, and sediment composition pertaining to other elements and to organic matter (determined through loss on ignition: LOI). On average, upland sediments have slightly but still significantly higher THg values (streams: 97.8 ± 1.4 SE; lakes: 113.2 ± 1.1 SE, ppb) than lowland sediments (streams: 90.1 ± 2.2 SE; lakes: 90.4 ± 0.2 SE, ppb). Lake sediment THg increases with increasing lake depth and decreasing lake area (p–value < 0.0001). Stream sediment THg increases with increasing stream depth and decreasing flow rate, order, and width (p–value < 0.0001). Mean sediment THg decreases from forests to tundra, barrens, and ice– and snow–covered basins (p–value < 0.0001). In wetland-dominated basins, sediment THg decreases by approximately a factor of two as the wet–area portion per basin increases from 0 to 40 %. Swamp dominated basins have higher sediment THg than marsh and bog/fen dominated basins (p–value < 0.0001). Highest sediment THg occurs downstream from high Hg–containing geogenic and anthropogenic sources, with sediment THg related to other heavy metals such as copper and zinc (p–value < 0.0001). The examination of 10th, 25th, 50th, 75th and 90th percentiles of sediment THg displays parallel trends with increasing LOI, being lowest at LOI = 0 and highest at 30 to 50 %. This suggests that geogenic THg contributions to sediments decrease as the organic THg contributions increase. The latter relates positively to mean annual atmospheric Hg deposition and precipitation rates, and more so for lakes than for streams. The regression coefficient between lake sediment THg and mean atmospheric Hg deposition and precipitation rates amounts to 0.432 (p–value < 0.0001) per National Topographic System (NTS, 1:250,000) tile. The standardized fish Hg concentration of the Fish Mercury Datalayer for Canada (FIMDAC) relate positively to lake sediment THg, but negatively to mean annual July temperature. This association explains 38.2 % of the mean fish Hg concentration variations per NTS tile. Hence, the climate and atmospheric Hg variations across Canada not only contribute to the Hg concentration variations in sediments but also in fish.