Trace element loads in the Great Lakes Basin: A reconnaissance

Water quality data for trace elements in the Great Lakes are relatively scarce, complicating the assessment of current trace element baselines and their distribution patterns. Here, we present concentration data for >40 major and trace elements in >100 samples from the Great Lakes connecting channels, surface waters, precipitation and select Canadian tributaries, to establish a high-level assessment of loading rates across the basin. Contrasting upstream-to-downstream trends were observed for the investigated trace elements, ranging from net-decreasing (>5-fold for e.g., Co, Tl, Y) to net-increasing surface water concentrations (>2-fold for e.g., Sb, U, As). Calculated loading rates reveal different, element-specific controls of runoff, connecting channel loads or precipitation on trace element occurrence. Lake-wide elemental mass-balances could be reasonably closed for conservative trace elements (e.g., Li, <53% residual) but not for others (e.g., rare earth elements with up to 5-fold discrepancies), reflective of general data scarcity and uncertainty in loading rates. In line with major water quality trends, spatial distribution patterns in Lakes Erie and Ontario display subtle near-shore to off-shore heterogeneity for a few trace elements (<1 order-of-magnitude for V or Se), but higher variability for trace elements with significant inputs derived from tributaries. This work provides important quantitative baseline data for trace elements in the Great Lakes that may help optimize surveillance and management strategies for the preservation of Great Lakes water quality.