Trace element diel variations and particulate pulses in perimeter freshwater wetlands of Great Salt Lake, Utah

Trace elements (including total and methyl Hg, Sb, Se, U, V, and Mn), field parameters (including dissolved oxygen, pH, and water temperature), and other constituents were monitored over a 24-hr period during August 20–21, 2008 and September 14–15, 2009 at the outlet of two freshwater wetland ponds located near the southeastern shoreline of Great Salt Lake, Utah, to determine whether pe- and/or pH-driven diel cycles in trace element concentrations would be observed. Unfiltered and filtered (< 0.45 μm) samples were collected hourly for all trace elements. Al, Fe, Hg, and Pb were predominantly (> 75% of mass) associated with the > 0.45 μm, or particulate, fraction. Cu, Cr, Cd, Mn, and Ti were also associated with the particulate fraction to a lesser extent (22–58% of mass). As, Co, Li, Ni, Sb, Se, Sr, U, and V were predominantly in the “dissolved” (< 0.45 μm) form. Over a 24-hr period, the particle-associated elements displayed up to a factor of 5 differences between minimum and maximum concentrations, most likely due to settling and resuspension of particles in the water column. Of the dissolved trace elements, Sb, Se, U, and V concentrations showed a diel variation that was positively correlated with variations in dissolved oxygen, pH and water temperature: increasing following daybreak and decreasing at night. Concentrations of the dissolved fraction of Mn displayed the opposite trend relative to Sb, Se, U, and V. Diel trends in dissolved trace elements are explained by increased sorption of Sb, Se, U, and V anions during nighttime hours, and increased sorption of the Mn cations during daytime hours, driven by changes in water pH and temperature. Diel variations in the anion-forming elements Sb, Se, U, and V, although small in magnitude, support established trends of the anion-forming As. Methylmercury, which has been shown to vary on a diel cycle in other nearby locations, was consistently low at both ponds and displayed no diel trend.

Research Highlights► We examined diel (24-hr) cycling of trace elements in a freshwater wetland. ► Sb, Se, U, and V cycles were positively correlated with pH cycles. ► Filtered Mn concentrations displayed an opposite cycle relative to Sb, Se, U, and V. ► Opposing cycles in anionic and cationic elements were explained by pH-driven sorption.