DIEL FLUCTUATION OF HEAVY METALS AND RARE EARTH ELEMENTS IN A MOUNTAIN STREAM WITH ACIDIC TO NEUTRAL PH

Three simultaneous 24-hour samplings were conducted to examine diel fluctuations in metal concentrations in Fisher Creek, a small mountain stream in Montana receiving acid mine drainage. Average pH’s at the upstream (F1), middle (F2), and downstream (F3) monitoring sites were 3.31, 5.46, and 6.80, respectively. The increase in pH was due to mixing with tributary streams and influent groundwater, and resulted in precipitation of hydrous ferric and aluminum oxides on the streambed. At the acidic upstream stations (F1 and F2), iron was the only solute to show significant diel variations, with daytime photo-reduction of Fe(III) to Fe(II) being the main process. A 7-fold increase in Fe²⁺ and total dissolved Fe was noted at F2 during the day. At the downstream, pH-neutral station (F3), dissolved Fe(II) no longer displayed a photo-reduction trend, but rather increased at night, along with Cu, Mn, Zn, Cd, and the rare earth elements (REE). Copper, the main element of concern in Fisher Creek, showed a 2.5-fold increase in concentration between sunset and sunrise. Dissolved REE increased 4-fold in the same time period. Shale-normalized REE profiles for F3 water samples have a weak slope, angled down across the periodic table (i.e., depleted in heavy REE), and show very little change in shape with time of day.

Whereas the 24-hour range in stream temperature was large (11°C), the diel range in pH at F3 was only 0.07 units. We propose that the observed diel trends in cationic metals at F3 are driven by temperature-dependent sorption onto hydrous ferric oxide (HFO) on the streambed, or suspended in the water column. Previous experimental studies show that adsorption of metal cations onto HFO is an endothermic process, and therefore is favored by increase in temperature. Sorption of Cu and Zn onto HFO at F3 was simulated using PHREEQCI 2.8. Satisfactory agreement was obtained between predicted vs. observed diel trends, but only if adsorption enthalpies are added to the thermodynamic database, to accommodate temperature changes. Whereas previous studies have stressed the importance of pH in diel metal cycling in streams, Fisher Creek may be the first clear example of a stream where diel metal fluctuations are driven mainly by temperature-dependent sorption reactions.