In some headwater streams in the Colorado Mineral Belt of the Rocky Mountains, the warmer and dryer summer conditions associated with climate change have accelerated the weathering of pyrite and other sulfidic minerals, causing dramatic (4-5 fold) increases acidity and the concentration of not only trace metals, but also of rare earth elements (REEs). Wetlands are also common features in the valley bottoms in this region and often have accumulated iron oxides in the sediments. These wetlands provide a natural laboratory for investigating the role of iron oxide and dissolved organic matter (DOM) photochemistry in mobilizing trace metals and rare earth elements (REEs). This study investigated how dynamic photochemical processes involving iron oxides and DOM control the diel cycling of trace metals and REEs in a wetland system downstream of the Pennsylvania Mine in the same valley in Summit County, Colorado, where summertime increases in trace metals and REEs have been documented. Three synoptic studies served to characterize the wetland and showed that concentrations of dissolved organic carbon (DOC), H₂O₂and REE were much higher than in the underlying shallow groundwater, and were comparable to those in the nearby stream, Peru Creek. In the diel study three sites were sampled, including the wetland surface water, the wetland groundwater, and Peru Creek downstream of the wetland. The changes in DOM concentration, H₂O₂, iron speciation (Fe²⁺/Fe³⁺), UVA/B, pH, anions, trace metal and REE concentrations were followed on an hourly basis. These results showed that REE concentrations increased in the morning in association with the rise in H₂O₂, suggesting that photochemical processes may result in the desorption and release of REEs from iron oxide surfaces in the wetland. Thus, photochemical interactions may account for the mobilization of REEs in the stream networks in the Colorado Mineral Belt.

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