Acceleration of Weathering of Trace Metals and Rare Earth Elements in Alpine Watersheds in the Colorado Mineral Belt

Many mountain watersheds in the Rocky Mountains in Colorado, USA, are impacted by natural acid rock drainage (ARD) and acid mine drainage (AMD), which mobilize trace metals and rare earth elements (REEs) into surface waters. Analysis of long-term water quality records for alpine tributaries receiving ARD in the Colorado Mineral Belt indicates that warmer summer conditions have been driving increases in the concentrations of sulfate, trace metals and rare earth elements (REEs). One example of these climate-change impacts is the Lincoln Creek Watershed, where a highly mineralized tributary contributes a substantial ARD loading into the headwaters, with an additional AMD contribution from a nearby abandoned mine, the Ruby Mine. We evaluated the transport, mixing, and attenuation of major solutes, trace metals and REEs across the snowmelt to fall period. Water samples from six main sites along a 7-km reach of Lincoln Creek below the ARD and AMD inflows were analyzed by Inductively-Coupled Plasma Mass Spectrometry (ICP-MS) and Ion Chromatography (IC) methods. The results were explored through transport calculations employing sulfate as a conservative natural tracer. Water chemistry in Lincoln Creek reveals distinct geochemical fingerprints for the Ruby Mine (enriched in Ca, Mg, Mn, and Cd) and the Mineralized Tributary (enriched in SO4, Fe, Al, and Cu). REE fractionation patterns and Ce anomalies further distinguish source contributions and processes, with the Mineralized Tributary displaying MREE enrichment from natural pyrite weathering and the Ruby Mine exhibiting HREE enrichment tied to mine derived flows. Most solutes exhibited conservative transport during mid-summer when the pH values were low, in the range of pH 4-4.5. During the higher flows associated with snowmelt instream losses of some trace metals and REEs was observed, which was also the case in the fall. These results indicate that both source composition, instream pH-dependent reactivity and hydrologic processes interact to control the downstream water quality impacts associated with these high mountain sources of ARD and AMD.