Potential photochemical controls on trace metals and rare earth elements in an acid mine drainage impacted wetland

Abstract

Recent climate trends in the Colorado Mineral Belt have intensified acid mine drainage (AMD) impacts, increasing the importance to understand trace metal and rare earth element (REE) cycling in affected watersheds. This diel study investigated biogeochemical and photochemical controls on metal and REE mobility in an AMD-impacted wetland below a large, abandoned mine. Daily photochemical cycling of H2O2 and iron species drove complex metal mobility patterns for both trace metals and REEs, with Cu, Cd, and Pb increasing during peak daylight hours (30%, 9%, and 113% respectively), while Zn, Mn, and Al decreased by 9%, 14% and 19%, respectively. REE concentrations frequently exceeded 100 µg/L for Ce, Nd, and Y, with both light REEs (LREEs) and heavy REEs (HREEs) exhibiting photochemically-driven diel fluctuations. Ce, Nd, Gd, Pr, and La concentrations increased by 3–10% during daylight hours, while Y and Dy decreased slightly (2–4%), and Sm decreased by 20%. Cerium anomaly calculations revealed distinct spatial patterns across the wetland-groundwater-creek continuum, with values ranging from 0.73 to 0.90, indicating ongoing oxidative processing of REEs throughout the system driven by retention time. These findings demonstrate that AMD-impacted wetlands are not simple flow-through systems, but rather complex environments where photochemical processes influence the cycling of both trace metals and REEs, with important implications for water quality management.