Five decades after the Clean Air Act, legacy metal contaminants in Northeast US surface waters document full recovery for the first time

Despite significant reductions in atmospheric pollutants following the Clean Air Act, few studies have documented the preceding recovery of inland waters in the northeastern United States. This study provides an updated assessment and the first documented cases of full recovery in a set of ponds sitting within the Adirondack Preserve, a region historically burdened by acid rain and atmospheric pollutants, and at the center of the Clean Air Act movement. Using sediment records from four ponds, we reconstruct a timeline of historical metal deposition, investigate the history and extent of disturbances on each watershed and lake, and demonstrate how these disturbances shaped the magnitude of deposition and their pathways to recovery. Records displayed the onset of metal contamination between 1900 and 1940 which peaked from 1960 to 1990, aligning with previous records in the region. Sites with histories of significant watershed disturbances displayed synergistic and substantial metal enrichment (e.g., Pb > 290 ppm), which are rarely observed. Although recovery has been slow, particularly for Pb, metal deposition in the ponds has declined significantly. Sites that have ceased major watershed activities have achieved a > 90 % return to baseline metal deposition in the last 5 years. This study is the first historical assessment from the Adirondack region to hypothesize the complex, synergistic factors leading to the variability in metal contamination concentrations seen throughout the region's waters. This study demonstrates the resilience of lake ecosystems, which we define as the ability to recover and regulate geochemical balance through natural watershed and internal processes, independent of direct human intervention. It also underlines challenges posed by legacy contaminants and highlights the nuances of setting realistic goals and timelines for legislative measures and recovery efforts.