Summary of methods.

California’s mining legacy continues to threaten water quality and ecosystem health throughout the state. This study focuses on mercury (Hg) releases from the former New Idria Mercury Mine, which was the second largest historic Hg producer in North America. San Carlos Creek, which flows adjacent to the mine, is impacted by acid mine drainage (AMD) as well as mining waste piles that sit at a high angle of repose over the channel. We documented total mercury (HgT) concentrations exceeding California’s 50 ng L-1 water quality objective for almost 10 km, from New Idria to a ~ 0.25 km2 perennial wetland at the confluence of San Carlos Creek and Silver Creek within the Panoche Creek watershed. During baseflow and low flow storm conditions, unfiltered total Hg (U-HgT) in creek water downstream from the mine typically ranged from 1,100–9,200 ng Hg L-1, with >90% of Hg in the particulate phase. U-HgT correlated (R2 = 0.6) with suspended particulate matter (SPM) that was presumably a mixture of AMD-derived flocculant, weathered calcines (i.e., roasted ore), and sediment from the local watershed. The flocculant is easily resuspended in baseflow conditions and scoured from the channel during high flow events, resulting in seasonal patterns of Hg transport in San Carlos Creek that do not always align with the stream hydrograph. Mercury from New Idria, along with AMD flocculant, metals, sulfate, and other anions, presumably accumulate or infiltrate at the Silver Creek wetland until larger storms remobilize sediments further downstream towards the ~ 0.65 km2 Panoche Creek Wetland, the Panoche Fan, and ultimately to the Fresno Slough, Mendota Wildlife Area, Mendota Pool and San Joaquin River.