Half-Century Observations Reveal Slow and Pulsed Recovery from Heavy Mercury Pollution in a Major Temperate River.

Rivers deliver substantial mercury to coastal oceans, significantly impacting seafood safety. However, the time frame for mercury levels and fluxes in large rivers to return to a stable, low-pollution state following long-term industrial pollution remains unclear. The Amur River, the world's fifth longest and one of the largest temperate rivers, originates in Mongolia and flows along the China-Russia border. Heavy industrial wastewater discharges in the 1960s-1970s severely contaminated the river, while strict controls implemented in the late 1970s created a rare opportunity to evaluate how quickly a large river recovers from severe anthropogenic mercury pollution. Here, we present an unprecedented 43-year time series of monthly observations of particulate mercury export to determine the river's recovery time frame. We find that mercury concentrations at the river mouth tripled due to wastewater discharges. After a 90% cut in wastewater discharges, mercury levels remained at peak levels for four years, returning to near-background within approximately 15 years, representing the initial flushing of mobile mercury from the drainage network ("Baseline Recovery"). However, after the initial flushing, legacy mercury from historic wastewater discharges was remobilized by hydrologic events, triggering episodic pulses ("Disturbance Recovery") that peaked ∼20 years after controls and reached up to three times the levels of the 1960s-1970s industrial peak. This remobilization was driven by agricultural expansion that enhanced soil disturbance and erosion, compounded by intensified droughts followed by heavy rainfall. This study provides evidence of slow and pulsed recovery of mercury fluxes in heavily polluted large rivers, highlighting that historical Hg pollution in some major rivers has likely been underestimated and reinforcing the need for sustainable, long-term management.