The recovery of river chemistry from acid rain in the Mississippi River basin amid intensifying anthropogenic activities and climate change

Acid rain has degraded the environmental health of many regions worldwide since the Industrial Revolution. Signatures of river chemistry recovery from acid rain since the Clean Air Act and similar legislation have been reported extensively in small streams but are often subdued or masked in large rivers by complex, co-occurring drivers. Here we assess the recovery of river chemistry from acid rain deposition in the Mississippi River Basin (MRB), the largest river basin in North America. We combine analysis of temporal trends of acid rain indicator solutes with Bayesian statistical models to assess the large-scale recovery from acid rain and characterize effects of anthropogenic activities. We found evidence of river chemistry recovery from acid rain; however, the effects of other anthropogenic activities, including fertilizer application and road salting, and changing climate, are likely intensifying. Trends of pH, alkalinity and SO₄ export suggest acid rain recovery at large in the MRB, with stronger evidence of recovery in the historically afflicted eastern region of the basin. The concentrations of acid rain indicators generally correlate positively to NO₃ and Cl, indicating that N-fertilizer application may have significantly increased weathering, and possibly acidification, and road salt application likely increased cation loss from catchments and contributed to SO₄ export. Temperature correlates positively with solute concentrations, possibly through respiration-driven weathering or evaporation. The concentrations of acid rain indicators correlate negatively and most strongly to discharge, indicating discharge as a predominant driver and that lower discharge during droughts can elevate concentrations of riverine solutes in a changing climate. Using long-term data, this study represents a rare, comprehensive assessment of the recovery from acid rain in a large river basin, taking into consideration the entangled effects of multiple human activities and climate change. Our results highlight the ever-present need for adaptive environmental management in a constantly changing world.