A review of long-term change in surface water natural organic matter concentration in the northern hemisphere and the implications for drinking water treatment

Reduced atmospheric acid deposition has given rise to recovery from acidification - defined as increasing pH, acid neu-tralization capacity (ANC), or alkalinity in surface waters. Strong evidence of recovery has been reported across North America and Europe, driving chemical responses. The primary chemical responses identified in this review were in-creasing concentration and changing character of natural organic matter (NOM) towards predominantly hydrophobic nature. The concentration of NOM also influenced trace metal cycling as many browning surface waters also reported increases in Fe and Al. Further, climate change and other factors (e.g., changing land use) act in concert with reduc-tions in atmospheric deposition to contribute to widespread browning and will have a more pronounced effect as de-position stabilizes. The observed water quality trends have presented challenges for drinking water treatment (e.g., increased chemical dosing, poor filter operations, formation of disinfection by-products) and many facilities may be under designed as a result. This comprehensive review has identified key research areas to be addressed, in-cluding 1) a need for comprehensive monitoring programs (e.g., larger timescales; consistency in measurements) to assess climate change impacts on recovery responses and NOM dynamics, and 2) a better understanding of drinking water treatment vulnerabilities and the transition towards robust treatment technologies and solutions that can adapt to climate change and other drivers of changing water quality.

Automated summary

Reduced atmospheric acid deposition has led to recovery in surface water quality, with increasing pH, acid neutralization capacity, and alkalinity. The recovery is accompanied by changes in natural organic matter concentration and composition, as well as trace metal cycling. These trends pose challenges for drinking water treatment, requiring comprehensive monitoring programs and a better understanding of treatment vulnerabilities and adaptive technologies.