Widespread chemical dilution of streams continues as long-term effects of acidic deposition slowly reverse

Studies of recovery from acidic deposition have focused on reversal of acidification and its associated effects, but as recovery proceeds slowly, chemical dilution of surface waters is emerging as a key factor in the recovery process that has significant chemical and biological implications. This investigation uses long-term chemical records from 130 streams in the Adirondack region of New York, USA, to evaluate the role of ongoing decreases in conductance, an index of dilution, in the recovery of these streams. Stream chemistry data spanning up to 40 years (1980s–2022) showed that acid-neutralizing capacity has increased in 92% of randomly selected streams, but that harmful levels of acidification still occur in 37% of these streams. Conductance and Ca2+ concentrations decreased in 79% of streams, and SO42− concentrations in streams continued to show strong decreases but remained several times higher than concentrations in precipitation. These changes were ongoing through 2022 even though acidic deposition levels were approaching those estimated for pre-industrialization. Further dilution is continuing through ongoing decreases in stream SO42−. Nevertheless, Ca2+ continued to be leached from soils by SO42−, organic acids and NO3−, limiting the replenishment of available soil Ca2+, a prerequisite to stem further dilution of stream water. As recovery from acidic deposition proceeds slowly, chemical dilution of surface waters emerges as a key factor in the recovery process.Chemical records from 130 streams in the Adirondack region of New York show widespread decreases in conductance that are ongoing even as acidic deposition levels approach those estimated for the early 1900s.Further dilution is likely as release of SO42− from soils at a gradually decreasing rate continues into the future. [Display omitted] •Chemical recovery from acidic deposition in Adirondack streams is widespread but incomplete.•Recovery remains slow despite atmospheric deposition nearing pre-industrial levels.•Dilution of stream chemistry is emerging as a key factor in recovery.•Accumulated effects of acidic deposition on soils are now driving dilution.•Dilution is slowing recovery by increasing dissolved organic carbon mobilization.