Comparison of northeastern and southeastern U.S. watershed response to the declines in atmospheric sulfur deposition

Since the implementation of the 1970 Clean Air Act and its 1990 amendments, sulfur dioxide (SO₂) emissions and subsequent sulfur (S) deposition have been declining across the eastern U.S. Although temporal patterns of deposition have been similar in the northeast (NE) and southeast (SE) regions of the U.S., stream and watershed responses have differed. This is primarily due to the spatial extent of the last glaciation, which terminated in northern Pennsylvania, rendering the soils in the SE more clay-rich and deeper than soils in the NE. Through the use of improved estimates of atmospheric deposition and more highly constrained estimates of stream export, we calculated S mass balances from New Hampshire to Georgia for 8 glaciated NE watersheds and 8 unglaciated SE watersheds that have high frequency (i.e. weekly) chemistry data available. Overall, total atmospheric S deposition declined by 70–90% from the early 1990s to the late 2010s across all sites. The resulting S budgets show that the timing of the conversion of the watersheds from net S retention to net S release differ between regions. The SE watersheds converted recently (late 2000s–2010s), while the NE watersheds have been net exporters of S since the 1990s to early 2000s. This delayed response in the SE is a consequence of the much higher sulfate (SO₄²⁻) adsorption capacity of the soils in this region. The flow normalized concentrations of SO₄²⁻, which eliminates variability due to discharge, shows that the majority of NE sites converged to similar concentrations by the mid to late 2010s, whereas the SE sites exhibit much greater spatial variability in concentrations. An evaluation of the storage and release of S within watersheds, using changes in adsorbed S and dissolved S (i.e. flow normalized concentration), suggests irreversibility of the S adsorption processes in NE and SE watersheds. The conversion of watersheds to a net release of S indicates improvements in watershed recovery due to the declines in acid deposition across the Eastern U.S.