Discerning the role of atmospheric deposition in long-term solute trends in large rivers of the United States

Abstract:

Trends in alkalinity and other solutes are evident in large rivers of the United States (U.S.) due to a variety of factors. Nutrient inputs, agricultural lime, decreasing acidic inputs, and changing flow conditions all contribute to trends in major solutes. In small, minimally disturbed headwater watersheds with limited acid neutralizing capacity, solute trends resulting from changes in acidic deposition are well documented. Larger rivers tend to have greater acid neutralizing capacity and are subject to a greater number of anthropogenic pressures. Therefore, the effects of acid deposition on larger rivers is expected to be minimal. In this study, trends in alkalinity and other solutes were determined in large rivers spanning a range of climate and land use characteristics in the U.S. Multi-decadal trend analyses were performed (1945-2010) using a robust flow-weighting scheme. Alkalinity increased in most rivers although the factors leading to the increases were highly diverse. In several rivers in the Northeastern U.S., the timing and magnitude of alkalinity trends indicated that changes in atmospheric deposition played a role in solute trends. Despite the recent increases in alkalinity, modern alkalinity concentrations in U.S. rivers are remarkably similar to those observed in the early 20^th century (1906-1910). On the other hand, sulfate and calcium concentrations remain elevated with respect to data from the early 20^th century suggesting that acidification processes are ongoing, although improvements occurred in many areas in recent decades.