Recovery From Acid Rain Deposition In The Mississippi River Basin

Acid rain deposition has been a widespread environmental concern since the mid-twentieth century and has negatively impacted ecosystems across the nation. Although efforts to reduce acid rain deposition itself have been successful, little comprehensive research has investigated recovery in surface waters. This study used United States Geological Survey (USGS) data and modelling program EGRET to investigate the in-stream recovery from acid rain deposition from 1974 to 2017. The Mississippi River, in this case, acts as a proxy for a large portion of the United States as it composes approximately 40% of the United States land area. Several chemical species and parameters were used in this study to complete this analysis: pH, sulfate (SO42-), divalent cations (Ca2+ and Mg2+), and nitrate as nitrogen (NO3-N). The findings of this study suggest recovery throughout the basin on average, as pH increases and sulfate (SO42-) concentrations and loads decrease over time. Divalent cation concentration and load increase over time, however, conflicting with recovery. The trend in divalent cation export is suggested to be primarily caused by chemical weathering via nitrogen fertilizers. Previous studies, largely conducted in smaller forested catchments, have found that decreased acid rain deposition rates result in decreased cation export. Comparing the relationship between trends in smaller catchments and large river basins provides a basis for assessing the roles of variables such as land use and catchment size. The findings of this study illustrate both the effectiveness of legislation as well as the environmental effects of agriculture on surface waters.