Assessing the ecological risks and benefits of national air quality programs and standards is not straightforward given the inherent complexity of natural systems.  It is often this ecological complexity that is difficult to set apart from the actual welfare effects.  For this reason, it is not easy to determine whether programs such as the Acid Rain Program (ARP) under Title 4 of the 1990 Amendments to the Clean Air Act (CAA) or the Secondary National Ambient Air Quality Standards (NAAQS) provide protection from negative public welfare effects.  This case study examines first the welfare benefits of the emission reduction associated with the ARP and its uncertainty, and second the current risk and uncertainty of the NAAQS for nitrogen and sulfur oxides (NOx and SOx) to aquatic systems from acidification.  The case study areas include the Adirondack Mountains of New York and the Appalachian Mountains of Virginia.  Acid deposition has affected hundreds of lakes and thousands of miles of headwater systems in these regions. The diversity of life in these acidic waters has been greatly reduced as a result of poor buffering capacity of the thin, acidic soils making these surface waters particularly susceptible to acidification.

Results/Conclusions

The critical load approach was used as a means to gauge the extent to which a water body has recovered from past acid deposition, the potential risk due to current deposition levels, and the degree to which uncertainty effects that determination. The critical load approach provides a quantitative estimate of the exposure to one or more pollutants below which significant harmful effects on specific sensitive elements of the environment do not occur according to present knowledge. The Steady-State Water Chemistry (SSWC) and Model of Acidification of Groundwater In Catchment (MAGIC) model were used to calculate the critical loads at three aquatic risk levels linked to surface water Acid Neutralizing Capacity (ANC) concentrations (ANC = 0, 20, 50 μeq/L) known to have negative ecological effects.  For the period from 2005 to 2007, at the three risk levels 36, 20, and 10% of modeled lakes within the Adirondack Mountains continued to receive levels of acid deposition that exceeded the lake’s critical load.  This compares to 85, 72, 52% for the Appalachian Mountains of Virginia.  Despite recent declines in acidic deposition, additional reductions are needed for recovery of these sensitive aquatic systems.