RECENT TRENDS IN THE ACID-BASE STATUS OF SURFACE WATERS IN MAINE, USA

Data from the EPA Long Term Monitoring Program lakes at the Tunk Mountain Watershed, Maine, indicate that decreases of less-than-or-equal-to 1 mueq L-1 vr-1 in SO4, and increases of less-than-or-equal-to 2 mueq L-1 yr-1 in ANC occurred in the 1980s. The sum of base cations also increased. These changes in aquatic chemistry were coincident with decreased concentrations of all solutes in precipitation during the 1980s. Other data on lakes and streams in Maine collected between the 1930s and 1990 generally confirm these trends and further indicate that larger increases in ANC may have occurred in some lowland lakes since 1940. Paleolimnologic studies indicate that decreases of 0.1 to 0.5 pH units occurred in a few small mountain lakes during the past 20 to 70 yr. However, ongoing acidification of lakes is indicated based on available data. Only lakes that were already at least marginally acidic (pH less-than-or-equal-to 5.8, ANC approximately 0) appear to have acidified.