An Evaluation of 20 Year Changes in Chemistry in the EPA Easter Lake Survey, A Statistical Population of Lakes in the Northeastern U.S.

In 1984, the U.S. Environmental Protection Agency conducted the Eastern Lake Survey (ELS-I) to assess the overall status of the eastern U.S. lake population, with particular attention to the relationship between acidic deposition and ANC. In 1986, a subset of the ELS-I lakes consisting of 145 lakes in the northeastern U.S. was re-sampled during ELS- 11. In 2004, we re-sampled the ELS-I1 lakes approximately on the 20th anniversary of their original sampling. The data produced by the two samplings allow for assessments of biologically relevant chemical changes that may reflect reductions in acidic deposition due to regulations in both the U.S. and Canada. Over 90% of the ELS-I1 lakes decreased in SO4 from an average of 1 10 peq/L in 1984 to 86 peq/L in 2004. Of particular interest were changes in base cation concentrations [Ca+Mg] that supply ANC to surface waters. Change in [Ca+Mg] corresponded to lake C1 concentration, a reflection of ion exchange due to anthropogenic sources of salt, such as road salt. Approximately half the ELS-I1 lakes were unaffected by anthropogenic salt. ELS-I1 lakes affected by anthropogenic salt were defined by lake [Cl] greater than 4 times [Cl] of precipitation. Over 70% of the lakes not affected by salt experienced a decrease in [Ca+Mg] with an average change of -6.9 yeq/L over 20 years. Eighty-five % of salt-affected lakes increased in [Ca+Mg], with an average increase of +62 yeq1L. ANC increased from an average of 86 peq/L in 1984 to 102 peq/L in 2004. Nearly 80% of the lakes in the low ANC lakes (ANC \u3c 25 yeqlL) experienced increases in EqpH from an average of 5.9 in 1984 to 6.2 in 2004. Approximately 90% of the population had decreases in total A1 with AlUC class I experiencing the greatest average change of -74 pg/L over 20 years. However, samples of total A1 were not filtered in 1984 while they were filtered in 2004 which may have resulted in an overestimation of decrease in total Al. DOC increased significantly with an average increase from 4.1mglL in 1984 to 5.6 mg/L in 2004. Reductions in S emissions have correlated with increased ANC and reduced A1 concentrations in the most acid sensitive surface waters. Reduced ANC and A1 are important indicators of improvement in biologically-relevant chemistry in northeastern U.S. lakes. However, decreases in base cation concentrations, particularly in low ANC systems unaffected by anthropogenic salt, slowed recovery from acidic deposition in some regions. Anthropogenic sources of salt throughout much of the northeastern U.S. complicated assessments of changes in lake chemistry, particularly by increasing [Ca+Mg]. While many lakes in the Adirondack region and in Maine were unaffected by anthropogenic sources of salt, there were no lakes in the statistical population from southern New England that were unaffected by anthropogenic salt. These data suggest that current models for selecting statistical lake populations are not appropriate and underscore the importance of site selection for drawing conclusions regarding the effects of deposition changes on lake chemistry