Evaluating scope and trends for the base cation decline in surface waters of the northeastern US

We propose to conduct the 20^th anniversary re-sampling of a subset of the EPA Eastern Lake Survey (ELS) lakes, originally done in 1984.  The lakes are the ELS-II statistical subset of 145 lakes, sampled in 1986.  The rationale for this research is to evaluate the chemical responses and mechanisms that underlie the regional decline in surface water concen­trations of base cations that has been widely reported from the entire northern hemi­sphere.  Two of our objectives will augment the 2003 assessment of aquatic trends in surface waters relating to the Clean Air Act (Stoddard et al., 2003) by 1) enhancing the statistical coverage of the region using the ELS-II sub-population; and 2) expanding the range of ANC in the target waters.  The ANC of waters in the 2003 Stoddard et al report was generally less than 100 µeq/L; the ANC in ELS lakes ranged up to 400 µeq/L.  Using these results, we will develop an empirical model for the rate of change in base cation concentrations as a function of ANC (or base cations).  We hypothesize that waters with higher ANC and base cations are not experiencing a decline in base cations.

These results are important to our interpretation of the responses of surface waters to past and future changes in atmospheric deposition.   In addition, the responses in surface waters are key indicators for our interpretation of processes in forest soils.  A widespread decline in base cations may indicate widespread cation depletion in forest soils, perhaps as a result of leaching by acidic deposition.  This conclusion would have negative implications for forest health.  Alternatively, the modest decline in precipitation acidity may be allowing soils to retain cations as ion-exchanges sites are replenished. This conclusion would represent a recovery mode, which would be positive for forest health. There are other possibilities, such as increased forest growth due to a fertilization by increased atmospheric CO₂, leading to increased uptake of cations by forests (and thus declines in surface waters). These data will provide, via a statistical sub-population of lakes, significant new understanding of the scope, magnitude, and trends in base cations in surface waters of the northeast.

Related Web site:

EPA’s Eastern Lakes Survey Data Sets