Evaluating scope and trends for decreasing base cations (and increasing diluteness) in surface waters of the northeast
Rosfjord, Catherine Harney. 2005. An Evaluation of 20 Year Changes in Chemistry in the EPA Easter Lake Survey, A Statistical Population of Lakes in the Northeastern U.S. M.S. Thesis, University of Maine, EES2005-010.
We will conduct the 20th 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. 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 the acid neutralizing capacity (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 the results from the propsoed study, we will develop and empirical model for the rate of change in base cation (Bc ) concentrations as a function of ANC (or base cations) as of 1986. We hypothesize that waters with higher ANC and base cations are not experiencing a decline in base cations. The results of this study are important in expanding our understanding of recovery of surface waters from acidification.
This proposal is a companion proposal to one already reviewed and funded by USDA. This proposal is simply requesting 50% of the funding for a graduate student who will make this project her thesis research, which was not possible to request from USDA due to funding limits.
Critical regional water problem:
This proposed research addresses key questions that remain unanswered regarding the major freshwater resource issue of acid deposition and the ensuing changes in surface water chemistry. The 1990 Clean Air Act addressed the issue of acid deposition by mandating reductions in sulfate emissions. Over the past two decades, sulfate emissions have been reduced, and deposition of the anion in surface waters has decreased significantly. However, the expected rebound in pH following the decrease in acid deposition has not occurred. For many lakes in New England, a further increase in pH is necessary to support the full potential of aquatic biota.
The decline in base cation concentrations in surface waters of the northeast has been offered as an explanation for the lack of ‘recovery’ from acid rain. One of the universal responses of low acid neutralizing capacity (ANC) watersheds to acid deposition is the mobilization of base cations from soils (e.g. Galloway et al., 1983). As rates of acidic deposition decline, the rates of cation mobilization are also expected to decrease, thereby slowing recovery from acid deposition.
Declines I base cations have been inferred from long-term monitoring of low ANC lakes and streams over thepast two decades (Stoddard et al. 1999;2003). Our research complements and broadens this monitoring by assessing the possible changes in base cations in high ANC lakes as well as low ANC. The observation of a widespread decline in base cations in the high ANC lakes of our study lakes would indicate similar processes of mobilization in both high and low ANC watersheds. Conversely, if base cation concentrations have not decreased in high ANC waters, it would indicate that the less sensitive watersheds have been buffered from acid rain and recovery will ensue at a faster rate.
Statement of Results and Benefits:
This research is important to our interpretation of the responses of surface waters to past and future changes in atmospheric deposition. While the process of acidification has been observed and documented, it is now important to assess the reverse process of deacidification and recovery. This study will shed light on the recovery processes in different types of watersheds, both sensitive and well buffered. It also provides information to interpret changes in zooplankton community structure over the past 18 years, funded through a separate funding source, which will provide further evidence to the biological recovery of lakes. This information is the bottom-line for the Clean Air Act: has biological condition improved as a result of our investment in reduced sulfur emissions? This study will allow policy makers to create protocol necessary to ensure recovery in regions with varying degrees of acid neutralizing capacity.
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-exchange 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 climatic warming or fertilization from atmospheric deposition of CO2 and N, that would lead 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.