A critical examination of chemical extremes in freshwater systems
Serediak, Nancy Anne
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The objectives of this thesis are to explore and identify: 1) the causative factors for extreme endpoints in freshwater chemistry, specifically eutrophication and acidification, and 2) the convergence of anthropogenic pollution, watershed composition and climate effects that could contribute to the occurrence of freshwater chemical extremes. Eutrophication is reviewed as a well-studied water quality issue that remains relevant as a management challenge. Extra focus is given to acidification, quantified as a decrease in acid neutralizing capacity (ANC), because it has a strong influence on physical properties such as nutrient (i.e., phosphorus, nitrogen) resuspension that can potentially leading to chemical extremes. Data from lakes in the western Great Lakes region are examined with respect to effects of acid inputs on in-lake ANC and pH response. Although drainage systems are discussed, special attention is paid to softwater seepage lakes as being the most sensitive with regards to acidification risk. The challenges of using data-intensive mass balance models in lakes with intermittent sampling histories lead to development of a simpler model for estimating open-water ANC in data-sparse locations. Acid input sources are compared as combinations of area-weighted charge balances using publicly available data from long term monitoring programs. Weighted data combinations are then analyzed using maximum likelihood methods suitable for use with observational data. The final model correctly predicted low ANC events (ANC < 25 ìeq L-1 ) 20 out of 24 times (R2 = 0.50 adjusted for small sample size; 168 observations), but underestimates the severity of the lowest extremes. Three factors stand out as being strongly related to acidification risk during the open water season: 1) volume of snowmelt, 2) in-lake ANC following spring turnover, and 3) pulsed runoff from associated wetland soils following drought and re-wetting events. Recommendations for future research focus on quantifying acid and nutrient content in pulsed runoff events and their impacts on freshwater systems given antecedent conditions in both lakes and connected wetlands.