Landscape factors that affect acid-base chemistry of Adirondack Long-Term Monitoring (ALTM) lakes

The Adirondack region has been affected by acid deposition for multiple decades. Atmospheric deposition of sulfur and nitrogen has decreased since the 1970s because of the 1970 and 1990 Clean Air Act Amendments (CAAA) as well as the NOX Budget Program. Previous studies have related the long-term changes in Adirondack lake water chemistry to decreases in atmospheric deposition and emission controls. However, relatively limited research has been conducted on the spatial patterns and the role of different landscape factors on the recovery of lake water chemistry in the Adirondacks. The objective of this study was to investigate the relations of volume-weighted concentrations and long-term changes of surface water chemistry of the Adirondack region with landscape characteristics of watersheds to improve the understanding of the factors that drive chemical recovery of surface water from declines in acid deposition. I delineated watersheds of Adirondack Long-Term Monitoring (ALTM) lakes using Geographic Information Systems, and quantified areal fraction of relevant landscape factors within each watershed. Using the GIS and statistical approaches, I compared volume-weighted concentrations and long-term changes of major solutes that affect acid-base chemistry of ALTM lakes (SO42-, NO3-, Ca2+, Al and DOC) with atmospheric deposition, topography, soil characteristics, surficial processes, wetland and canopy coverage, lake watershed vegetation, and forest disturbance. I observed that changes in NO3- concentrations in precipitation after 2004 had a very similar pattern as SO42- at all three National Atmospheric Deposition Program (NADP) sites in the Adirondacks (NY20, NY29, and NY98), and that more than 80 percent of the variation in annual volume-weighted H+ concentrations in atmospheric deposition can be explained by the variation in the sum of SO42- and NO3- concentrations. I also interpolated from 8 NADP sites using the Kriging method to achieve continuous maps of precipitation concentrations of major ions in the Adirondacks, and validated the results against the measurements from an additional NADP site (NY29) at Moss Lake. Results of this study indicated that spatial variation in volume-weighted lake SO42- concentrations and lake SO42- decreasing rates were influenced by wet atmospheric deposition of SO42- but were also affected a combination of landscape factors including elevation, lake hydraulic residence time, wetland and canopy coverage, and thickness of glacial till deposits. Higher volume weighted concentrations of SO42- were found in drainage lakes than seepage lakes and lakes of higher elevation, shorter hydraulic residence time, lesser wetland coverage, and thicker deposits of glacial till. I found a similar relationship between lake SO42- and Cl- which suggests the pattern of higher lake SO42- in drainage lakes may be linked to evapotranspiration. Greater wetland coverage within ALTM lake watersheds resulted in more rapid decreases in SO42- probably due to the bacterial assimilatory SO42- reduction. Watersheds with greater canopy coverage may be linked to interception of greater dry S deposition. More rapid decreases in SO42- concentrations were also observed in lakes of higher elevation. Volume weighted NO3- concentrations and NO3- decreasing rates in ALTM lakes, in contrast to SO42-, showed no relationship with atmospheric deposition. Regression analyses indicated that higher volume weighted NO3- concentrations were found in drainage lakes at higher elevation, with steeper land slope, or lower chlorophyll a concentrations. The study also demonstrated that forest disturbance and land slope were the factors most responsible for the spatial variation in rates of NO3- decreases. Drainage lake watersheds with shallower land slope or greater area affected by the 1995 and 1950 forest disturbance showed more rapid decreases in NO3- concentrations likely due to vegetation uptake of NO3- for regrowth following forest disturbance. The spatial variation in volume weighted Ca2+ concentrations and rates of Ca2+ decreases in ALTM lakes were attributed to the processes of weathering and leaching from soil exchange complex. I used lake Na+ and dissolved SiO2 as indicators of weathering input. Regression analyses demonstrated that ALTM lake watersheds with greater input from weathering and higher concentrations of soil exchangeable Ca2+ had higher volume weighted Ca2+ concentrations. I also observed more rapid decreases of Ca2+ in lake watersheds of more rapid changes in weathering and leaching processes. Results of this study suggested that spatial variation in volume weighted acid neutralizing capacity (ANC) of ALTM seepage lakes were primarily driven by wet deposition of SO42- + NO3- and in-lake processes. Volume weighted ANC of ALTM drainage lakes were influenced by atmospheric deposition of SO42- + NO3-, but were also affected by a combination of landscape factors including wetland coverage, land slope, cation weathering, and cation exchange. No significant relationship was found for ANC increases in ALTM seepage lakes with atmospheric deposition or landscape factors. The regression analysis for ANC increases at ALTM drainage lakes indicated that, in addition to decreases in the sum of SO42- and NO3- from atmospheric deposition, elevation, wetland coverage, and forest disturbance history play important roles. Marked decreases in lake concentrations of Al were evident, particularly among thin till drainage lakes. Lakes which are characterized by the highest concentrations of inorganic monomeric Al showed the greatest rates of decrease of inorganic monomeric Al. Higher volume-weighted DOC was found in lakes with lower concentrations of SO42- and NO3-, higher O horizon C:N ratio, B horizon base saturation, and greater color. More rapid increases in DOC concentrations were observed in lakes with more rapid decreases in SO42- and NO3-, more rapid changes in color of lake water, and higher O horizon exchangeable Al and Fe