IDENTIFYING SOURCES OF SNOWMELT ACIDIFICATION WITH A WATERSHED MIXING MODEL

We used ionic tracers to estimate the volume of old (soil and ground) water interacting with snowmelt in eleven Adirondack, NY watersheds. The contribution of old water varied from 66 to 90%, with no general relationship between old water % and soil depth to till. This approach also discriminated between watershed retention and release of particular ions to lake outlet water during snowmelt. Most watersheds released NO3- during snowmelt, in addition to the snowpack NO3-. Nitrification of snowpack NH4+ explained part of the additional NO3- in lake out outlet water, but some NO3- was likely mineralized nitrogen from soil organic matter. All watersheds retained NH4+ as well. Nitrogen release was greatest in the acidic watersheds in the southwestern Adirondacks, a region thought to be impacted by anthropogenic deposition. During snowmelt, Ca2+ and Mg2+ ions (presumably from soil exchange sites) were also released from most watersheds. In watersheds with acidic (minimum pH < 4.6) lake outlet water, Al was also released during snowmelt. Thus, lake outlet water acidification during snowmelt was both buffered by cation release, and intensified by NO3- release. If the soil exchangeable cation pools were not replenished prior to snowmelt, or NO3- mobilization were increased, acidification during snowmelt would intensify.