EPISODIC ACIDIFICATION DURING SNOWMELT OF HIGH-ELEVATION LAKES IN THESIERRA-NEVADA MOUNTAINS OF CALIFORNIA

Atmospheric loads to dilute lakes in the Sierra Nevada mountains of Ca lifornia are very low, and fall almost entirely as snow. When acidic a nions preferentially elute from melting snow, these low loads may nont heless be enough to acidify low ANC lakes. Two of the ten lakes includ ed in the Sierra Episodes Study are discussed here: High Lake, the onl y lake in the study to become acidic during snowmelt; and Treasure Lak e, typical of the remainder of the lakes. All lakes exhibited increase s in NO3- concentrations during early snowmelt these were accompanied by increases in base cations, primarily Ca2+. In the first few days of snowmelt, NO3- concentrations at High Lake increased more rapidly tha n concentrations of base cations, resulting in ANC values below zero. Export of both NO3- and SO42- from the watersheds exceeded the inputs from the snowpack, suggesting that other sources (e.g., watershed mine rals, stored inputs from the previous summer, transformations of other inputs) of these anions are important.