Surface Water Chemistry in the ILWAS Basins

Abstract

Alkalinity and pH differences observed between the three ILWAS lakes (Panther, Sagamore, and Woods lakes) are primarily a result of inherent watershed differences in base cation supply rates, relative to comparable strong acid input levels. The relatively high proportion of base rich ground water input to Panther Lake results in high pH and alkalinity (annual mean pH 6.2*, alkalinity 147 μeq L^-1). In contrast, shallow interflow with excess strong acid and high Al levels dominates the Woods Lake basin (annual mean pH 4.7*, alkalinity -10 μeq L^-1). Temporal acidification, observed in all three basins during snowmelt, occurs as a result of base cation dilution (particularly in Panther Lake) and increased strong acid anion levels. These marked changes in surface water chemistry are related to an upward shift in flow paths from ground water dominated base flow to shallow interflow during increased snowmelt discharge. Elevated nitrate and Al levels observed during these episodes suggest that HNO₃ from the snowpack and soil nitrification triggers acidification and Al mobilization. On an annual basis, Al export rates were less than 2% of total base cation output from the Panther basin and 23% of base cation output from the Woods basin. Woods Lake itself serves as a secondary sink for Al exported from the soils of the watershed, particularly during the summer months when increasing pH levels induce Al precipitation. Annually, an estimated 43% of the Al entering the lake is retained. Strong acid neutralization in the ILWAS basins appears to be a two-stage process, with initial Al mobilization in upper soil horizons followed by primary mineral dissolution and alkalinity production in deeper soil horizons. Separation of these processes in either time or space results in incomplete neutralization, acidification, and export of inorganic Al to surface waters.