The episodic acidification of Adirondack Lakes during snowmelt

Maximum values of acid neutralizing capacity (ANC) in Adirondack, New York lake outlets generally occur during summer and autumn. During spring snowmelt, transport of acidic water through acid‐sensitive watersheds causes depression of upper lake water ANC. In some systems lake outlet ANC reaches negative values. We examined outlet water chemistry from 11 Adirondack lakes during 1986 and 1987 snowmelts. In these lakes, SO₄²⁻ concentrations were diluted during snowmelt and did not depress ANC. For lakes with high baseline ANC values, springtime ANC depressions were primarily accompanied by basic cation dilution. For lakes with low baseline ANC, NO₃⁻ increases dominated ANC depressions. Lakes with intermediate baseline ANC were affected by both processes and exhibited larger ANC depressions. Ammonium dilution only affected wetland systems. A model predicting a linear relationship between outlet water ANC minima and autumn ANC was inappropriate. To assess watershed response to episodic acidification, hydrologic flow paths must be considered.