Predicting regional episodic acidification of surface waters using empirical models

Studies of individual lakes and streams have documented the occurrence of transient, short‐term acidification of surface waters during hydrologic events, but a regional assessment of episodic chemical effects has not been made. Application of a two‐box mixing model, together with regional chemistry and deposition data, indicates that acidic episodes (acid neutralizing capacity <0) are likely an important regional phenomenon. Population estimates of the total proportion of acidic stream reaches increased by 40–640% in six subrogions of the eastern United States when episodes were taken into account. Data from a small sample of lakes in the Adirondacks (which appear to be representative of the lake population) show that fall “index” acid neutralizing capacity is an excellent predictor of the minimum episodic ANC measured at the outlets of these lakes during spring snowmelt. While 11% of the Adirondack lakes were acidic at fall overturn, a linear regression model predicts that more than 35% were acidic at their outlets during the spring of 1986.