Episodic stream acidification caused by atmospheric deposition of sea salts at Acadia National Park, Maine, United States

Major episodic acidifications were observed on several occasions in first‐order brooks at Acadia National Park, Mount Desert Island, Maine. Short‐term declines of up to 2 p H units and 130 μeq L −1 acid‐neutralizing capacity were caused by HCl from soil solutions, rather than by H 2 SO 4 or HNO 3 from precipitation, because (1) SO 4 concentrations were constant or decreased during the p H depression, (2) Cl concentrations were greatest at the time of lowest p H, and (3) Na:Cl ratios decreased from values much greater than those in precipitation (a result of chemical weathering), to values equal to or less than those in precipitation. Dilution, increases in NO 3 concentrations, or increased export of organic acidity from soils were insufficient to cause the observed decreases in p H. These data represent surface water acidifications due primarily to an ion exchange “salt effect” of Na + for H + in soil solution, and secondarily to dilution, neither of which is a consequence of acidic deposition. The requisite conditions for a major episodic salt effect acidification include acidic soils, and either an especially salt‐laden wet precipitation event, or a period of accumulation of marine salts from dry deposition, followed by wet inputs.