Mercury mobilization and episodic stream acidification during snowmelt: Role of hydrologic flow paths, source areas, and supply of dissolved organic carbon

Abstract

We quantified hydrologic source areas and flow paths, acid‐base and aluminum chemistry, dissolved organic carbon dynamics, and mercury mobilization during snowmelt at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA. Here we show (1) episodic acidification during snowmelt at the HBEF is controlled by multiple mechanisms (base cation dilution, nitrate and aluminum acidity, and natural organic acids) and persists despite long‐term decreases in acidic deposition; (2) episodic acidification continues to result in mobilization of inorganic monomeric aluminum to concentrations toxic to fish; (3) DOC mobilized from shallow organic soils during snowmelt results in the mobilization of mercury from these same sources; (4) methyl mercury may be produced in the forest floor over winter and flushed from soils during snowmelt; (5) the amount of mercury released during snowmelt likely represents a large portion of annual mercury export; and (6) hydrologic source areas and flow paths, as well as DOC dynamics, strongly influence episodic acidification and the mobilization of mercury, even in a watershed with low stream water DOC concentrations and export.