Proton production by transformations of aluminium and iron in lakes

The effect of in-lake transformations of Al and Fe species on H+ production was studied in strongly atmospherically acidified Plešné Lake, Czech Republic, in 2005. We developed a model that quantifies the impact of individual processes (changes in metal concentration and charge, precipitation, and liberation from organic complexes). The net H+ production associated with Al and Fe transformations was 252 and 1 meq m−2 yr−1 (on the lake area basis), respectively, reflecting fluxes of ionic, organic, and particulate forms into and out of the lake and the pH gradient between the inlet and outlet. The greater importance of Al was due to two orders of magnitude higher Al than Fe fluxes. The most important H+-producing processes were precipitation of Al(OH)3 from ionic Al (Ali) (producing 166 meq H+ m−2 yr−1), and hydrolysis of inlet Ali (130 meq H+ m−2 yr−1) that decreased the average charge of Ali due to pH increase from 4.3 in the inlets to 4.7 at the outlet. The liberation of organically bound Al (Alo) was the most important H+ sink among the metal transforming processes (44 meq H+ m−2 yr−1). The H+ production in the lake associated with the change in Ali storage and its charge due to pH change between the end and start of the mass budget periods were quantitatively unimportant (4 and −4 meq H+ m−2 yr−1, respectively).