Evidence for a hydrologically controlled iron cycle in acidic and iron rich sediments

Retention of ferrous iron at the interface between ground- and surface water is crucial for the acidity balance of lakes influenced by acid mine drainage. Iron budgets were developed for two sediments in areas of differential groundwater inflow (ca. 1 and 10 L m(-2) d(-1)). In both areas iron was sedimented as schwertmannite (Fe8O8(OH)(x)(SO4)(y) , 8 - x = 2y, 1,0 < x < 1,75) at rates of 5.5-5.9 mmol m(-2) d(-1) leading to iron(III) enriched sediments (3.9-6.2 mmol g(-1) dry weight). Compared to the surface water, the inflowing groundwater had higher pH (4.5 vs. 3), ferrous iron (6-20 mmol L-1 vs. 0.8-2.0 mmol L-1) and sulfate (5-60 mmol L-1 vs. 8-13 mmol L-1) concentrations. The inflow of the groundwater caused a change in sediment pore water chemistry and an increase in pH to above 5.5. The pH increase was probably mostly due to decreased transformation rates of schwertmannite to goethite (0.27 mmol m(-2) d(-1) vs. 5.6 mmol m(-2) d(-1)), also decreasing the production of H+ in the sediment. Compared to the control, in the area with groundwater inflow solid phase iron sulfide (0.011 mmol m(-2) d(-1)vs. 0.0019 mmol m(-2) d(-1)) and carbonate were formed at a higher rate, and more sulfate was reduced in incubation experiments. This finding can be explained by saturation indexes of siderite and by sulfate reduction becoming thermodynamically more competitive by about 40 kJ eq(-1) compared to iron reduction. However, only a small fraction of the reduced ferrous iron and sulfide was retained in the sediment, emphasizing the importance of re-oxidation processes. The study demonstrates the existence of biogeochemical patterns in lake sediments due to variations in hydrologic boundary conditions in the adjacent aquifer.