Groundwater inflow controls acidity fluxes in an iron rich and acidic lake

To ascertain the influence of hydrological boundary conditions on acidity fluxes in lakes influenced by acid mine drainage, acidity budgets were developed for two sediments in areas of differential groundwater inflow (approx. 1 L m(-2) d(-1) and 10 L m(-2) d(-1)). In both sediments iron was deposited as schwertmannite leading to iron(III) enriched sediments (3.9... 6.2 mmol g(-1), referred to 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 changed the sediment pore water chemistry and triggered a further increase in pH to above 5.5. In both sediments acidity generation in the surface water (10... 30 mol m(-2) a(-1)) strongly prevailed over acidity consumption in the sediments (> -0.6 Mol m(-2) a(-1)). With advective groundwater inflow, however, more acidity was consumed due to TRIS formation (-0.12 mol m(-2) a(-1) vs. -0.017 mol m(-2) a(-1)), iron carbonate burial (upper estimate: -0.14 mol m(-2) a(-1) vs. -0.022 Mol m(-2) a(-1)), and unspecific ferrous iron retention (-0.39 Mol m(-2) a(-1) vs. -0.08 Mol m(-2) a(-1)). Also, less acidity was generated due to schwertmannite transformation (-2.4 mol m(-2) a(-1) vs. -0.11 Mol m(-2) a(-1)). The acidity balance of internal processes in the sediment with groundwater inflow was negative, whereas it was positive in the other sediment. The study demonstrates that in acidic and iron rich lakes the hydrological boundary conditions strongly affect geochemical processes as subsumed in acidity fluxes.