Spatial patterns of groundwater-lake exchange – implications for acid neutralization processes in an acid mine lake

摘要

Exchange of groundwater and lake water with typically quite different chemical composition is an important driver fornbiogeochemical processes at the groundwater-lake interface, which can affect the water quality of lakes. This is of particularnrelevance in mine lakes where anoxic and slightly acidic groundwater mixes with oxic and acidic lake water (pH<3). Tonidentify links between groundwater-lake exchange rates and acid neutralization processes in the sediments, exchange rates werenquantified and related to pore-water pH, sulfate and iron concentrations as well as sulfate reduction rates within the sediment.nSeepage rates measured with seepage meters (u00012.5 to 5.8 L m-2 d-1) were in reasonable agreement with rates inverted fromnmodeled chloride profiles (u00011.8 to 8.1 L m-2 d-1). Large-scale exchange patterns were defined by the (hydro)geologic setting butnsuperimposed by smaller scale variations caused by variability in sediment texture. Sites characterized by groundwater upwellingn(flow into the lake) and sites where flow alternated between upwelling and downwelling were identified. Observed chloridenprofiles at the alternating sites reflected the transient flow regime. Seepage direction, as well as seepage rate, were found toninfluence pH, sulfate and iron profiles and the associated sulfate reduction rates. Under alternating conditions proton-consumingnprocesses, for example, sulfate reduction, were slowed. In the uppermost layer of the sediment (max. 5 cm), sulfate reductionnrates were significantly higher at upwelling (>330 nmol g-1 d-1) compared to alternating sites (<220 nmol g-1 d-1). Althoughndifferences in sulfate reduction rates could not be explained solely by different flux rates, they were clearly related to thenprevailing groundwater-lake exchange patterns and the associated pH conditions. Our findings strongly suggest thatngroundwater-lake exchange has significant effects on the biogeochemical processes that are coupled to sulfate reduction such asnacidity retention and precipitation of iron sulfides. Copyright © 2012 John Wiley & Sons, Ltd.