Characterizing water and dissolved iron mass discharge within a groundwater-ferruginous lake system, Brownie Lake, Minnesota

Ferruginous, meromictic lakes have upper oxic and lower anoxic, iron-rich layers that do not mix seasonally. It is important to create accurate iron budgets for these lakes to better understand how iron is used in the larger carbon cycle. Brownie Lake, MN, is a ferruginous meromictic lake that is likely receiving large amounts of dissolved iron from groundwater; however, the scale and spatial distribution of dissolved iron in groundwater flow, or iron mass discharge, needs to be quantified to improve estimates of the groundwater component in iron budgets. The goal of this research was to estimate the scale and distribution of iron concentrations and iron mass discharge within the aquifer surrounding Brownie Lake. This was done by 1) developing a conceptual site model for the groundwater flow system, and 2) estimating iron mass discharge across a cross section of the aquifer using high resolution data collected by a tool traditionally applied to contaminant hydrogeologic investigations. The groundwater flow system was found to be strongly influenced by the geometry of underlying glacial deposits and by strong hydraulic conductivity contrasts. These local features suggested that groundwater-surface water interaction may be more complex than previously indicated by regional scale models. Iron concentration and iron mass discharge were found to be variable within the aquifer. This improved understanding of the scale and spatial distribution of iron mass discharge is critical to creating more accurate iron budget estimations. This study also demonstrated that tools developed for contaminant studies can be valuable in biogeochemical cycling studies.