CHARACTERIZING DISSOLVED IRON CONCENTRATIONS AND GROUNDWATER FLUXES IN AN AQUIFER DISCHARGING TO A FERRUGINOUS, MEROMICTIC LAKE

Ferruginous, meromictic lakes contain a mixolimnion layer and an anoxic, iron-rich monimolimnion layer that remain stratified throughout the year. Brownie Lake, MN, is a ferruginous meromictic lake that is likely receiving significant amounts of dissolved iron from groundwater; however, the magnitude and heterogeneity of iron flux has not yet been quantified. Constraining the iron budget is critical to understanding how iron participates in the carbon cycle. This study uses tools and techniques traditionally applied in contaminant hydrogeology to inform the magnitude of heterogeneity in groundwater iron mass flux and possible controls on this heterogeneity. Three co-located sediment cores and water chemistry profiles were collected along a 200 m transect parallel to the lake shore using a Geoprobe and a direct push groundwater profiling tip. A total of 44 intervals across the water chemistry profiles were monitored for DO, pH/ORP, temperature, and specific conductance, then sampled for major cations, anions, δ18O-H2O, δ2H-H2O, and alkalinity. Over 150 samples were collected from the sediment cores (avg. depth 9.14 m) targeting each distinct lithologic unit for grain size, permeameter, minerology, and XRF analysis. The core data will support characterization of hydraulic conductivity across the transect which will then be combined with hydraulic gradients informed by temporary drive point wells to estimate groundwater fluxes. Groundwater fluxes and iron concentrations contoured across the transected will be combined to estimate iron mass fluxes. We expect to see variations in iron flux based on heterogeneities within the glacial sediments and a general increase in concentration with depth as the transect intersects longer flow paths with higher residence times and more reducing conditions. Quantifying the heterogeneity of iron flux will constrain the iron budget, better inform processes occurring at the transition zone, and improve our understanding of the coupled iron-carbon biogeochemical cycles. Elucidating iron’s role in the carbon biogeochemical cycle will advance understanding of microbial function, essential nutrient/element availability, and climate change.