INSIGHTS FROM SEEPAGE VELOCITY MEASUREMENTS AT THE AQUIFER-LAKE BASIN INTERFACE BENEATH A MUCK-FILLED LAKE

In 2019, work was begun to investigate the groundwater-surface water (GWSW) interactions at the bottom of Unnamed Lake, located several hundred meters downgradient from a pipeline oil spill near Bemidji, MN. The site has been investigated by researchers led by the USGS since the early 1980s. Recently concern over the fate of partially degraded oil products reaching the lake been growing. The 2019 investigation involved the use of Darcy-based data gathering (slug testing, head measurements and Darcy’s Law), seepage meters, and the StreamBed Point Velocity Probes (SBPVP). The presence of the muck impeded seepage meter measurements, but the SBPVP and Darcy approaches were successful. The initial work revealed that a thick layer of muck in the lake basin – up to 30 feet thick in some locations – might be serving as an aquitard that prevented water from entering the lake freely at the GWSW interface. Between 2020 and 2024 the site was revisited to obtain transects of seepage velocity beneath the muck layer (100 to 125 m across), north and south, and along two additional transects that ran for about 15 m each along the eastern shoreline.

Unnamed Lake is regarded as a ‘flow-through’ lake. The lake-wide transects differed from flow pattern expectations in two primary ways: first, the muck redirected flow beneath the lake. This resulting seepage velocities at the top of the sandy aquifer and the bottom of the muck were very high, about two orders of magnitude or more than in the surrounding aquifer. The detailed shoreline transects revealed highly variable patterns of seepage that appeared to depend on shoreline features such as beaver trails.

The work performed here suggests that a conceptual model for Unnamed Lake is one with highest rates of flow entering the lake several meters from shore, then being diverted around the lower boundary of the basing by muck. The work also showed that shoreline features can exert pronounced effects on the direction and magnitude of seepage velocity entering or leaving the lake. These findings are particularly relevant for the transport and fate of oil metabolites that are currently arriving at the lake or soon will be.