PFAS FATE AND TRANSPORT AT THE GROUNDWATER-SURFACE WATER INTERFACE ALONG THE ROGUE RIVER IN NORTHERN KENT COUNTY, MICHIGAN

PFAS (per- and poly-fluoroalkyl substances) are emerging contaminants of concern that are harmful to humans at extremely low concentrations. They bioaccumulate in the food chain and are recalcitrant in groundwater. In the 1960’s, PFAS compounds were dumped in the House Street Disposal Site (HSDS), an unlined landfill located on the crest of a glacial end moraine in Kent County, Michigan. A network of monitoring wells delineates a PFAS plume that migrated 3 km (2 mi) downgradient toward the Rogue River over the past 50 years.

Today, the Michigan Department of Natural Resources (MDNR) operates fish-rearing ponds in the area where the PFAS plume is interpreted to intersect the groundwater-surface water interface (GSI). Each year, the MDNR fills the man-made ponds with water from a nearby creek. At the end of the summer, the ponds do not drain completely because they contain natural springs. In this investigation, we sampled surface water and modeled groundwater flow to ascertain the effect of the ephemeral ponds on PFAS transport at the GSI.

Surface water samples from the inlet stream, walleye pond, fathead minnow pond, a wetland seep, and an adjacent stream were analyzed for PFAS compounds by LC-MS/MS using EPA draft method 1633. 4 to 8 carbon chain PFAS compounds were detected in all samples, with the exception of the seep, which contained only a 4 carbon chain compound. Total PFAS concentrations ranged from 50-60 ng/L (ppt) in the ponds and the adjacent stream. However, PFAS concentrations in shallow monitoring wells along the Rogue River average 400 ng/L. A simple mixing model suggests that ~1/8 of the water in the ponds comes from springs (the GSI), whereas 7/8 is sourced by surface water containing PFAS at lower concentrations.

We developed a 100 km² (40 mi²) MODFLOW model for the site with nine layers representing the aquifer system underlying the local glacial terrain. Advective transport modeling using MODPATH (forward particles) closely followed the path of the plume between the HSDS and the Rogue River discharge area. However, adding the fishponds to the model as constant head boundaries did not substantially change the predicted flow paths or model calibration curve, leading us to conclude that PFAS transport is dominated by the regional flow system with limited influence from seasonal changes to boundary conditions at the GSI.