Greg H. LeFevre, Jessica R. Meyer, Dana W. Kolpin

Muddy Creek is a small wastewater effluent dominated stream in eastern Iowa that has undergone extensive monitoring and study over the last decade and serves as a ‘field laboratory’ for understanding the transport, attenuation, ecological exposure, and effects of complex mixtures of contaminants including PFAS, pharmaceuticals, and pesticides. Recently, we began investigating PFAS in groundwater at this field laboratory following installation of new monitoring infrastructure at three sites: US-1 (42 m upstream of the wastewater treatment plant outfall), DS-1(0.2 km downstream of the outfall), and DS-2 (5 km downstream of the outfall). The infrastructure at the sites includes drive-point multilevel systems (MLSs), shallow and deep (~ 0.6 and 2 m below the bed respectively) drive point piezometers, stilling wells, and stream bank monitoring wells (depths between 2 and 4 m below ground surface). Each drive-point MLS includes either two or four monitoring intervals between 0.6 and 2 m below the bed. Hydraulic head and stream stage were monitored continuously over a two-year period in the drive-point piezometers, stilling wells, and stream bank monitoring wells at all sites. Samples were collected for PFAS analysis from the surface water (grab) and groundwater (drive-point MLSs and stream bank monitoring wells) three times over a one-year period. Hydraulic gradients during baseflow conditions at all three sites were upward, from the aquifer to the stream. As previously documented, wastewater was a point source of long- and short-chained PFAS compounds to this system. Unexpectedly, the levels of short-chained PFAS (PFBA, for example) in surface water at US-1 were similar to effluent concentrations. The source of these short-chained PFAS at US-1 is currently unknown. Groundwater samples collected at DS-1 from the streambed and banks indicated only short-chained PFAS (PFBS and PFBA, for example) were present whereas a mix of long- and short-chained PFAS were present up to 0.7 m below the bed and in the banks at DS-2 with more short- than long-chained PFAS detected. The results indicate the distance between the outfall and DS-1 may not have been sufficient to drive effluent derived PFAS into the stream bed but that hyporheic processes operating at a variety of scales over the distance between DS-1 and DS-2 did result in exchange of PFAS from the surface water into the groundwater. In addition, the relative lack of long-chained compounds in groundwater at DS-2 indicates subsurface migration caused separation of the PFAS mixture with the short-chained compounds showing greater apparent mobility and the long-chained compounds having a more limited extent. This work has important implications for understanding PFAS transport, attenuation, and fate in effluent dominated streams with local implications for mass flux loading of PFAS to the aquifer that serves as a drinking water source to Iowa City.