Transport and fate of PFAS in the subsurface
Per- and polyfluoroalkyl substances (PFAS) are a diverse group of fluorinated organic contaminants in the environment resulting from use at firefighting facilities, in consumer products, waste at manufacturing sites, and disposal in landfills. Leaching of per- and polyfluoroalkyl substances (PFAS) currently retained in the shallow subsurface above the water table are a persistent source of groundwater contamination. This retention process is very complex because the timing and extent of PFAS mobility in the shallow subsurface are controlled by soil properties, climate and weather events, and site specific hydrogeologic conditions. Despite important advances in understanding the migration and retention behavior of PFAS, there remains a critical knowledge gap on how spatial variation in geologic properties—that have a strong impact on groundwater flow processes—impact how, when, and where PFAS contamination migrates in the subsurface. Our group is currently involved in three projects focused on understanding multiscale transport of PFAS in the vadose zone.
- A collaborative NSF-funded project utilizing highly instrumented columns and recent advances in numerical models to quantify dynamic, spatially variable PFAS adsorption in heterogeneous unsaturated geologic porous media. The specific aims are to measure in situ adsorption of two representative PFAS in saturated and partially saturated heterogeneous columns based on our recent theoretical study on PFAS immobilization under unsaturated conditions. These measurements will then be used to provide direct validation to advanced spatially-resolved numerical models and enable the development of upscaled models of PFAS transport under heterogeneous subsurface conditions. The project is anticipated to provide unprecedented in situ transport and adsorption measurements, upscaling approaches, and parameterization of a newly-developed numerical model used to describe and predict PFAS leaching through soils to groundwater aquifers.
- A project funded by the Wisconsin Department of Natural Resources focused on understanding how repeated wetting and drainage cycles, driven by recharge events and a fluctuating water table, impact the short- and long-term mobility of PFAAs and precursor compounds in the unsaturated zone.
- A field data collection and numerical modeling study to support the Town of Campbell’s investigation to transition to a centralized municipal water supply to serve its residents. PFAS groundwater contamination is associated with the use of aqueous film-forming foams (AFFFs) as firefighting agents at the La Crosse Regional Airport. PFAS contamination is now directly impacting approximately 4,300 residents of the Town of Campbell, in all regions of French Island. Impacted residents are currently dependent on groundwater from shallow wells (30 – 80 feet below land surface, on average) that tap the surficial alluvial sand and gravel aquifer for drinking water supply. This project is a collaboration with researchers from the USGS, the Wisconsin Geological and Natural History Survey (WGNHS), and University of Wisconsin-Madison (UW-Madison) to identify safe, sustainable sources of drinking water for the Town of Campbell and adjacent communities.
Combined, these studies are expected to advance our ability to quantitatively measure and model PFAS migration and retention under realistic geologic conditions. Improved understanding of PFAS migration in the subsurface is anticipated to enable future progress in PFAS modeling, prediction, and remediation strategy development.
Quantifying the impact of spatial and temporal variation in hyporheic zone fluxes
on phosphorus transport and release in Wisconsin streams and rivers
River bed sediments can be an important source of phosphorus to Wisconsin waterways, driving eutrophication and negatively impacting aquatic health, human health, and local economies. There is limited understanding of how groundwater–surface water exchange impacts river sediment phosphorus storage, and this study aims to quantify these processes. Students will characterize phosphorus and subsurface hydrology in stream sediments at two sites in central Wisconsin and conduct batch and column experiments on sediment samples to evaluate which biogeochemical conditions promote storage and release of phosphorus. This work is supported by Freshwater Collaborative of Wisconsin. The University of Wisconsin-Madison is a member of the Freshwater Collaborative of Wisconsin, a partnership that provides students with opportunities for water-related coursework, hands-on experiences, internships and research opportunities at campuses throughout the UW System. Learn more at freshwater.wisconsin.edu.
Measurement of bacterial transport and immobilization in variably saturated geologic materials of WI
The occurrence of bacteria and other microbial agents in private wells across the state of Wisconsin poses significant risks to human health. Discrepancies between bacteria measurements in drinking water wells compared with the expected near-surface immobilization from laboratory experiments in homogeneous geologic materials indicate an incomplete understanding of dynamic processes of bacteria transport and immobilization in realistic geologic systems. The overall objective of this collaborative project funded by the University of Wisconsin Water Resources Institute is to quantify preferential bacterial breakthrough and immobilization through heterogenous saturated and unsaturated columns representative of soils in the Central Sands Region of Wisconsin. To achieve this objective, we are using positron emission tomography (PET), an imaging modality capable of quantifying the migration of radiolabeled bacteria (E. coli) through columns of geologic materials. Imaging data will provide three-dimensional time-lapse observations of the role of geologic heterogeneity and fluid saturation conditions on the advection, dispersion, and immobilization of radiolabeled bacteria. The expected outcomes will be unprecedented experimental measurements and uniquely constrained analytical and numerical models of bacterial transport and immobilization in geologic porous media. These measurements and models will provide more accurate and geologically specific information about how bacteria and microbial agents contaminate groundwater sources. This information will complement existing contamination risk parameters such as depth-to-bedrock measurements, soil type, and climate and weather conditions to improve guidance, regulations, and risk assessments of bacterial contamination of drinking water wells in Wisconsin.
Fluid channelization and fracture-matrix interaction
Understanding flow and transport behavior in fractures is important for geothermal energy recovery, geologic carbon storage security, and unconventional resource recovery. The focus of research projects in this area is to utilize positron emission tomography (PET) to quantify fracture connectivity, fluid channelization, and fracture-matrix interaction in complex fracture networks. An example of this type of imaging data is shown in the video below, highlighting how aqueous radiotracer transport occurs almost exclusively in pre-existing fractures. This data is used to constrain a range of models addressing questions such as:
- How does fracture roughness impact flow channelization and how does this change with mechanical displacement?
- How does solute diffusion into the rock matrix change as a function of flow conditions and fracture alteration?
Time-lapse image of solute traveling through a naturally fractured shale core (from right to left) over the course of six hours.