Using the Min-Trap® to Assess PFAS Partitioning
It has been demonstrated that a number of processes affect the transport of per- and polyfluoroalkyl substance (PFAS) in groundwater. PFAS can sorb to organic carbon, metal minerals and exhibit high affinity for air-water interfaces. The magnitude of these processes can vary in time and space, particularly at sites with complex geology and dynamic conditions within the groundwater table fluctuation zone. From a practical perspective, this phenomenon may make it challenging to reliably discern concentration trends, design robust and reliable remediation systems, predict long-term plume behavior, and assess risks to potential receptors.
How Min-Traps® Work
The Min-Trap® (Mineral Trap Sampler) is an in situ passive sampling technology that was originally intended to capture and sample reactive minerals, such as iron sulfide, actively forming in an aquifer. In an exciting development, the application of the Min-Trap® has recently been expanded to characterize contaminant distribution between groundwater and solid (i.e., aquifer) phases under actual field conditions. The Min-Trap® consists of a slotted PVC housing containing mesh pockets filled with a selected solid matrix (e.g., site soil, or engineered media such as colloidal carbon). Min-Traps® are deployed directly in existing monitoring wells, allowing groundwater to circulate freely through the media during the deployment period. Following incubation and retrieval, the solid matrix is analyzed (e.g. via EPA Method 1633A) to quantify accumulated contaminant mass. A corresponding groundwater sample is collected concurrently to determine aqueous concentrations.
Calculating an Effective Field Partition Coefficient
This approach provides users with a practical means to assess effective contaminant partitioning behavior as a result of all relevant partitioning processes under real-world field conditions. The Min-Trap® provides an effective field partition coefficient (kd) (calculated by dividing the quantity sorbed to soil (µg/kg) by the concentration in water (µg/L)). Calculating kd is necessary for estimating the fraction of contaminant mass bound up in the solid site soil and sediment rather than basing site management decisions on what is being detected in the groundwater.
Additionally, the retardation coefficient R, which describes the rate of contaminant travel relative to water, can be calculated from kd as follows:
R = 1 + (ρb*kd) / ϴ
where, R is the retardation coefficient (dimensionless),
ρb is the bulk density of the dry aquifer solids (g/cm3 aquifer),
kd is the solids to water distribution coefficient (mL/g), and
ϴ is the water-filled porosity (cm3 water/cm3 aquifer)
Key Advantages of Min-Traps®
Because of their ease of deployment and relatively low costs, multiple Min-Traps® can be deployed within existing monitoring wells across a site to assess effective field kd in different parts of the plume providing more robust information on spatial variations transport behavior. This information can support more robust conceptual and quantitative models. Additionally, Min-Traps® can help assess the performance of sorption-based amendments such as colloidal carbon, activated carbon, biochar, or zero-valent iron. The data provided by Min-Traps® offer a practical field-based means of characterizing subsurface contaminant behavior, evaluating remedial media performance, and supporting decisions regarding treatment optimization and long-term monitoring.
Microbial Insights is grateful to Dr. Craig Divine of Arcadis U.S., Inc. for sharing his expertise and insights, and we’re honored to feature his work here. For questions, please reach out to Dr. Divine at [email protected].
