PFAS Impact Area Analysis

Summary: John and Jane operate a successful, luxurious resort, but their groundwater supply is threatened by a major underground toxic PFAS plume found upstream of their community well. Initially, they were told the plume is outside of the 10-year “wellhead protection area”, or area of contribution. But now they aren't so sure as more data about the complex subsurface has become available. You be the judge...what is the risk to John and Jane's business?

Background & Questions

John and his wife, Jane, operate a successful, luxurious resort – and business is booming! During the winter, visitors frequent their ski slopes and wood lodges and cabins. In the summer, the resort is a popular destination for all kinds of water sports: from white water rafting, swimming, and fishing, to canoeing and kayaking.

But John and Jane were recently entangled in a messy dispute. The resort, which uses groundwater for drinking water supply, is downstream of the nearby air force base that was recently found to be the source of a major underground toxic chemical plume. The toxic chemical – called PFAS – was used to make firefighting foams at the base. (The chemical is also routinely used in daily life – everything from clothes to cookware, to cleaning products and personal care products). Experts recently linked PFAS chemicals to a litany of ailments, including cancers. 

Although the wells at the resort currently are not contaminated, the PFAS plume is migrating toward their drinking water supply. Based on the latest monitoring data, the PFAS plume is still outside of their 10-year “wellhead protection area”, or area of contribution. One of their friends, a regular skier at the resort who happens to be a geochemist, warned the couple: 

“this wellhead protection area – I know you spent almost $40,000 on a consultant to make it – is likely very approximate. It was delineated based on very limited measurements around the site, reflecting only the so called near-field, or the area immediately around the resort. The entire aquifer was assumed to homogeneous and groundwater flow uniform…but the subsurface environment within the entire wellhead protection area is anything but homogeneous. Flow is probably never uniform, especially for an aquifer in a glacial landscape. Heterogeneity and preferential channels are the norm, not exceptions.  Heterogeneity can dramatically impact the PFAS transport in the subsurface. The existence of preferential channels could mean dramatically reduced travel times.”

This really freaks out John and his wife. The couple thought they had about 10 years to work with to protect their water supply. But it sounds like it could be dramatically less. They decided to sue the air force base for adversely impacting their exclusive source of water and the value of their business. Before they make it too public (which by itself is risky), they want to do some serious homework; they want to make sure they get the facts right. So, they have hired you as an expert witness. They want the following questions addressed, taking advantage of the Freedom of Information Act and the extensive amount of new data collected recently at the site:

• How long would it take for the PFAS plume to reach their wells?
• What is the risk?
• What is the worst case scenario?
• What is the shortest possible time to reach the wells?

Objectives and Deliverables:

Address the questions posed by John and Jane through the use of stochastic groundwater modeling and the available site data. Include a probabilistic wellhead protection area delineation, taking into account the heterogeneity at the site. 

Prepare a 1-2 page report that summarizes your approach and findings. You should discuss your findings with regards to responsibility for the contamination. Include any detailed model results / graphics in support of your conclusions in an appendix.  

Given Information

A map of the site is shown below. The community well and the air force station are situated between the Slow-time Steam and Rapid River. The large Lost Lake sits just southeast of the resort. A thick layer of clay and silts lies directly below the land surface, resulting in an aquifer that is confined. The aquifer itself is a highly mixed collection of sands, gravels, silts and clays. The stage difference between the surface water bodies provides the driving force for groundwater flow. Both Rapid River and Slow-time Stream are fully-connected to the aquifer.

The well supplying water to John and Jane’s resort sits approximately 1800 ft from Rapid River. The PFAS contamination originates 12,000 ft upstream of the community well. The community well operates at 600 GPM.

Field Data

The detailed geostatistical analysis at the site shows:

• Geometric mean hydraulic conductivity (K): 57.6 m/day
• Variance of lnK: 2.01
• Correlation length scales of 46.0 m

Other pertinent information about the site:

• Top aquifer elevation: -10 ft
• Bottom aquifer elevation: -80 ft
• Effective porosity: 0.2 
  

MAGNET/Modeling hints

• Use ‘Synthetic mode’ in MAGNET to create a model domain with the same dimensions as shown in the map
• Overlay the provided SiteMap image file included in the problem description. Choose ‘Use Domain Extent’ to fit the image to the established domain size.
• Conceptualize the model as 1-layer, confined aquifer. Conceptualize the River and Stream features as constant head boundary conditions
• Treat the domain boundaries as ‘no-flow’ boundaries. The lateral boundaries are sufficiently far from the pumping well to not be impacted.
• Use a zone feature of the entire aquifer area to assign the hydraulic conductivity as a random field following the statistical parameters provided above.
• Use particle tracking applications on your simulated flow patterns to track the movement of particles under different realizations of the hydraulic conductivity field