Case Study

PFAS Remediation Design for Fire Training Site

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iFLUX Sampler
PFAS
  Urban
  Regenesis, mott macdonald
   United kingdom

Our value

High spreading risk of PFAS contamination

Design PFAS barrier and measure its efficiency

Optimize liquid carbon injections depths by identifying high flux zones.

Situation

iFLUX has been mandated to carry out PFAS mass flux measurements on a site impacted by a PFAS pollution and for which a treatment is in progress. 

The contamination was characterized  and delineated in earlier soil survey. Additional flux measurements are used to design the barrier to prevent a PFAS pollution from spreading with the groundwater flow. Based on mass flux measurements the amount of liquid Activated Carbon to inject at different depths is calculated.

The soil is heterogeneous with gravely to sandy layers, calcareous clay, occasionally with shell fragments and at depth mudstone.

Sampling

Mass flux measurements are executed at several depths in two existing wells (diameter 51/63 mm). The wells are sampled. Concentrations (order of magnitude) are mentioned in the table below.

filter depth (mbgl)

PFBA

PFDA

PFDoDA

PFHpA

PFHxA

FHxS

PFNA

PFOA

PFOS

PFOSA

PFPA

PFUnDA

undeep

1500

<1

<1

600

3000

70

10

300

10

<2

300

<1

deep

800

<1

<1

600

2500

70

10

200

10

<2

2500

<1

Mass flux measurement are performed to assess the potential stratification of PFAS fluxes in groundwater due to large differences in hydraulic conductivity of the different soil layers. The measurements cover the zones of high to medium hydraulic conductivity as well as a zone of lower conductivity, identified based on available HPT profiles. In total mass flux measurements are executed at 6 depths.

Canva Design DAGFdrA-x78

"Use of iFLUX allows for more accurate PlumeStop designs and increases the likelihood of groundwater treatment success"

Challenges

How to avoid PFAS from spreading:
  • How to limit plume expansion of persistent and mobile contaminant?
  • Injection is expensive
  • Is active barrier effective?
  • Large, dilute plumes (> 1 mile in length, 75% of plume is < 10µg/l)

The Solutions

The PFAS cartridge in collaboration with Cyclopure to optimize remedial design and measure its effectiveness:
  • Flux measurements to optimize focused injection on high flux zones
  • More efficient use of liquid activated carbon based on measured data
  • Pro-active follow-up with flux measurements
  • iFLUX PFAS cartridges with the Cyclopure resin has a detection limit of 5-10µg/m²/day and 5-80 ng/l and measures accumulation over time.

The Result

As expected, the highest PFAS fluxes were measured at depths corresponding to the targeted thinner soil layers of higher hydraulic conductivity, as identified by the HPT measurements, at depths around 3 mbgl, 5 mbgl, 6 mbls and 11 mbgl. 

In the layer of lower hydraulic conductivity between 5m and 6m-bgl, the PFAS flux was significantly lower. 

Averaged seepage velocities can be estimated based on the measured mass flux and groundwater concentration data. Depending on the substance, estimated averaged seepage velocities range from 20 to 70m/yr for the low hydraulic conductivity zone (between 5 and 6 m-bgl) and from 60 to 120 m/yr for the higher hydraulic conductivity zones.

Overzicht Detail-1

A second measurement campaign is planned, after installation of the barrier. Upstream a monitoring of the Mass flux  in one or two zones of higher hydraulic conductivity and with a higher PFAS flux, as well at 11 mbgl is planned. 

To evaluate the barrier on longer term flux measurements downstream of the barrier can be of added value.

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