PFAS migration in groundwater –hydrogeological assessment and long-term modelling of plume evolutionto inform environmental management options

ASLAN PERWICK – PATTLE DELAMORE PARTNERS LTD

THANK YOU TO MANY OTHERS – NERENA RHODES, ANDREW RUMSBY, JAMES CONWAY, ALAN PATTLE, HAMISH WILSON, AMBER SIMMONDS, MANY MORE

Scope

• Understanding long-term fate & transport of key PFAS compounds in groundwater – why the need?

• Existing Plume – what is the situation?

• Plume predictions are needed - How could such a task be approached?

• The predictions and what do they mean?

• What are the learnings?

• What are the limitations?

PFAS Fate & Transport –Why the need to understand?

• PFAS compounds can have long residence times within the environment – risks could require long-term management.

• Leaching of source mass from the Source Zones (Soil) can continue for many decades – risk could require long-term management.

• Ultimately – a basis for Environmental Risk Management & decision making.

PFAS Fate & Transport –Present Situation

• PFAS use started on the site in the 1970s, continued for circa 35-yrs.

• Large existing PFOS+PFHxS Plume (the focus of this study)

• PFOS + PFHxS use has ceased within the last decade

• Area of PFOS + PFHxS >0.07 ug/L –likely greater than 1000 ha

PFAS Fate & Transport –How did we get here?

• Only 2-years of time-series data available –little data available for trend analysis

• Estimate that surface water system is a key migration method

• Plume estimated to have advanced quickly along key surface water routes, due to PFAS surface run-off.

• Gradual PFAS soil leaching to groundwater, and then advection and growth of a groundwater plume

• Highly dispersive hydrogeology, but layered, and numerous private groundwater takes

PFAS Fate & Transport – 3D Prediction of Plume Evolution: 1970s to Today

ELASPED YEARS @ 0, 1, 2, 3, 5, 10, 15, 20, 25, 30, 35

PFAS Fate & Transport – How did we get to today’s plume?

• Minimum plume mass of PFOS + PFHxS estimated at 45 kg

• Based on observed GW data + estimates of plume extent + hydrogeological properties

• Estimate of vertical extent (evidence pointing towards most mass being held shallow)

• Conservative Transport? or Retarded Transport?

PFAS Fate & Transport –Questions to Answer:

• How much contaminant mass (PFOS + PFHxS) may be present in the existing plume?

• How could the plume evolve into the future – shape and mass?

• What and where are the likely key receptors of the plume?

• How long before the plume may decay to be below (current) drinking water guideline?

PFAS Fate & Transport –What about the Unknowns?• Many unknowns – need to take

‘best-estimate’ approach

• Numerous ‘scenarios’ conducted to provide a range of predictions:

• Instantaneous Source Remove [Faster End]

• Source Depletion Scenario – ‘25-year’ half-life [Base Estimate]

• Source Depletion Scenario – ‘50-year’ half-life [Longer End]

• Conservative Transport

• Retarded Transport

?

?

?

?

?

PFAS Fate & Transport – Predicted Plume Mass Evolution:

• Short-Medium Term: Mass of groundwater plume likely to increase or stay steady – despite cease of PFAS use

• Even if Soil Source Zones could be instantly removed, existing groundwater PFAS mass is likely slow to deplete

• Soil Source Zones predicted to continue leaching PFAS mass to groundwater for many decades

• Virtually all groundwater PFAS mass to exit groundwater system via surface water

PFAS Fate & Transport – Predicted Plume Area Evolution:

• All scenarios indicate plume likely to increase in area …

• Then enter a phase of gradual depletion

• Scenarios ranged between 20-yrs to 150-yrs for plume to decrease in area (from today)

• ‘Best estimate’ is between 75-yrs to 100-yrs to decrease in area (from today)

• Even the ‘Instant Source Remove’ estimates 20-yrs to 60-yrs for area reduction

PFAS Fate & Transport – ‘Instant Source Remove’ – Present to 125-years:

FUTURE YEARS @ 1, 2, 3, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 70, 75, 80, 90, 100, 113, 125

PFAS Fate & Transport – ‘Depletion Soil Source – 25-year HL’ – Present to 125-years:

FUTURE YEARS @ 1, 2, 3, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 70, 75, 80, 90, 100, 113, 125

PFAS Fate & Transport – ‘Depletion Soil Source – 25-year HL’ – Present to 125-years:

FUTURE YEARS @ 1, 2, 3, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 70, 75, 80, 90, 100, 113, 125

PFAS Fate & Transport – ‘Depletion Soil Source – 50-year HL’ – Present to 250-years:

FUTURE YEARS @ 10, 20, 40, 50, 60, 70, 80, 90, 100, 110, 120, 140, 150, 160, 180, 200 , 210, 220, 240, 250

Prediction Summary:• The existing plume extent is large, and is likely to remain large for multiple

decades, before a gradual depletion over multiple more decades.

• Existing plume mass is likely to increase before a gradual depletion over multiple decades.

• Key receptors are interpreted to be surface water bodies, and these largely constrain the ultimate extent of the plume

• Shallow groundwater bores downgradient of the existing plume will likely require risk management measures in the future

Learnings:• Multi-disciplinary input needed when attempting a PFAS groundwater

assessment of this scale and complexity.

• Surface water transport route, and SW-GW pathway, a key migration method for this site (and likely others).

• Even with numerous uncertainties, 5D modelling (X,Y,Z, Time, Conc.) can still provide a useful basis to assist with decision making / risk management approach e.g. likelihood of risk or not?, understanding big-picture trends, null-hypothesis testing …

Limitations:

• For more certainty/refinement of predictions, some more field data would be needed e.g. Additional time-series, aerial delineation, and vertical delineation.

• PFAS compounds are tricky devils – behavior & chemical transformations in the sub-surface are not fully understood, could be significant.

• Soil source zone concentrations and depletion rates need to be estimated –underlying limitation

• Dealing with nanograms! PFAS sampling & analysis can show large variation between sampling rounds

• Many More!

Thank You & Questions

PFAS Fate & Transport – Predicted Plume Mass Evolution:

• 50-yr half-life source depletion predicted contaminant mass

PFAS Fate & Transport – 3D Geology Model:

• Constructed from Regional Council borehole database records