X-rays get to the bottom of biopile success

A series of dirt piles - biopiles - constructed at Casey research station.
Biopiles at Casey are about 1.5 metres high and measure between 25-40 metres long by six metres wide. Fertiliser, moisture and air are circulated throughout the pile to assist the native microbes to degrade fuel in the soil resulting from spills. (Photo: Rebecca McWatters)
The clay liner under X-ray showing variations in hydration (white) and voids (black) from desiccation and cracking. An X-ray of a clay liner showing finer cracks.A schematic showing how a biopile works.Biopiles covered by snow in winter.

X-rays have been harnessed to investigate the performance of materials used to help clean up fuel contaminated soils at Casey research station.

Australian Antarctic Division remediation experts, Dr Rebecca McWatters, Mr Tim Spedding and their team*, in collaboration with the Division’s Polar Medicine Unit, are using the station’s medical X-ray machine to examine the effect of moisture, drying, freezing and thawing, on clay liners used at the base of ‘biopiles’.

“Biopiles are mounds of fuel-contaminated soil that rely on native soil microorganisms to break down the fuel,” Mr Spedding said.

To contain the soil and prevent fuel leaching out while it is undergoing treatment, a composite liner system is used beneath the biopiles, in a similar design to that used in Australian landfills.

The system consists of a clay liner (a layer of powdered bentonite sandwiched between two geotextiles) which absorbs moisture from the compacted and levelled ground beneath the biopile, and swells to form a barrier against the migration of contaminants from the biopile above. A heavy-duty plastic sits on top of the clay liner and adds an additional barrier against the migration of contaminants from above. Finally, a geotextile helps protect the plastic from being punctured by any rocks in the biopile soil.

“The biopile system has been employed since 2010 to successfully remediate contaminated soil, which is then reused around the station,” Mr Spedding said.

“While biopiles have been used in the Arctic and extensively in temperate environments, the Australian Antarctic Program is the first to use large biopiles in Antarctica.”

An important part of the team’s work is to monitor the liner system throughout the soil remediation process to ensure it is performing as designed in the relatively untested Antarctic conditions.

Through the monitoring process the team, in collaboration with Queen’s University in Canada, has been studying different combinations of clay and plastic liner materials to test their long-term performance in containing fuel within the biopiles, and their durability in the Antarctic climate.

“We’re looking for the right combination of technologies that will do the job with minimal cost, maintenance and management, and that will outlast the remediation process and allow for long term containment,” Dr McWatters said.

“While the plastic liners are good at stopping fluid migration, they can get punctured by rocks in the soil. The clay liners, on the other hand, self-heal if they’re punctured, but they operate best at about 60 per cent hydration. As Antarctica is very dry, the clay liner can often be unevenly hydrated or even desiccated.”

In the first five years of biopile operations the team included small ‘coupons’ of each material beneath them, which they removed each year to see how they changed in response to freezing, thawing, puncturing by rocks, and exposure to contaminants and ultraviolet (UV) light.

The team also has test plots of the materials on the ground and on top of shipping containers, where materials are exposed to wind, snow and UV. These materials are regularly inspected and tested to see how each is performing.

“The tests are essential to demonstrating that these materials are performing as intended and preventing fuel contaminants from entering the environment,” Dr McWatters said.

“They have also allowed us to identify the best combinations of liner materials for Antarctic conditions, which are used in the construction of new biopiles.”

The tests include X-ray analysis of the clay liners to see deep into the structure of the clay and understand how it changes with freezing, thawing, wetting and drying.

If the clay has dried out after being hydrated above 60 per cent, for example, hexagonal patterns appear. If the clay has not been hydrated at all, it appears as black areas of powdered bentonite. Hydrated clay appears white.

“We used to send our clay liner samples to Queen’s University to X-ray, but it meant we couldn’t do the sampling in real time,” Dr McWatters said.

“So we approached Polar Medicine to see if we could use the Casey station machine.”

Chief Medical Officer, Dr Jeff Ayton, obtained the required approvals from the Australian Radiation Protection and Nuclear Safety Agency, for the station doctors to use the medical diagnostic X-ray equipment to analyse scientific samples.

“Now we can determine the moisture content and structural change in samples immediately, which means greater accuracy,” Dr McWatters said.

“We can also X-ray clay liner samples from the test plot on a weekly basis to see what happens to them over the summer season, and test other samples under repeated freezing, drying and rehydration.”

The team has found that even within one biopile, the clay liner has areas that are frozen, wet, partially hydrated, desiccated (wet and then dried) and dry (never wet) - demonstrating the challenges of an Antarctic site.

“The risk with clay liners is that they don’t hydrate in the first place,” Mr Spedding said.

“But when they do and then they dry out again, how does that affect performance? And how does freezing compound any changes in performance? These are questions that are very rarely studied in the field.”

Future testing will help to answer these questions. So far though, the research has shown that the variation in bentonite content in the liner (the amount and type of clay) is more important to its performance than it is in temperate conditions, and that the preparation, hydration and drainage of the subgrade on which the biopiles sit, is critical.

“This is the first and only detailed study of these liners in Antarctic conditions and its showing in real time how these liners are performing,” Mr Spedding said.

“Because of the performance testing and monitoring conducted to date, these liners can be confidently reused for new biopiles, which minimises the cost of rebuilding.

“This work is relevant for future clean-up projects and any engineering project where geosynthetics are used in the Antarctic, and it’s also contributing to industry development of new products.”

Wendy Pyper
Australian Antarctic Division

*Australian Antarctic Science Project 4036