Nature based solutions for the remediation of contaminated soil and the restoration of soil functions

Related theme: Future-proof Site (Re) Development & Transformation

Vermilion is an oil and gas producer which acquired existing oil assets in France. Part of the assets included a former production and storage site of oil drilling sludge. With that acquisition Vermilion took on responsibility to remediate the site, restore the quality of the soil and make it fit for further use. 

Challenge

The site is in a rural area and had been used 40-50 years ago for natural gas extraction. It consisted of an extraction well that resulted in the production of 16,000m3 of hydrocarbon-polluted soil stored in sealed cells on the site.   

The volume of soil to be remediated and the remote location of the site made customary remediation techniques challenging. Biodegradation using aeration techniques would be difficult and costly as it was difficult to get equipment on site and due to the lack of energy sources. Excavating and removing the contaminated soil in those volumes was not considered feasible or cost effective. TAUW was asked to find another, low-tech solution. 

Solution

TAUW conducted studies on nature-based solutions in a project lasting 3-4 years. 

Nature-based remediation with Alfalfa 
The team used an agricultural technique called vegetated windrow to treat the polluted soil on site. The polluted soil was excavated and spread on the surface in 100m3 piles which were planted with grass and alfalfa. Initially grass helped as rapid soil cover to retain moisture, the growing vegetation was later trimmed and used as extra cover and amendment. Inoculated Alfalfa has remarkable soil-improving properties. It captures nitrogen from the atmosphere, thus eliminating the need for nitrate addition, and transfers it to the soil microorganisms through its deep root system, effectively enhancing hydrocarbons degradation in soil.

Study outcomes
In agreement with competent authorities, surface soil contamination needed to be reduced by 90%. The research team conducted on-site pilot tests to validate the treatment conditions and monitor the degradation rates, and changes in biological soil indicators. Their findings showed a 91% reduction of long-chain and short-chain hydrocarbons in a 3–4-year period.  
 
The study also computed the carbon footprint of this project; comparing traditional soil disposal to off-site facilities or mechanically aerated biopile with the carbon footprint of these vegetated phytogenic piles. The findings showed that this study had a lower carbon footprint, cost less energy and the growth of plants even converted them into a carbon sink.  

Impact 

Beautiful biodiversity
This highly innovative study shows the value of nature-based solutions and wider benefits to soil health and ecology. The contaminants can be remediated on-site, preserving valuable soil resources. The revitalised soil increases biodiversity below ground (bacteria, fungi, nematodes, etc.) and a variety of vegetation, flowers, insects, birds, and mammals above ground.  

The study also yielded interesting scientific data, such as consolidated information on degradation rates for hydrocarbon molecules, which is of interest to scientists in this field. The project is a fitting example of how a constraint can be turned into an opportunity: phytoremediation is a cost-effective, ecological, and replicable solution.  

Lastly the study won the award for climate action in the field of Polluted Sites and Soils by UPDS, the Union of Site Remediation Professionals in France. 

Partnership

This study brought together various areas of expertise in TAUW as well as close cooperation with scientists, local seed manufacturers and landscape architects.