Nature even better at cleaning up soil than expected

Published: 16 February 2021

The general approach to contaminated soil is “isolate and monitor”. Because removing the soil is often not an option, either because of the depth of the pollution or other causes. In those cases, biodegradation by “toxophile” bacteria can be a solution. Johan van Leeuwen wondered about the exact conditions in which specific substances are degraded and whether we can encourage the bacteria to do this more efficiently. His research shows that remediation with bacteria can be better and faster than previously thought. “For example by controlling the nitrate levels in the soil.” He will defend his doctorate thesis at Utrecht University on 17 February.

New openings for the biodegradation of toxic substances

Van Leeuwen is a geohydrologist and civil engineer at Utrecht University and Deltares. At Deltares, he works in the joint biochemistry lab Castel, a laboratory that is shared with Utrecht University and TNO. This is where he simulated the composition of the soil in order to test, in perfect conditions, which other substances get the bacteria to work more efficiently.

Grift Park

Van Leeuwen: “I saw in the lab that adding certain substances furthered biodegradation and led to an increase in the size of the bacterial population. But a simulation in the lab is not the same thing as real soil. There were plenty of opportunities for field research (approximately 250,000 locations) but fieldwork nearby in Amersfoort, and later in Grift Park in Utrecht, where there used to be gas factories, provided the link to practice.”

New degradation pathway

Stimulating the soil bacteria on site produces measurable positive effects after one year. Nitrate is a good example of a substance that contributes to this process. “It is important to keep the location isolated and to continue monitoring so that the nitrate cannot enter the surrounding groundwater and surface water, but that is also the case with the toxins at the location. The facilities required are often in place already.”  The researcher does emphasise that new degradation pathways depend on the conditions:  the particular location, the level of pollution and what else is in the soil.


Soil contamination can be found on the sites of both old and modern factories: mono- and polyaromatic hydrocarbons such as benzene, toluene, indene and naphthalene can be present in aquifers below the surface. They were left behind on old factory sites as a result of discharges when little was known about them; on new sites, incidents such as leaks are often the cause.

Follow-up research

Van Leeuwen will soon be awarded his doctorate but he is continuing to look for difficult subsurface puzzles to solve. A second phase of the study will begin this year at the former Grift Park gasworks. In addition, Van Leeuwen is also developing a study at Deltares into plastic pollution in the soil and banks.

Doctorate: Biodegradation of mono- and poly aromatic hydrocarbons in a contaminated aquifer originating from a former Pintsch gas factory site. Laboratory and field investigations – News – Utrecht University (