3 Summary and conclusions

Fytoremediering af olie- og benzinforurening på nedlagt benzinstation

I order to evaluate the potential of phytoremediation of gasoline and diesel spills at former gas stations, a full scale field test was undertaken at a former gas filling station in Axelved, Rønnede, Denmark.

The site was chosen primarily because the polluted area was well defined and the pollution was near the surface.

In spring 1999 an area of 1.400 m² was planted with 2.500 willow and 500 poplar cuttings, surrounded by a shelterbelt.

The test was financed by Oliebranchens Miljøpulje (Danish Oil Industries' Environmental Clean-up Association) and the Danish Environmental Protection Agency. The total cost was app. 200.000€.

The purpose of the test was:

to examine how the hydrologic cycle was affected by the plantings,
to what extend the microbial degradation of hydrocarbons was affected by the trees,
to examine what end levels of contamination should be expected
to document the cost of preparing, planting and monitoring similar phytoremediation projects,
to establish a timescale of phytoremediation of gasoline and diesel.

The field test included:

Screening of soil pollution
Collecting climate data, such as precipitation, wind and sun radiation
Monitoring groundwater levels at different depth in the subsoil
Monitoring hydrocarbon concentrations and inorganic ions in the groundwater
Plant growth

In the period from October 1999 to November 2005, 14 monitoring rounds were performed.

Groundwater chemical data suggest a reduction of hydrocarbons in the near surface groundwater, the main reduction being in the first two years where the rooting was poor. This is ascribed to the accelerated microbial activity due to the 40 tons of fertilizer that was applied.

Screenings suggests a substantial reduction of hydrocarbons in the soil to levels between 37 % and 50 % in the period from 2001 to 2005. However in some smaller areas the soil still contains very high concentrations of hydrocarbons and mobile free phase.

The trees were not able to lower the groundwater table significantly and thus not able to increase the oxygen level in the contaminated soil.

In the period from 1999 to 2005 the trees has grown from 50 cm cuttings to trees with a height of 7 to 10 meters. There is apparently no correlation between growth and the contamination level in the soil