Summary and conclusions

Fytoremediering af PAH- og olieforurening på nedlagt asfaltværk

In the period 1999 to 2006, a phyto-remediation project was performed at the site of a former asphalt plant in Vassingerød, Allerød, by a partnership between the Danish EPA and the County of Frederiksborg,.

An area in the northern part of the site is contaminated with mainly diesel oil , and an area in the southern part is contaminated with tar components (PAH’s). The diesel pollution has been found in concentrations of up to 1,600 mg/kg soil dw (dry weight) at a depth of approx. 1.5-2 m b.s. (below surface). The tar-pollution has been found in concentrations of up to 2,000 mg/kg soil dw at a depth of approx. 2 m b.s., and has been registered to at least 5-6 m b.s. Furthermore, there are high concentrations of hydrocarbons, BTEX’s and phenols in the surface-near groundwater. The soil at the site consists mainly of sandy clay/sandy till.

This site was chosen for the phyto-remediation project because it was contaminated with both diesel oil and tar, and further it was possible to investigate the method’s effect on the groundwater.

In 1999, willows and poplars were planted in rows, with a distance of 1 m between rows, and 0.5 m between individual trees. The cost of planting, re-planting where necessary, weeding and fertilization amounted to DKK 121,000, excl. VAT (moms), for a planted area of 3500 m², over a period of 6 years.

Throughout the period, a number of pollution components and parameters of groundwater chemistry were analyzed in the surface-near groundwater, as a measure of the changes in the concentrations of pollution in the surrounding soil. Also, three times throughout the period, soil samples were taken for analysis from two small areas in respectively the northern and the southern polluted areas of the site. Also, it was monitored, whether the pollution could be detected in the primary groundwater.

The results of the soil analyses show that, with the conditions of this site, it was not possible to get a clear picture of the changes in the soil contamination over time by doing repeated sampling within small lateral distances. Large variations over time were also seen in the concentrations of pollution components in the surface-near groundwater. It is estimated that generally a reduction in the concentrations of BTEX’s and naphthalene occurred. Possibly a reduction in the concentrations of phenols and total hydrocarbons also occurred. No reduction in the concentrations of other PAH’s or NSO compounds were observed. Because of the large variations, it was not possible to calculate degradation rates.

The changes in the parameters of groundwater chemistry indicate great biological activity and a tendency towards less reduced conditions in the surface-near groundwater.

It is not known, whether the reduced level of pollution in the surface-near groundwater is caused by the planting at the site, or whether it would have occurred without planting, because of natural attenuation. The most recent soil analyses also show that the soil at the site is still contaminated with hydrocarbons and PAH’s in concentrations which are considerably above the criteria of the Danish EPA for clean soil. Therefore, it can be concluded that the time period of 6 years is far from enough for phyto-remediation of this site.