2 English resumé

Fytoremediering af kulbrinte- og anden forurening på nedlagt slamdepot

The purpose of this project has been to test plant-assisted clean-up, phytoremediation, as a possible technology for the remediation of organic contaminants in soil.

On the specific site, the Municipality of Copenhagen wanted to establish a knowledge base regarding the use of willows and poplars on sites contaminated with oil and PAHs. The specific purposes were:

To investigate the influence of the plants on degradation of oil and PAH in soil
To investigate how low the end concentration of oil and PAH can be brought using the specific plants
To document costs related to a plant-assisted clean-up
To decide the time necessary for plant-assisted clean-up technology.

The project was started in the spring of 1999 and ran until the end of 2005.

The site was a former sludge disposal site with very high concentrations of both organic and inorganic contaminants, a high content of organic matter and a very shallow aerobic zone. The project encompassed planting two test areas with willows and poplars, respectively, and comparing developments with respect to aeration, root development, plant health, degradation of oil and PAH, and uptake of metals in comparison to a reference area where nothing specific was planted, and weeding and maintenance was carried out in a similar manner to the planted areas.

The project has given the following results:

In the first year a great deal of the newly planted trees died due to lack of sufficient weeding and maintenance, so new trees had to be planted in 2000. After this, a number of trees died during the first years (app. 15% in each area), partly due to insufficient weeding, but probably also due to the very high concentrations of metals on the site, especially copper. The remaining trees have shown good growth throughout the project period. After six years the willows are 3 to 4 meters high, while the poplars are 4 to 6 meters high. Visual inspections of the leaves has shown however that at the end of the season several of the trees have wrinkled and spotted leaves, indicating less resistance to diseases. This apparently does not influence the overall growth of the trees.
Leveling of the site has shown that a settling of the site has taken place since 1999. The settling is greatest in the planted areas, where it is about 10 cm, while the reference area has only settled 3 cm. The difference can probably be ascribed to the greater biological activity in the planted areas.
Soil-gas sampling at different depths during the project period showed that the oxygen content in the upper soil layers has gone up in all three areas of the site (two planted and one reference). A comparison between the planted areas showed the oxygen content is highest in the area planted with poplars.

There is no obvious relation between soil-gas content, root development and degradation of contaminants.
Soil profiles in all three test areas showed that the “border” between the aerobic and anaerobic zone has moved downward during the project period.
The concentration of total hydrocarbons has decreased in all three test areas during the project period. The decrease is relatively higher in the planted areas (50 to 55 %) than in the reference area (app. 20 %). This indicates that the planting of the trees has enhanced the degradation of the hydrocarbons in the soil.
PAH-analyses showed no obvious trend with respect to either composition or content when comparing the planted areas with the reference area.
There is also no obvious trend in the content of the sludge related components (DEHP, NPE and LAS).
Analysis of nutrients in the soil showed generally higher content in the reference area, demonstrating that the planting of trees enhances the turnover of nutrients.
There is a very high level of contamination with heavy metals in all three test areas, with concentrations that place them in class 4 (the most contaminated) according to the soil classification system adopted by all counties and municipalities on Zealand.
Root counts in the 2 planted areas showed that for both areas the root intensity is higher in the aerobic zone than in the anaerobic zone. At the same time, the aerobic zone increased during the project period (and thus the root intensity) along with increasing root intensity in the anaerobic zone.
Sampling and analysis of groundwater samples showed no obvious effect from the plants. This was to be expected since the planted areas are much smaller than the contaminated area as a whole. However, a comparison of pore water samples taken in the root zone showed lower concentrations than those found in the groundwater as a whole.
Analysis of the metal content of the leaves indicated a relationship between the concentration level and the general health of the plants, since the majority of the metals are found in the highest concentrations in the leaves showing signs of disease.
Analysis of weeds (nettles) from all three test areas showed (as expected) that the highest concentrations of metals are in the roots. A comparison between leaves from nettles and from the trees shows that there are no obvious differences with the exception of copper, where the content is significantly higher in the nettles than the trees.
Uptake of metals was investigated by cutting and parting of two trees from each planted area. The analyses showed that the metal concentration was highest in the leaves, lower in the branches and lowest in the trunk.

Generally the metal content was higher in the willows than in the poplars, but there were large variations between the two trees from each area.

An estimation of the total uptake of metals in the trees showed that the uptake will not significantly reduce the metal content found in the soil. The exception is the cadmium uptake in willows, which over a 30-year period would lead to a lowering of the content to below the criteria for clean soil. This is comparable to results found in other investigations of cadmium uptake in willows.
The positive effect of the planting of the trees on the content of total hydrocarbons is probably primarily due to increased degradation of the components in the established root zone, maybe combined with some uptake into the trees.
With the very high concentrations found on the site, it has not been possible within the project period to give an estimate of the remediation time necessary to reach acceptably low concentrations (if at all possible). How low the resulting concentrations will be depends partly on the initial concentrations, and partly on the composition of the contamination present, including possible presence of inhibitory concentrations of e.g. heavy metals.
Clean-up using the tested technology and with an assumed clean up time of 10 years can be estimated to cost somewhere between 400 and 500 Danish kroner per treated tonne of soil. This does not include initial investigations but does include a fairly extensive monitoring program in order to document progress and point out possible needs for modifications (such as need for nutrients, etc.).

In general, the investigations carried out show that the planting of both willows and poplars does have an effect on the concentration of total hydrocarbons in the soil, since the reduction was substantially higher in the planted areas than in the reference area, It was also shown that very high metal concentrations will have an inhibitory effect on the development of a healthy stand of trees. In the light of the difficult conditions on the test site, including very high contaminant concentrations and anaerobic conditions at shallow depths, conditions which are not normally considered for phytoremediation, the planting of trees has had a surprisingly large effect compared to the non-planted reference area.