Naturlig nedbrydning af olie og chlorerede opløsningsmidler i grundvandet på Drejøgade 3-5.

Summary and conclusions

This project comprises natural attenuation of oil components and chlorinated solvents in the groundwater at the locality Drejøgade 3-5, Copenhagen. Four sources of contamination have been observed at the site: One oil source and one PCE source, which coincide at the centre of the site, and two PCE sources on the south-western part of the site.

Purpose

The purpose of the project has been to perform a detailed redox characterisation of the groundwater at the site. The degradation processes which take place at the site can then be characterised and quantified, and the degradation rates for the given processes estimated. Furthermore, the intention of the project has been to evaluate selected methods to analyse and sample oxygen, iron and methane in groundwater.

Background data

The background data for the investigations is field measurements and lab-analyses, carried out during two monitoring campaigns in 1998. The data has been used to map the redox conditions in the groundwater reservoir, to assess and quantify the oil degradation, to assess the degradation of the chlorinated solvents, and furthermore to correlate the spreading of the contamination plume, the redox conditions and the extent of degradation at the locality.

Sampling

For sampling of very volatile components, it is recommended that methods preventing contact with atmospheric air are adopted. Furthermore, it is recommended that oxygen is measured directly in the field in a flowcell, and that iron is filtrated during the sampling procedure in the field.

Redox conditions...

In Drejøgade, strongly reduced conditions (sulphate reduction and methanogenesis) are found at the oil source, iron and sulphate reduction is seen in the plume down streams and aerobic and nitrate reducing conditions are seen in the remaining part of the site.

...and contaminants at Drejøgade

The oil components are degraded from a concentration level at 12 mg/l at source to 0.5 mg/l total hydrocarbons approx. 60 m downstream from the source. It is assessed that 25-75 kg oil/year are degraded at and downstream of the source. First order degradation rates are estimated at 0.008 – 0.01 day^-1 for total BTEXs and 0.005 – 0.007 day^-1 for total hydrocarbons. The degradation causes the sulphate reducing and the methanogenic conditions, which are observed at the oil hotspot.

Contaminant plumes

Two types of chlorinated solvent plumes are observed: A plume coinciding with the oil plume and one solely containing chlorinated solvents. In the oil plume, where there are strongly reduced conditions, a significant reductive dechlorination takes place, in that 99.5% of the content of PCE is transformed into degradation products. 3.27% of the PCE is completely transformed into ethene. In the other plume with aerobic and nitrate reducing conditions, the dechlorination process is not as effective, since only 4.9% of the PCE is transformed into degradation products. No PCE is completely degraded into ethene.

For both oil components and chlorinated solvents, the groundwater concentrations are observed to decrease with depth (down to 15 mbgl) , which is why the characteristic redox zonation observed in the upper (white) filter (9,5-10 mbgl) in the saturated zone is not seen in the deeper groundwater filter.

The investigations in Drejøgade have clearly confirmed the expected connection between the oil spreading, the redox chemistry, and the degradation potential for chlorinated solvents in the aquifer. The most reduced conditions are found where the oil concentrations are the greatest, while the less reducing or the oxidating conditions are found in the groundwater unaffected by the oil plume. In the strongly reduced zone, most PCE is degraded, and complete dechlorination occurs. Here, the chlorinated compounds mainly comprise degradation products. In the slightly reduced zone, only a little amount of PCE is degraded, and complete dechlorination does not occur. Mainly higher chlorinated compounds (PCE and TCE) are found in this zone.

For the chlorinated solvents there is, as mentioned earlier, a close correlation between the redox conditions in the aquifer and the extent of the dechlorination. If the amount of organic matter is insufficient for the bacteria to generate the strongly reduced conditions in the aquifer, the degradation will only occur to a certain extend, and toxic degradation products (espec. vinyl chloride) will be accumulated. As PCE and TCE are not or only slowly degradable under aerobic conditions, it is a prerequisite for the application of natural attenuation as a remediation technology that these two components are degraded within the strongly reduced section of the contaminated plume.