Summary

Sammenligning af oprensningseffekt ved airsparging og kemisk oxidation

This project has been carried out under the Danish EPA's Technology Programme for soil and groundwater contamination in cooperation with the County of Funen.

At a former dry-cleaning establishment in Faaborg, remediation of tetrachlorethylene (PCE) contamination has been carried out using air sparging and vacuum extraction. Remediation left rather heavy contamination in the soil and groundwater. Attempts have subsequently been made to clean up this contamination using chemical oxidation by injection of potassium permanganate as the oxidation agent.

The remediation was carried out with the purpose of securing the indoor climate in the flats above. In general, the purpose of the Technology Programme has been to illustrate the differences in clean-up effects of air sparging and chemical oxidation in the saturated zone.

Before the clean-up was started, the contents of PCE in the groundwater were measured at to up to 64,000 mg/l and up to 1,300 mg PCE/kg TS in the soil. The secondary aquifer below the buildings consists of fine to coarse-grained melt water sand with local elements of silt. The layer below the aquifer is moraine clay.

After air sparging for 16 months, a remarkable reduction of the contamination could be observed, but heavy contamination in the source area was still observed consisting of up to 13,000 mg PCE/l in the groundwater and 160 mg PCE/kg TS in the soil in the saturated zone. Subsequently, it was decided to supplement the remediation with chemical oxidation.

For this project, an injection strategy of adding potassium permanganate in several rounds of doses was chosen. A total of 82 m³ potassium permanganate solution equivalent to 2,700 kg pure potassium permanganate was injected.

In order to investigate the clean-up effect and spread of potassium permanganate to the groundwater, water samples were taken regularly to examine colour changes, chlorinated solvents, inorganic parameters, heavy metals and bromide traces.

The injection of potassium permanganate was more difficult than expected primarily because of the risk of inflow to a leaky sewer below the basement in Grønnegade 41. This meant that the planned volume of permanganate could not be added. However, in general, good horizontal and vertical distribution of potassium permanganate in and immediately downstream the hot spot was seen.

In the source area and immediately downstream, a significant reduction in the contamination level was achieved using chemical oxidation. It is assessed that the residual free phase in this area has, on the whole, been fully remediated. However, a small amount of residual contamination is still present in the source area, but measurements of the indoor climate show that this contamination does not pose a risk for the indoor climate in the property.

In areas outside the actual injection area, where the distribution of permanganate was not been quite so good, the effect of the clean-up is also not so good, as the permanganate has spread - primarily at the bottom of the aquifer. This indicates that the permanganate, similar to the contamination, sinks to the bottom of the aquifer of the area.

A comparison of the clean-up effect of air sparging and chemical oxidation in the saturated zone shows that the by far largest reduction of the contamination level occurs when using chemical oxidation. It is assessed that in general the chemical oxidation is more effective than air sparging in relation to clean-up in both the horizontal as well as the vertical level. For both methods, the limitations in accessibility makes it difficult to carry out a complete clean-up in the saturated zone.

Addition of potassium permanganate causes diluted heavy metals to be added to the groundwater from the injection fluid. Apart from chrome-6, the contents of heavy metals are, however, relatively low. Furthermore, mobilisation of chrome and zinc also takes place because of the chemical oxidation. However, the results of the measurements of heavy metals show that a substantial spreading of heavy metals in the groundwater downstream does not take place.

In general, experience with chemical oxidation in the area shows that the distribution of permanganate is the focus point. If, for a longer period of time, the distribution of permanganate is optimal, it is possible to clean-up even very heavily contaminated areas.

On the basis of the experience from this project, it is assessed that the saturated zone could have been cleaned up more effectively and cheaply using only chemical oxidation (without air sparging). Some of the costs of the air sparging could, instead, have been used to set up a more effective injection system for chemical oxidation.