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Thermally Assisted Vacuum Extraction of PCE. Main Report

Summary and Conclusions

This project was carried out within the Danish Environmental Protection Agency technology programme fpr soil and groundwater contamination in cooperation with the Funen County.

An investigation in 1999 revealed a serious tetrachloroethylene (PCE) contamination of soil and groundwater under a cleaning establishment at 28 Vesterbro in central Odense. In the soil vapour, contamination has spread over an area larger than 10,000 m2 and has affected the indoor climate in hundreds of flats and houses. The contamination level was so high that the health inspector declared several flats and houses unfit for humans.

The soil contamination of the unsaturated zone was cleaned by vacuum extraction, supplemented by heating of the source area by means of steam injection. The groundwater contamination was subsequently cleaned by chemical oxidation by injection of potassium permanganate.

The purpose of the preventive measures was to secure the land uses allocated in the residents affected by contamination from the premises at 28 Vesterbro. The principal purpose of the technology project was to document whether the heating of the unsaturated zone furthered the remediation compared to ordinary "cold" vacuum ventilation, whether it was possible to control the heating, and whether the heating resulted in an inappropriate spreading of the contamination.

Geology and geohydrology

The strata of the location are 1-2 m filling, underneath approx. 8 m sand with some silt and deeper down moraine clay. The groundwater level is free and 6-7 m below surface.

Contamination

The investigations carried out show a serious contamination by chlorinated solvents in soil, soil vapour and groundwater. In the source area the soil contained free PCE phase with maximum contamination of soil up to 26,000 mg/kg TS. The highest concentrations are related to the silted strata from depth 1 to 5 m. Also in the pore vapour very high contents of PCE with concentrations up to 66,000 mg PCE/m3 were found. In the upper secondary reservoir (groundwater close to surface level) a contamination of PCE contents up to 13,000 µg/l was found. Contamination with PCE contents of up to 3-4,000 µ/m3 from the cleaning establishment affects the indoor climate in hundreds of flats in the central part of Odense. For comparison the quality criterion is 6 μ/m3.

Preventive strategy

Contamination in the unsaturated zone was cleaned by means of vacuum ventilation. In order to achieve a rapid release of PCE from the small-grained strata, cleanup of the unsaturated zone was supplemented by heating of the soil by steam injection. As the contamination was only 1-2 m below the existing houses, it would be expensive - perhaps impossible - to carry out the heating by injection of the steam from the outside and towards the source area without an unacceptable impact on the installations in the soil and the floors in the houses. Consequently the strategy was to heat the most polluted part of the soil from below, mainly by heat conduction. Air was added to the injected steam in order to minimise possible condensation of PCE.

In the saturated zone polluted groundwater was pumped up for plume check during the steam cleanup operation. Potassium permanganate was injected subsequently for cleanup of the groundwater contamination in the source area and immediately downstream.

The cleanup strategy applied, involving steam injection, has caused some concern, as there was a risk of uncontrolled spreading of contamination both vertically and horizontally and spreading of steam to the above buildings. Another concern was whether the heating would result in subsidence of the buildings or damage to the underground installations. These conditions were studied in the design phase, and the cleanup was planned to meet these problems. During the design, modelling of the steam spreading was made to support the dimensioning of injection quantities and depths. The critical temperature was estimated to be 50°C in a 1.5 m depth. Further, it was calculated that the maximum extent of the steam zone must not exceed 3 m below surface level.

Treatment facility

Because of difficult access conditions the vacuum extraction was carried out by means of two 40 m long horizontal borings, positioned approx. 4 m below surface level. The borings were carried out as horizontal borings and from behind - the so-called blind-hole technique - each with four 50 mm steel filters to improve the distribution of the airflow over the long filter stretch. Furthermore, two vertical ventilation borings were established in the most polluted areas. As the heating of the soil is considerable, the borings affected by the steam injection were carried out in heat-resistant material (steel or polypropylene). For tightening of the borings cement-stabilised bentonite was used. The choice of material and the application of cement-stabilised bentonite functioned satisfactorily. None of the borings were damaged by the heat, and sealing by means of the Great-Belt-mixture was highly efficient, preventing spreading of steam upwards through the borings.

The extracted air was cleaned through charcoal filters. Ventilation pump, compressor, cooling plant, water separator, manifold system, flow meters, manometers, frequency converters and other equipment were built into a silenced 20" container. The steam was generated from a mobile steam plant with oil-fuelled boiler. The entire plant was equipped with a control and monitoring system.

Monitoring

As the steam injection took place under a residential area, a comprehensive monitoring programme was established in order to monitor the extension of the temperature increase and possible geotechnical changes of the soil. Furthermore, comprehensive monitoring of contamination in the saturated and the unsaturated zones was made to ascertain possible inappropriate spreading of contamination components as a consequence of the steam injections. Additionally detailed monitoring of the substance removal was made by measuring the contamination contents in the extracted soil vapour and groundwater.

Operation

The operation of the in-situ cleanup operation was initiated on 4 January 2001. The first phase of the in-situ cleanup included traditional vacuum extraction from the 10 vacuum filters and pumping up of groundwater downstream the source area. Approx. 500 m3 soil vapour was extracted per hour at continuous operation. After approx. 4½ months‘ operation of the vacuum ventilation the injection of steam was made from one boring in the source area through a 1.0 m filter, which was placed around the groundwater level (6-7 m depth). In total the steam plant was operating during 3½ months, and in total 231 t steam was injected. The injection rate was typically 150-200 kg/h. Compressed air was added to the injected steam in order to reduce condensation of PCE in the steam front and to minimise the loss of PCE to the saturated zone.

The temperature measurements show that an area of approx. 400 m2 was affected by the steam injection; the temperature in an area of approx. 300 m2 was heated to temperatures exceeding 90°C (radius of approx. 10 m from injection boring). The steam front was spread mainly horizontally off the steam boring. Heating was heaviest from the groundwater level at 6.5-7 m depth to 4 m below surface level. In the hotspot area the steam zone had spread to a depth of 3.5 m. Heating above 3.5 m depth took place primarily by heat conduction. The measurements of the extension of the temperature increase show that the temperature of the steam/air mixture was controlled in such a way that the vertical condensation front did not exceed 3.5-4 m below surface level. According to model predictions it can be generally concluded that the extension of the steam zone can be widely controlled by means of the flow caused by the vacuum extraction. It should, however, be noted that this conclusion is only valid in cases where the vacuum extraction is carried out immediately above the injection.

Operational problems

The project was not carried out without difficulties. The most significant operational problems were to achieve sufficient cooling during the steam injection. Further, noise from the cooling compressor made it difficult to prevent noise levels above 40 dB in the night. Other problems were clogging of the steam injection boring, frequent inexplicable stops of the steam generator and poor capacity of some of the horizontal vacuum extraction filters. In spite of the above problems it was possible to keep the time schedule for the cleanup, and the intensive cleanup of the source areas was completed 3 months ahead of schedule.

Effect of the cleanup

A total of approx. 3,580,000 m3 soil vapour was extracted, corresponding to an exchange of the soil vapour of approx. 2,400 times. There was an exponential decrease of the PCE concentrations from 5,500 mg/m3 to 4-5 mg/m3 during the period before the steam injection. During the steam injection the PCE contents increased to 120 mg/m3. When the steam injection ceased, the PCE contents were approx. 10 mg/m3. After 16 months‘ operation period, the PCE contents were below 1 mg/m3. The process was stopped, and the equipment disassembled in June 2002.

Before the steam injection approx. 250 kg PCE was removed by vacuum extraction, and 3½ months later approx. 280 kg had been removed, i.e. an increase of 30 kg. If no steam had been injected, 5 kg PCE would have been removed during the same period, i.e. the substance removal gain has been approx. 25 kg, corresponding to a reduction of the working time of at least two years. The relative part of PCE removed during the steam injection was smaller than expected, representing only approx. 10% of the total substance removal at the vacuum extraction. The calculation of the substance removal is of course somewhat uncertain, as only random sampling has been made of the contamination in the extracted soil vapour. Boiling of free phase PCE might have taken place within short periods of time and has not necessarily been registered in connection with COWI‘s measurements. Consequently, it cannot be ruled out that the substance removal is underestimated. Further, it is not known whether wet oxidation of PCE has taken place as a result of the heating, during which PCE is transformed to carbon dioxide and water. Although only about 10% of the contamination was removed during the steam injection, experience from other in-situ projects shows that it is in fact the remaining 10% of the contamination that causes problems with repercussion and prevents many in-situ projects from being completed within a reasonable time frame.

The results show that the contamination contents in the soil vapour within the influence area were reduced very efficiently. After 1 year‘s operation the contents had been reduced from up to 66,000 mg/m3 in the source area to below 5 mg/m3. Thus, the results show that no inappropriate spreading of PCE in the unsaturated zone had taken place as a result of the steam injection. Outside the theoretical influence radius of the ventilation (i.e. areas with vacuum below 1 mbar) a significant reduction of up to factor 100 had also taken place as a consequence of the vacuum ventilation.

The results of the repercussion measurements show that the contamination levels in the source area are within the interval of 1.3 to 5.7 mg/m3, which is very low compared to the contamination level before the cleanup, when contents up to 66,000 mg/m3 were registered. The source area below the spillage container has been cleaned up, but minor contamination in the area of a more diffuse character remains.

The contamination contributed by PCE from the soil contamination to the indoor climate in 30 Vesterbro is now far below the quality criterion, which is 6 µg/m3. It is assessed that the indoor climate problems in the entire area of Vesterbro have been solved.

The contamination level in the pumped-up groundwater shows a general reduction of the contamination level. The substance removal through the pumped-up groundwater was very small, less than 1 kg.

During the cleanup operation the contamination contents in the very source area around the spillage container was reduced significantly, from a level of 10-15,000 µg/l to less than 100 µg/l. The results of the steam injection show that — contrary to expectations - there was no loss to the groundwater in the source area as a result of the steam injection. It is assessed that owing to the addition of air to the steam, the released PCE contamination in the unsaturated zone was led to the vacuum extraction plant and not condensed and led to the ground water.

Radon measurements

The investigation demonstrates that radon can be used as trace gas for soil vapour penetration into houses situated on sites polluted by cleaning establishments and for assessment of the influence zone of vacuum plants. Application of continuous radon measurements seems especially suitable for investigations of areas, where preventive measures have already been installed (i.e. where "on and off" measurements are possible).

Geotechnical conditions

Precision levelling shows that there was a general elevation of the foundations closest to the steam injection of 1-3 mm during heating. During cooling the level was rapidly reduced to the initial level. Subsequently a subsidence of typically 1-5 mm compared to the initial level before heating has taken place.

Environmental assessments

For heating, approx. 20,000 l fuel oil was used for the injection of 230 t steam. For the operation of vacuum pump, cooling plant and compressor approx. 123,000 kWh was used, of which approx. 50,000 kWh results from steam heating.

It appears from the environmental assessment that the most serious environmental effects are the greenhouse effect, bulk waste and hazardous waste. The environmental gains are persistent and human-toxic effects. In general, it is assessed that the environmental impacts in connection with the remediation strategy correspond fairly to the benefits gained. It is, however, assessed that impacts exceed the gains slightly. On the basis of the calculations and assessments made, the environmental budget is exceeded, and, thus, there is no significant gain to be achieved by cleanup by the remediation strategy. Environmental impacts result primarily from the large energy consumption. The method does not allow for psychological aspects related to affected houseowners or financial conditions. Taking into consideration that hundreds of persons have been affected by the contamination both financially and psychologically, the cleanup is considered to present a considerable social gain.

Economy

The total expenses for the cleanup operation was approx. 5.8 million DKK excluding VAT. This amount includes expenses paid for by the Technology Fund, amounting to 1.3 million DKK. A corresponding commercial cleanup is assessed to amount to 4 million DKK today.

Putting into perspective

Recent years‘ testing of many new preventive measures that turn up constantly shows that there are more than one efficient method to solve a contamination problem. The Vesterbro project is one example of how a problematic contamination under difficult access conditions is handled by combining several different preventive measures in a favourable way. It is assessed that the proper combination of different methods is the key to solving the contamination problems in soil and groundwater.


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