Steam-enhanced remediation of contaminated site
Summary and conclusions
Removal of soil- and groundwater contamination using steam-enhanced remediation has gained ground through the latest years, especially in the USA. Here in Denmark, five steam-enhanced remediations have been started up and/or concluded through the latest 5 years. Under the Programme for Technology Development, the Danish Environmental Protection Agency has financed special activities for development and documentation of this technique on four of these sites. A picture emerges of a promising technique with an efficient and fast source removal in the unsaturated zone as well as in the saturated zone. But a picture is also emerging of a technique with special requirements to design, operations and monitoring.
In this report, a steam-enhanced remediation of a former dry cleaner site is described. NIRAS Consulting Engineers and Planners A/S and Ove Arkil A/S carried out the remediation in autumn 2000 for the County of Northern Jutland.
The dry cleaning facility has been in operation for 54 years until 1990 and has caused a significant soil and groundwater contamination with turpentine (stoddard solvent) and PCE. The strata on the locality consist of sandy filling down to approx. 1.5 meters below ground surface, hereafter sand down to approx. 3.5 meters under the ground surface. Hereunder Yoldia clay with an expected thickness of minimum 5 meters. An unconfined groundwater aquifer with water table surface approx. 1.7 meters under the ground surface has been located.
The contamination is found both in the unsaturated zone and in the saturated zone down to approx. 3.5 meters under the ground surface. The largest part of the contamination is situated under buildings.
The steam-enhanced remediation is carried out by means of 11 injection wells. 8 of 11 injection wells are placed in the periphery of the contamination, whereas three injection wells are placed centrally in the source zone in order to ensure a sufficient heating in the middle of the contamination at the end of the operation period. Groundwater and soil vapour/steam were extracted from two wells in the centre of the source zone.
The steam-enhanced remediation has been carried out over a period of approx. 4 months - 3 months of continuous operation and 1 month of cyclic operation. At the conclusion of the remediation approx. 1000 kg of contamination - mostly turpentine - were removed, which is evaluated to be the major part of the contamination on the site.
The actual distribution of the steam fronts has continuously been monitored during the remediation by means of temperature measurements in and around the remediation area. Also removal of contamination has been monitored and a series of technical operation data have been registered. Furthermore, the following activities, financed by the Danish Environmental Protection Agency (the Technology Development Programme), have been carried out as part of the remediation:
 | Extended temperature monitoring |
| Monitoring of vertical pressure distribution |
| Determination of soil parameters |
| Evaluation of the extent of hydrous pyrolysis oxidation (HPO) |
| Extended documentation of possible vertical mobilisation of free phase |
| Monitoring of relative humidity in the extracted vapour phase |
| Extended monitoring during cyclic operation. |
In the following, a series of important conclusions and experiences from the steam-enhanced remediation, among these the monitoring carried out under the Technology Development Programme, are summarised:
Steam stripping is an aggressive technique that requires intensive monitoring and supervision, and a fast decision-making from site owner, consultant and contractor. Furthermore unintentional stops of operation of the extraction system can cause an unintended heating outside the remediation area.
When the steam stripping is performed close to the surface and below buildings there is a high risk of formation of cracks in walls and ceilings. Furthermore the sewer system can be heated, which can cause obnoxious smells in the area around the site.
The extended horizontal and vertical temperature monitoring, turned out to be extremely valuable for the continuous evaluation of the functionality of the system and as basis for changes in the operation strategy. At similar remediations, a corresponding detailed monitoring programme is recommended established to ensure sufficient control and adjustment of the operation.
On the basis of the experiences in this project, it has been evaluated that sensors to be used for pressure measurements in and around steam zones as well as sensors for humidity measurements under conditions as in the extracted vapour phase are not commercially accessible.
The monitoring of the CO2 gives indications that a certain HPO of the contamination has taken place during the steam injection on the site. The amount of the contamination removed by HPO might be of the same magnitude as the contamination removal by means of extraction of water and gas phase and as free phase liquid.
In this project there are found no signs of significant vertical spreading of contamination into the lower clay layer. However, on other sites with fractured or sandy clay it can not be precluded that such a spreading can occur.
The monitoring of the contamination removal during the cyclic operation was carried out manually with a PI-detector twice a day and with regular laboratory analyses. The manual measurements in the extracted vapour phase were of vital importance to the cyclic operation of the system and formed the only reliable basis for decisions about the duration of each phase.