Chlorinated Compounds in Soil-gas
- Fig 1. The soil gas probe with the sampling tube connected
Contaminated soil is a serious and wide spread problem that normally has to be solved to avoid pollution of the ground water reservoir. Many remedial technologies are available for solving the problem – which to choose depends on the pollution level, chemical substances in the pollution and the geological structure of the ground. Chlorinated solvents in the sub-surface are very common pollutants, and especially the two solvents PCE (Tetrachloroethylene) and TCE (Trichloroethylene) are often observed. A source of these pollutants is, for example, dry cleaners. At an actual site, a very cost effective method for in situ treatment of contaminated soil was tested. The operating agent for the project was NIRAS Consulting Engineers and Planners. It was sponsored by three Danish counties and the Danish EPA under the Technology Demonstration and Evaluation Programme.
The Monitoring Need
To be able to evaluate the efficiency of the remedial system, the amount of pollutant removed from the soil has to be monitored. This is done by measuring the flow and the gas concentration of the soil-gas leaving the one-way valve. As different compounds might be cleaned up at different speeds, it was decided to monitor 4 gases, simultaneously, during the test period. The Photoacoustic Multi-gas Monitor - INNOVA 1312-5 was chosen for this application. The low detection limit and the long term stability of the INNOVA 1312-5 are characteristics of the photoacoustic principle. Low drift (as low as one detection limt per 3 month) is another advantage of the photoacoustic principle.
The 4 gases were:
- Tetrachloroethylene (PCE)
- Trichloroethylene (TCE)
- 1,1,1-Trichloroethane (TCA)
- Chloroform (TCM)
The tested Technique
- Fig. 2 The working principle of passive soil vapour extraction. (where P = pressure) (Drawing courtesy of NIRAS)
Passive soil vapour extraction relies on naturally occurring variations in barometric pressure. In Denmark, this oscillation primarily results from the passage of weather fronts and can be as high as 25 mBar over a 24 hour period. As the barometric pressure rises air is driven into the soil. When the pressure drops, soil-gas will ascend into the atmosphere. When low permeable layers are present above the polluted volume, one can create a one directional flow from the perimeter of the polluted soil volume towards its centre by use of a number of bore holes and a one-way valve. This principle is outlined in Fig. 2.
The Solution
- The Photoacoustic Multi-gas Monitor - INNOVA 1312
The monitoring period lasted 34 days during which the soil-gas was measured for chlorinated compounds every 15 minutes. The following maximum concentrations were observed:
- PCE 15 mg/m3
- TCE 350 mg/m3
- TCA 61 mg/m3
- TCM 5 mg/m3
The measurement showed that the dominating substances in the soil-gas were TCE and TCA. By multiplying the flow rate in the soil-gas stream by the gas concentrations, the mass removal rate of the four substances was calculated. The mass removal rate of the TCE/TCA was 45g/24hrs, or 180 - 200g TCE/TCA for the total measuring period.
At this site, the removal rate of volatile compounds (primarily PCE and TCE) is expected to be 5kg pr. year. A quarterly monitoring of the gas concentrations and ground water concentrations is scheduled to indicate the long term effect of the remedial actions.
For more information please download the complete Application Note below - or contact LumaSense.