TEMA 19. Descontaminación de suelos contaminados
4 Casos prácticos
Practice case 2:
Soil vapour extraction and lowering of groundwater level, Denekamp
General
Laboratory tests or pilot plant investigations were not necessary for this project, as the design was based on expert judgement and knowledge. A straightforward in situ remediation approach was chosen.
This project demonstrates that not all remediation projects need to be preceded by extensive studies and investigations. The need for them depends on the contamination situation and the soil structure/stratification.
Contamination situation
The petrol station in Denekamp that caused the contamination had been situated there for a long time. After the company ceased operations, a soil investigation was carried out in order to find out whether the site was contaminated with mineral oil.
The investigation proved that the soil at the former service station and beneath the existing showroom was contaminated with mineral oil and aromatic compounds in concentrations exceeding the Dutch Intervention Value. The contamination extended to a depth of ca. 2.5 m -g1.
The groundwater also contained mineral oil and aromatic compounds in concentrations exceeding the intervention values. This contamination extended to a depth of 5 m -g1. The groundwater table is situated at 0.9 to 1.4 m -g1. The soil consists of moderately fine sand.
Weighing of the remediation options
It was decided to apply a combination of in situ remediation techniques and excavation. A part of the contamination outside the buildings was excavated, as it was easily accessible and a fuel tank present there had to be removed anyway. The remainder of the contamination, which could not be excavated because of the presence of buildings, was remediated by means of soil vapour extraction. For this purpose, the groundwater table was lowered to 2.5 m -g1.
In this project the groundwater table was lowered to below the soil contamination, in order to remove the entire contamination by soil vapour extraction. This was done because in this project the desired lowering of the groundwater table could be achieved by a low extraction rate (2 to 3 m3/h), and because installing air injection filters beneath the showroom would have been quite inconvenient for the owner.
Detailed design and installation
Given the extensive knowledge available on the biological degradation of mineral oil and aromatic compounds, it was decided that additional investigations were not necessary. Another reason for this was the availability of well-documented practice cases where soil vapour extraction had been applied in comparable types of soil.
The installation of horizontal drains by means of horizontally directed drillings is a proven technique. Therefore, both the soil vapour extraction drains and the groundwater withdrawal drains were installed beneath the showroom by means of this technique.
A cross section of the system is presented in Figure 5.4.
Figure 5.4 In situ remediation system
Four horizontal drains were installed to extract the soil vapour. The calculated air flow rate was ca. 50 m3/h at an underpressure of 50 mb. The initial total hydrocarbon concentrations in the soil vapour were estimated at 2 to 4 gr/m3. The remediation was expected to take two years.
Six horizontal drains were installed to extract groundwater for the in situ soil and groundwater remediation. The extraction rate was 2 to 3 m3/h, and maximum initial concentrations of mineral oil were 250 µg/l, and of aromatic compounds 3,000 µg/l.
Both groundwater and soil vapour were treated by means of a combined biological treatment installation.
Execution
The excavation works and installation of the in situ system took place in March and April 1994. In May 1994, a start was made with lowering the groundwater table, and in June 1994 the soil vapour extraction began.
In August 1995 the target values for soil and groundwater were reached and the system was stopped. The remediation took less time than the estimated two years. Due to (among other things) the higher soil vapour extraction rate (100 m3/h instead of 50 m3/h), the contaminants volatilized more quickly, which made the in situ remediation proceed faster than predicted.
Soil vapour
Figure 5.5 shows the concentration the extracted soil vapour.
Figure 5.5 Contaminant levels in the soil vapour
Chemical analyses results show that the initial concentrations of aromatic compounds and volatile hydrocarbons in the extracted air were 1.2 and 1.9 g/m3, respectively. This is in line with the estimated initial total hydrocarbon concentration, which was 2 to 4 g/m3.
The final total hydrocarbon concentration of the soil vapour was 20 mg/l
Ground water remediation
Figure 5.6 shows the BTEX concentration in the extracted groundwater
Figure 5.6 BTEX in extracted groundwater
The initial concentration in the pumped-up water was 480 µg/l, which turned out to be much lower than the 3,000 µg/l expected.
The end concentrations were below the detection limit of 0.1 µg/l, both in the extracted water and in the observation wells.
Soil quality
During the in situ remediation, the contaminant concentrations of the soil decreased from 2,300 mg/kg mineral oil to below the detection limit ( < 10 mg/kg), and from 480 mg/kg aromatics to below the detection limit ( < 0.1 mg/kg).
In total, the cost of the remedial operation amounted to USD 300,000.
Concluding remarks
This project has demonstrated that - owing to the experiences gained with this type of soil contamination and soil structure - the in situ soil remediation could be dimensioned and performed without carrying out extensive soil investigations in advance.
It also demonstrated that under certain circumstances the Dutch target values can be attained by means of in situ remediation techniques.
This project was carried out by Middelbrink en van Breukelen
B.V.; Tauw Milieu was responsible for the project design and management.