TEMA 19. Descontaminación de suelos contaminados

4 Casos prácticos

Practice case 4:

In-situ biorestoration of a phthalates contamination using only sparging

General

This practice case comprises the in situ cleanup of a site contaminated with phthalates. The remediation is currently in its final stages. As phthalates are nonvolatile contaminants, this is an example of in situ biorestoration during which the contaminants are removed purely by biological degradation. Furthermore, sparging is used without it being necessary to employ soil vapour extraction to capture any stripped contaminants.

Contamination situation

During an exploratory investigation of the company site, the soil and groundwater proved to be contaminated with phthalates. The contamination - mostly consisting of di-isobutyl-phthalates (DIBF) - migrated with the groundwater to beyond the site borders. The groundwater contamination extended to a depth of ca. 5 m-gl, and had a surface area of ca. 600 m2. Maximum concentrations in the soil were 10,000 mg/kg, and 7,300 µg/l in the groundwater.

Almost half of the contamination was situated beneath a warehouse that is still in use.

Weighing of the remediation alternatives

If the contamination had been removed by excavation, the warehouse - which had steel foundations - would have had to be demolished, or moved to another location. In situ bioremediation seemed like a good option, as according to literature phthalates are easily biodegradable [Kurane et al., 1978; Engelhardt & Wallnofer, 1 978].

As phthalates are non -volatile and can not be stripped from ground water, in this situation the addition of oxygen by means of sparging was chosen. There was no need for soil vapour extraction, since the contaminants were non-volatile. During the in situ remediation, groundwater was contained by a deepwell placed just outside the contaminated area. The pumped-up water was only slightly contaminated, and could therefore be discharged directly onto the sewage system.

In order to stimulate biological degradation, warm water ( + 30° C) was infiltrated upstream. This also allowed the possibility to add nutrients, or correct the pH, if necessary. The latter may be required as the pH of the soil may change considerably as a consequence of the degradation of the phthalates.

The remediation duration was estimated at 1.5 years.

Design and dimensioning

Compressed air was injected by three sparging wells (6 m-gl) at a flow rate of 120 Nm3/hour.

Groundwater was extracted at a flow rate of 120 m3/d, and 60 m3 of warm water per day was infiltrated. The groundwater could be discharged directly onto the sewage system. In addition to the sparging wells, the water infiltration filters (7) and the deepwell, six monitoring wells were installed to monitor the remediation progress. Furthermore, several air monitoring filters were installed to monitor whether there was uncontrolled spreading of the contamination via the soil vapour.

Figure 5.8 Cross section of the installation.

Execution

During the remediation, samples were taken from the pumped-up groundwater, the monitoring wells, and - periodically - of the soil vapour.

In addition, oxygen concentrations in the groundwater, as well as the temperature course were monitored. The remediation was begun in March 1995.

Pumped-up groundwater

Only during the first week were phthalates detected in the groundwater, albeit in low concentrations. After that, the concentration dropped to below detection level (2 µg/l).

Concentrations in groundwater

Before the remediation started, the monitoring wells were sampled in order to determine the baseline situation. A maximum of 1,000 µg of phthalates per litre was measured. After two months of remedial activities, no phthalates could be detected in the groundwater of the monitoring well. Nor were they detected during a re-sampling round. In order to ascertain whether subsequent supply would occur, the in situ remediation was interrupted, and after 2 and then 3 months of stand-still the concentration levels were again determined. Again, no phthalates were encountered. Remediation had been achieved well within the estimated time.

Concentrations in soil

The soil was sampled before and after the remediation. After the groundwater samples proved to be clean, the soil was resampled. The concentrations in the soil and the vadose zone were now below detection limit, with a maximum concentration in the saturated zone of 590 mg/kg.

Other parameters

The pH of the groundwater remained constant during the remediation. The temperature was 14° C to begin with, then rose to 23° C.

After completion

The phthalates contamination of the groundwater had been removed. Phtalate concentrations in the soil had decreased to below the intervention value. Test proved that there were no risks of spreading, as the phthalates did not leach from the soil into the groundwater.

The competent authorities have agreed that the aboveground installation can be removed. After 1 year, groundwater will again be sampled, and if it is clean, the remediation will be regarded as finished. If the water is not clean, however, it will be assessed whether a short period of sparging is required to clean up the remaining contamination.

The costs of the project were ca. USD 200,000. It was executed by Heymans Milieutechniek B.V. The design, dimensioning and supervision were conducted by Tauw Milieu B.V.

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