Fig. 1, Overview of Bologna train station and surrounding urban area
A widespread plume of chlorinated solvents (CHC) has been monitored for more than a decade underneath the train station and extending out under the surrounding urban area of Bologna, Italy. One area has concentrations exceeding 1,000 μg/l, but most of the chlorinated solvent concentrations are low (100μg/l or less) yet persistent and with no effective natural attenuation occurring. The main contaminants are TCE and cis-1,2-DCE, which are present in two aquifers with different geological characteristics: The superficial aquifer has a thickness of 6 to 8 meters and consists of fine sands and silts.
The deeper aquifer has a thickness of 2 to 4 meters and consists of higher permeable sand. This contamination had previously been considered as practically untreatable, due to its low concentrations, location and size.
Fig. 2, Section illustration showing the superficial and deeper contaminated aquifers
The University of Rome “La Sapienza” had been commissioned to evaluate strategies for the remediation of this large urban site. Based on the results from site investigation, lab testing and microcosm studies, biological reductive dechlorination was recognised as a potential approach.
However, biological degradation was considered impractical for treatment on the site, due to the potential inefficiencies at such low contaminant concentrations. But with the creation of our new PlumeStop technology, another treatment approach became an option.
Fig. 3, Soil testing during the pilot phase
PlumeStop allows for the effective treatment of widespread, low concentration plumes, as it works by combining in situ adsorption and enhanced biological degradation. The technology provides a rapid and sustained treatment to very low concentration targets from a single, simple and safe injection into the subsurface.
There is no need for mechanical plant installation and also no requirement for repeated applications, making it an ideal solution for this large, complex site. This minimised the activities on a site comprising a busy railway station, residential neighbourhood and ongoing redevelopment works.
Fig. 4, Drilling wells prior to injection in Area 2
The intention on this site was to treat all of the accessible ‘hot spots’ present within the plume, with the first area being used as a pilot test to confirm dose and distribution.
Fig. 5, Injection of PlumeStop on the platform of the railway station, in Area 3
Fig. 6, Direct push injections in the road at Area 4
For the pilot test, REGENESIS installed 6 multi-level injection wells and applied specific doses of PlumeStop at the target depths. On-site works for the pilot test were completed in 3 weeks and included the execution of a range of tests in order to con rm the treatment effectiveness and create an accurate design for the full scale application.
Full scale treatment comprised injection into the four identified ‘hot spot’ areas (see figure 7). In Areas 1 and 2, multi-level injection wells were used, while in areas with restricted access: Area 3 (rail way station) and Area 4 (residential area) , a direct push approach has been employed.
In all treatment areas, PlumeStop has been co-injected with REGENESIS electron donors HRC® and HRC Primer®, to provide initial biostimulation and promote continual in situ bio-regeneration of the Liquid Activated Carbon.
Fig. 7, Full-scale treatment locations: targeting the 4 ‘hot spots’ within the widespread, low concentration chlorinated solvent plume
Treatment results show a rapid reduction in CHC concentrations inside the target areas (see examples in figures 8, 9 and 10). In the majority of monitoring wells concentrations of all chlorinated compounds are below stringent regulatory standards (‘CSC’) and in many cases also below the detection limit. In most groundwater monitoring results, daughter products, cis-1,2-DCE and VC, show a decreasing trend from the start of treatment, rather than a sequential increase and then decrease in concentration, as would be expected in a typical ERD approach. This is due to full reductive dechlorination occurring on the surface of the PlumeStop biomatrix, rather than in the groundwater itself. It is expected that the concentrations will be maintained at low levels without any rebound effect, due to the continuous regeneration of the activated carbon provided by the biodegradation process.