Summary and conclusions

Funktionen af den reaktive væg på Vapokon grunden - 7 år efter etableringen

The Vapokon-site is located in Søndersø on the island of Funen in Denmark. From 1976-1996 the company Vapokon Petrokemisk Industri A/S operated from the site. Activities by the company resulted in widespread soil and groundwater contamination, primarily comprised of aromatic and chlorinated solvents. In 1999 a reactive barrier containing zero-valent iron was installed at the Vapokon-site in order to clean up the contamination. Since the reactive barrier was established, several investigations of the site have been carried out and several of these show that the barrier works effectively against chlorinated solvents such as PCE, TCA and 1,1,1-TCA. Furthermore, some of the investigations have indicated that the barrier might be clogged in some areas and when considering the flow pattern, this indicates that the groundwater is primarily passing through a limited area of the barrier. Moreover, groundwater analyses upstream of the barrier have revealed relatively high concentrations of chlorinated solvents in a limited area. This is probably due to the presence of DNAPL (dense nonaqueous phase liquid), which in consequence will result in a heterogeneous impact of the barrier.

Flux-based monitoring is necessary to examine the impact of the barrier and the variation over the cross-sectional area . This could moreover provide an estimate of the mass of chlorinated solvents removed by the reactive barrier. As further aim of the flux analysis is to look into whether a faction of the contaminants will diffuse out of the barrier with the hydrogen gas produced by the dechlorination in the barrier. The final aim of the present investigation is to evaluate the effectiveness of the reactive barrier seven years after installation, together with an evaluation of the extent of the potential clogging of the barrier.

The flux analysis is based on a groundwater analysis and on an analysis of the hydraulic conditions at two sets of cross-sections located parallel to the barrier. Each set of cross-sections consists of one cross-section upstream of the barrier and one downstream. Furthermore, the two sets of cross-sections have different levels of detail. In two areas just above the barrier gas probes have been installed and gas samples have been collected. The general status and effectiveness of the reactive barrier are evaluated on the basis of iron cores taken from the barrier. These cores are taken in two different areas of the barrier and for each location cores from two different depths have been collected. The iron cores are used to set up a degradation experiment and to measure the production of hydrogen. Furthermore, microbiological and mineralogical characterizations of the cores are performed.

The flux analysis shows that 26kg of chlorinated solvents pass through the reactive barrier at the Vapokon-site per year, and that the barrier reduces this mass by 99%. A problem concerning the barrier is that a large flux of the chlorinated degradation product cis-DCE is found just downstream of the barrier. This means that a total of 5.7 kg cis-DCE leave the barrier per year. Moreover, the flux analysis shows that the primary flux through the barrier is limited to an area of 14m², which is equivalent to 11% of the overall cross-sectional area. This could have an impact on the function of the barrier in this area due to the high load of chlorinated solvents. The transport of contaminants is probably limited to a small are of the barrier due to the hydrogeological conditions and the presence of DNAPL resulting in narrow plumes of contamination containing high concentrations.

The iron cores collected from the barrier at the Vapokon-site show no signs of clogging due to mineral precipitation. The measurements of the hydrogen production performed using iron from the barrier, on the other hand, show that part of the pore volume might be clogged due to the presence of hydrogen gas, as the mean production is around 4,4m³ per day. Part of this gas may be expected to disappear from the barrier either by diffusion or by being consumed by hydrogen-oxidizing bacteria in the aerobic zone above groundwater level. Measurements of the hydrogen concentration in the pore space above the barrier do not show particularly high concentrations of hydrogen. This indicates that some of the hydrogen produced leaves the barrier through the sides. Analyses of the air in the pore space above the barrier do not reveal the presence of any chlorinated compounds, but in view of the relatively high hydrogen production, a fraction of these compounds might be leaving the barrier together with the hydrogen gas.

The iron cores are also used to investigate the reactivity of the barrier and the results are compared to the reactivity of virgin iron of the same kind as the iron cores installed at the Vapokon-site. In one of the investigated areas the reactivity of the iron was generally lower compared to virgin iron, while the degradation rates from the other area were similar to those for virgin iron. The explanation for the difference between the reactivity in the two areas can probably be found in the microbiological analyses, which show the presence of halorespiring bacteria in the iron in the most reactive area. This illustrates that degradation in parts of the barrier most likely is a combination of degradation with zero-valent iron and biological degradation.

The overall evaluation of the condition and the reactivity of the reactive barrier at the Vapokon-site after seven years of operation is that the barrier is effective at removing chlorinated compounds. The concentration of cis-DCE downstream of the barrier greatly exceeds the given criteria and a great majority of these compounds are leaving the barrier and something has to be done to solve this problem. The barrier is not clogged by mineral precipitation, but investigations of the hydrogen production show that it might be clogged by hydrogen gas. The extent of this clogging and the faith of the hydrogen in the barrier could not be determined by the investigation.