At the Kerkstraat 37 and 39 in Zaltbommel the activities of a metallurgical holding have contaminated the soil and groundwater with CVOC. Between 1987 and 1998, the topsoil has been excavated and the contaminated groundwaterhas been treated through pump & treat. Between 1999 and 2007, BioSoil has performed an in situ remediation by stimulating the biodegradation (substrate addition for a reductive dechlorination).
Full report available in Dutch (see section Conclusion and Recommendations)
The goal of this HIP pilot is to demonstrate a procedure for using advanced monitoring techniques, which can demonstrate the presence or absence of a stable end situation.
The site is located in the old town of Zaltbommel, within the ancient city walls. The soil consists of sand and clay layers. Between 4.7 m to 10 m below surface a clay layer is situated.
Figure: expected groundwater movement.
During one year (March 2007-March 2008) the site was monitored during multiple occasions. The pollution situation, the redox conditions and availability of electron donors were determined in 12 monitoring wells. Based on monitoring data, trends in the pollution situation were determined and the progress of the natural degradation processes were monitored.
Currently, at one of the two source areas a residual contamination is still present. Partly because of the impact of leaking sewers in the alley and street the contamination is spread horizontally, though the direction is not fixed. Since there is an infiltration situation, the contaminants are partly spread vertically towards the first aquifer. The contamination is partly vertically enclosed. It is not known untill which depth the contaminants in the soil have spread.
Although in the topsoil and Holocene clay layer of the garden natural degradation occurs, there is no clear increase in dechlorination. This is probably due to continuous spread from the topsoil in the backyard and the relatively unfavorable redox conditions for the complete degradation of CVOC. Also, there appears a likely shortage of electron donor for complete natural degradation in the source area.
In the groundwater contaminated area, the topsoil and at the alley of the Kerkstraat, total degradation of the contamination occurs. There appears to be both favorable redox conditions as a sufficient presence of electron donor which leads to a sustainable natural attenuation.
Since the contamination is hardly limited in the vertical spread, upto the Holocene clay layer and the first aquifer, it is possible that the source area is much larger than previously asumed. The lack of information about groundwater flow conditions and soil structure and the limited degree of vertical information of the contamination, it is impossible to determine how much spread still occurs. It is therefore not possible to determine if natural biodegradation occurs sufficiently in order to establish a stable end situation.
During the excavation two source areas where found leaking pure product from the sewer in the alley:
1. At the corner of the backyard and the alley
2. At the corner of the Kerkstraat and the alley
According to the statement, the contaminants have spread eastward from these two source areas, allowing contaminants to end up in the backyard.
Currently, a residual contamination is present at the corner of the Kerkstraat and the alley. Partly because of the influence of the leaking sewer in the alley and in the Kerkstraat, the contaminants have spread horizontally. The direction is not fixed. Since there is infiltration, the contaminants have partly spread vertically towards the first aquifer. The vertical enclosement of the contamination is limited. It is not known untill what depth the contaminants have reached in the soil profile.
In general it can be recommended that before designing a remediation system, a thorough understanding of the soil structure, permeability, diffusion paths, pollution situation and natural degradation potential is very important.