AquaConSoil 2015
DNAPL source zone treatment with ZVI soil mixing
Introduction: Dense Non Aquious Phase Liquid (DNAPL) source zone remediation remains a challenge. The known heterogeneous distribution of chlorinated hydrocarbons (CHC) and the formation of difficult to locate DNAPL pools and/or residual DNAPL makes the successful remediation of CHC source zones even more challenging.
Challenge: In the historic city center of a small town near Rotterdam (The Netherlands) the activities of a former dry cleaner led to contamination of soil and groundwater. The soil stratigraphy consists of mainly clay with heterogeneous distributed layers of peat and sand. The soil is contaminated down to 10 meters below surface with CHC (PCE, TCE, DCE, VC) which feeds a plume in the first aquifer. In order to make the site suitable for redevelopment the client was looking for a remedial solution that is capable of reducing the contaminant mass in the source zone in a short period of time and stops the distribution of the contaminants towards the plume.
Several remedial options for the source zone were evaluated such as biological treatment, chemical oxidation and excavation. All these remedial approaches were judged not feasible, because of the very sensitive historical buildings in close vicinity to the source zone, the soil stratigraphy and the probable presence of DNAPL as well as the wish of the client for rapid redevelopment of the site.
Approach: ARCADIS proposed soil mixing with zero valent iron (ZVI) as best option for the site and was granted the contract. This innovative technology combines the benefits of a chemical treatment with optimal reagent distribution. The application is vibration-free and fast. Since the reagent slurry comprised of a combination of ZVI and bentonite the introduced clay also assists in additional reduction of the permeability which subsequently leads to further reduction of leaching of residual contamination to the plume.
Result: Prior to the field application ARCADIS executed a feasibility study in order to develop an optimum reagent mixture. For the full-scale remediation, a double auger system best suited to the location specific constraints was developed. In a five weeks timeframe, 600 overlapping columns were drilled to treat the 1.200 m3 source zone. With the implementation of a thorough quality control protocol, issues such as ZVI quality and mixing performance could immediately be identified and corrected. Six months after application the contaminant mass in the treated soil volume was already reduced by more than 90% while further reduction is expected.
Presentation: We will present details of the applied ZVI soil mixing technology in general and for this specific urban site in particular. Also several issues that have to be taken into account during application will be further discussed.
