ZVI-Clay remediation of a chlorinated solvent source zone, Skuldelev, Denmark: 1. Site description and contaminant source mass reduction

Field investigations on the effects of ZVI-Clay soil mixing were conducted at a small DNAPL source zone with PCE as the parent compound. In a one-year monitoring program, soil samples were collected at three horizontal sampling planes (2.5, 5.0 and 7.5 m bgs.). PCE was found to have a pseudo first-order degradation half-life of 47 days resulting in more than 99% depletion of the source mass after one year. The main degradation product was ethene, while only low concentrations of the primarily biotic sequential degradation products (cDCE, VC) were detected. The soil mixing resulted in more homogeneous vertical conditions, while the horizontal homogenization was very limited. Iron was delivered in the full targeted depth with an average iron enrichment of 3.1%, and an average decline in the oxidation–reduction potential of more than 500 mV. Due to the applied top-down addition of ZVI, the iron content decreased from 4.6% to 2.1% on average over a depth of 5 m; hence, there is a potential for optimization of the delivery method. Most in situ technologies are limited by subsurface heterogeneities, whereby the successful dispersion of geological units and contaminants holds great promise for remediation of DNAPL source zones with ZVI-Clay soil mixing.

► The ZVI-Clay remediation technology was used at a DNAPL source zone (PCE).
► Iron was delivered in the full targeted depth by soil mixing.
► The average iron enrichment was 3.1% and the decline in ORP was more than 500 mV.
► Within one year more than 99% of the contaminant mass was depleted.
► The main degradation product of tetrachloroethene (PCE) was ethene.