Jet grouting is a technique to deliver oxidizing or reducing reagents directly into source zones in heavily contaminated industrial sites. The advantage is a forced mixing of contaminant and reagent, eliminating various risks posed by an inhomogeneous, layered subsoil. The feasibility of this approach was demonstrated at a former chemical laundry in Zwölfaxing, near Vienna, Austria. After two extensive investigations, two remediation campaigns were conducted with extensive monitoring and sampling in between to study the effects of locally set measures on the overall site. Samples taken from core drillings and liner samples as well as membrane interface probes (MIP) were examined and compared to data obtained from concentration measurements at observation wells. This contribution demonstrates the effectiveness of jet grouting and presents a novel approach using MIP sounding for assessing remediation success. This sounding technique enables investigating a zone of highly contaminated subsoil before and after the treatment, with measurements directly showing the dechlorination success.