Geostatistics meets a growing interest for the remediation forecast of potentially contaminated sites (both chemical/radiological): pollution mapping, contaminated volumes estimation, integration of auxiliary information, setup of adaptive sampling strategies. This interest has been supported in France by the GeoSiPol association (Geostatistics for Polluted Sites) created in 2004 and which is gathering together geostatistical specialists and actors involved in the characterization of polluted sites: institutional organizations, industrial and consulting offices.
As part of demonstration studies carried out for GeoSiPol and with the financial contribution of ADEME, geostatistics has been applied for the detailed diagnosis of a former TOTAL oil deposit in France. Several investigation campaigns put into evidence high hydrocarbons grades in the lower part of a backfill layer covering almost entirely the site. Given the future use of the site, health risks led to the definition of a remediation threshold on Total Hydrocarbon. As a consequence, the location of areas presenting hydrocarbons grades above the remediation threshold had to be estimated. Geostatistics allowed to estimate pessimistic / probable / optimistic scenarios for the contaminated volumes and to quantify the risks associated to the remediation: financial risk to excavate clean soils, sanitary risk to leave in place contaminated soils. After a first mapping, an iterative approach led to collect additional samples in uncertain areas previously identified by geostatistics. Estimated volumes were then updated, leading to a probable contaminated volume of 12 100 m3, associated to a 90% confidence interval lying between 10 000 and 15 400 m3.
Today, the effective remediation of the site allows to provide an experience feedback about the geostatistical methodology. In particular, the estimated volumes can be compared to the real one equal to 13 100 m3 of contaminated soil. This value is close to the probable estimated volume and widely included in the 90% confidence interval.
The study shows that geostatistics is a well-suited approach for the remediation forecast of such contaminated sites. Moreover it provides a framework for both uncertainty assessment and cost-benefit analyses, in particular regarding the relevance of collecting additional data versus starting the remediation.