Geophysical Surveys At The Oitti Dnapl Spill Site, Southern Finland

In the 1950’s and 1960’s about 10,000 liters of dry cleaning solvents from a dry cleaning establishment near the town<br>center at Oitti, southern Finland were disposed of in a well and in pits. The groundwater was found to be contaminated in<br>1992 when a regional groundwater quality survey was done in the region. Among the solvents are trichloroethylene and<br>tetrachloroethylene. Although the major part of the total amount of solvents were deposited in a condensed form into a<br>landfill, the storage sites at the dry cleaning establishment became injection wells by default, because of the permeable<br>nature of the underlying strata (porous sand and gravel, typical of the glacial esker formation overlying the area). The<br>groundwater has been found to be contaminated within a total area of several square kilometres and the amount of<br>polluted groundwater in the esker formation is estimated to be 8 million cubic metres. The main water supply well for<br>the town became contaminated and had to be closed because of high levels of tri- and tetrachloroethylene.<br>Since 1992, the spill site and the whole esker area have been subject to diverse studies aiming at the delineation of the<br>polluted area and at the planning of the remediation methods. In 1995 it was decided to perform geophysical surveys at<br>the site, with two main objectives in mind: first, to delineate the unknown bedrock topography underlying the esker<br>formation, and secondly, to map variations in the groundwater depth in the vicinity of the establishment. This<br>information is essential in the delineation of the probable contaminant flow directions. The main geophysical methods<br>applied were ground penetrating radar and seismic refraction sounding. To a lesser extent, earth resistivity sounding and<br>inductive sounding (EM3 1) techniques were also applied. On the basis of seismic refraction surveys the depth to bedrock<br>surface could be determined on the flanks of the esker and results from the ground penetrating radar surveys gave<br>information about the groundwater depth and overburden quality variations on and around the spill site. A geostatistical<br>analysis, based on the well-water analyses of the contaminants, proved to be a useful tool in the delineation of the<br>probable contaminant flow directions.