Evaluation of the hydraulic performance of a full-scale permeable reactive barrier by a tracer study

Investigation of the hydraulic performance of a fannel-and-gate permeable reactive barrier (PRB), packed with zero-valent iron (Fe-0) and installed at the Vapokon site, Demnark, has been conducted with a natural gradient tracer study. The purposes of this study were to evaluate the flow pattern and determine the seepage velocity (v(x)) of groundwater through the PRB. After collecting and analysing about 13 000 groundwater samples over a period of 10 months, the moving path of the tracer (i.e. lithium, Li+) was identified in which the Li+ plume was observed passing through the reactive barrier. However, probably owing to clogging caused by mineral precipitates, there was a preferential path inside the Fe-0 PRB. Comparison of the water table contour in the Vapokon site obtained in March 2000 and January 2003 showed formation of a low permeability zone within the reactive barrier, thereby further verifying clogging caused by mineral precipitates. Spatial moments analysis of the Li+ distribution illustrates the similarity of the maximum Li+ mass passing through the upgradient and downgradient locations of the Fe-0 PRB and thereby indicated the conservative of the Li+ across the reactive barrier as well as the absence of bypassing flow. Based upon the results of first moment analysis, a v(x) of 99.5 in year(-1) within the PRB was calculated. The v, just upgradient of the reactive barrier, however, was only about 6.86 in year(-1), most likely owing to the effect of disturbance exerted by the clogged upgradient interface of the upper part of the Fe-0 PRB.