Potential consolidation-induced NAPL migration from coal tar impacted river sediment under a remedial sand cap

Subaqueous sediment, if capped for remediation purposes, may undergo consolidation due to the increased effective weight of the capping material. The standard Atterberg limits test and a modified drained three-dimensional consolidation test (DTCT) were performed on sediment collected from a river adjacent to a former manufactured gas plant site that contains high concentrations of coal tar. The plastic limit of five sediment samples ranged between 72 and 89%, and the liquid limit ranged between 123 and 194%. The plasticity index ranged from 51 to 122%, with the values among the sediments correlating with the coal tar content (r² = 0.93). DTCT experiment was performed on 5 cm sediment overlain with 5 cm sand to a maximum applied effective cell pressure of 41.4 kPa. Consolidation increased almost linearly at lower pressures (<13.8 kPa); however, as higher pressures were imposed, the ratio of consolidation per applied pressure decreased. The results of this study suggest that porewater advection, resulting from sediment consolidation, will occur from the sediment to the capping material. Because this water will contain numerous polycyclic aromatic hydrocarbons, measures, such as adding sorptive materials, should be taken to reduce the flux of these compounds.