Characterization of contaminant migration potential in the vicinity of an in-place sand cap

Purpose: This study characterized the chemical transport potential of polycyclic aromatic hydrocarbons (PAHs) and total petroleum hydrocarbons (TPH) in the vicinity of a sand cap placed in the nearshore zone of a tidal marine embayment. Materials and methods: Groundwater seepage was investigated along the perimeter and within the footprint of the sand cap, with results verifying the presence of significant freshwater upwelling shoreward of the sand cap boundary. The depth distribution of PAHs and TPH was assessed in sediment cores collected from within the cap area footprint. Results and discussion: The depth distribution of PAHs and TPH demonstrated a spatial pattern of elevated chemical concentrations in the shoreward zone of the capped area, consistent with the spatial pattern of elevated freshwater flux. Visual inspection of recovered cores confirmed the presence of a fine-grained, low-conductivity sediment layer underlying the sand cap, with material properties of this layer potentially suggesting compaction following placement of the sand cap. This fine-grained sediment layer was not evident in the shoreward zone of the capped area. Conclusions: The presence of the aquitard under the sand cap, coupled with the apparent erosion of this fine-grained layer in the higher energy shoreward zone, suggests the potential for enhanced groundwater seepage in the shoreward zone of the sand cap. It is hypothesized that enhanced groundwater flux is responsible for the elevated concentrations of PAH and TPH observed in core profiles collected from the zone characterized by elevated freshwater seepage and tidal pumping and that the fine-grained sediment layer that serves as an aquitard impedes groundwater flux within the cap area footprint. In effect, the absence of groundwater seepage observed for those stations within the footprint of the sand cap has likely resulted from compaction of the native sediment strata, whether or not compaction resulted directly from cap placement. © Springer-Verlag 2010.