Abstract
A 4-D monitoring GPR survey was carried out in order to outline the light non-aqueous phase liquid (LNAPL) plume underground during a remediation process in the area of an impacted gas station in Fortaleza, NE Brazil. The complex GPR signature of the plume was well characterized. Low reflectivity zone corresponds to hydrocarbon vapor phase in the vadose zone. Enhanced reflections are associated with free and residual products directly above the water table. A theoretical model based on geochemical zonation of the LNAPL was calculated using a finite difference time domain (FDTD) forward modeling code, whose synthetic radargrams provide useful insight into the nature of the reflection events related to LNAPL contamination. Hydrogeologic and hydrochemical data from installed monitoring wells provided additional support for interpreting of the GPR response. Moreover, a network of radar sections collected during fifteen months at the polluted site show clearly temporal changes in the plume dimensions, induced by regular pumping of groundwater. This corrective action caused an important drop of the water table that was followed by a rapid shrinking of the impacted area in the vadose zone, generating pooling of vapor products. A steady restore of the reflectivity in the shadow zone suggests a decrease of LNAPL saturation along the GPR survey.
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