Three-Dimensional Laboratory Experiments on Fate and Transport of LNAPL under Varying Groundwater Flow Conditions

Variations in groundwater flow regimes due to direct draining/pumping and surrounding climatic variabilities may significantly affect the spatial and temporal distribution of hydrocarbon compounds in the subsurface. The fate and transport of these contaminants have been studied quite adequately; however, the behavior of these pollutants under varying groundwater flow regimes has not been investigated in the past. Therefore, an extensive experimental investigation was made to study the effect of changes in groundwater flow velocity on the fate and transport of light nonaqueous phase liquids (LNAPLs) using a three-dimensional (3D) sand tank setup. The tank setup of size $60\mathrm{cm}\mathrm{long}\times 30\mathrm{cm}\text{wide}\times 60\mathrm{cm}$ deep embedded with sampling ports was packed with homogeneous sand having grain size in the range 0.5–1.0 mm. Pure phase toluene, a representative LNAPL, was released from the top port of the tank setup to create a pure phase pool of the LNAPL around the groundwater table. A constant water flux was allowed to flow first to maintain a base groundwater flow velocity in the horizontal direction. The flow velocity was then increased or decreased by changing the water flux passing through the saturated zone by keeping the water table location at the same height. Groundwater samples were collected routinely and were analyzed using gas chromatography–mass spectrometry (GC-MS). A series of batch experiments were also performed using the same groundwater and porous media used in the tank setup to estimate the biodegradation rate at different dissolved LNAPL concentrations. It was observed that the biodegradation rate increases up to 50 ppm concentration and remains almost the same until 100 ppm and then decreases with increasing concentration of LNAPL. The biodegradation rates corresponding to the observed concentration of LNAPLs in the tank setup were then used to conduct the simulation experiments. Results show that the dissolution rate of LNAPL increases linearly with groundwater velocity and was estimated for the three different groundwater flow regimes varying from 0.083 to $0.129\mathrm{cm}/\mathrm{h}$. The observed high rate of degradation of LNAPL for faster flow velocities shows the dependency of the degradation kinetics on dissolved LNAPL concentration. The observed breakthrough curves at different ports showed that horizontal and transverse transport of LNAPL was more prominent compared to its vertical movement. The observed concentration of dissolved toluene compared well with the simulated curves for the considered cases of groundwater flow velocities. The results of this study are of direct use in applying bioremediation techniques for field problems subjected to dynamic groundwater flow conditions.