Dissolution of a Toluene Pool Under Constant and Variable Hydraulic Gradients with Implications for Aquifer Remediation

Factors affecting the dissolution of a toluene pool floating at the water table were investigated using an experimental aquifer 85 cm long, 30 cm high, and 20 cm wide. Under conditions of steady, uniform flow, the dissolved toluene concentration decreased with increasing vertical distance from the free phase pool, resulting in steep concentration gradients. The concentration gradient steepened with increasing ground‐water velocity. The thickness of the dissolved plume increased with increasing distance along the pool and decreasing ground‐water velocity. In all cases, measured concentrations were a small fraction of toluene solubility. In pulsed pumping experiments, dissolved toluene concentrations in all sampling ports increased significantly during zero flow conditions and, when the flow was resumed, they gradually decreased to the same levels as before the flow was stopped. Under the experimental conditions used, comparison between the pulsed and continuous pumping showed that the total mass removal would be higher for continuous pumping and would result in shorter toluene removal times. However, the total volume of water pumped and needed to be treated would be lower by 50% for pulsed pumping. The latter would result in a higher by ∼ 20% mass removed per volume of water pumped, suggesting that pulsed pumping would be a more efficient operation compared to continuous pumping, when considering removal of dissolved toluene from the aquifer.