Impact of the water table fluctuations on the apparent thickness of light non-aqueous phase liquids

Abstract

The light non-aqueous phase liquids (LNAPLs) percolating into the subsurface from damaged underground storage tanks, pipelines cisterns and from the unsealed landfills present a threat to soil and groundwater. If a layer of LNAPL floats on the groundwater table the initial step of remediation should be its recovery. In order to a proper design of LNAPL recovery an assessment of the mobile LNAPL volume is required. This volume can be determined on the base of a thickness of LNAPL layer in the porous medium (the actual thickness) or on the base of LNAPL specific volume. Then the LNAPL actual thickness and specific volume are estimated on the base of the LNAPL thickness measured in the monitoring well (the apparent thickness). Unfortunately, the actual LNAPL thickness is always different from the apparent LNAPL thickness. This difference depends on the properties of soil and the amount and properties of LNAPL. Additional factors influencing significantly the difference between apparent and actual thicknesses are the fluctuations of the water table level (the hydraulic head fluctuations). The major objectives of this study became to investigate the impact of hydraulic head fluctuations on the measured apparent and actual LNAPL thicknesses. Obtained results show that when the hydraulic head diminishes, the apparent thickness of LNAPL increases and the actual thickness decreases. Instead, when the hydraulic head again rises, the apparent thickness decreases, and the actual thickness increases. When the hydraulic head rises considerably it can take place that the free product don’t be present in the observation well. The results affirm that hydraulic head fluctuations complicate considerably the estimation of the actual thickness and the mobile volume of LNAPL on the base of the apparent LNAPL thickness measured in the observation well.