A recently developed Multifunctional Well has been implemented for the remediation of subsurface contamination caused by hydrocarbon spills. Four different remediation processes can be operated within the same well. At optimal efficiency of each remediation process, successive treatment of the capillary fringes, the vadose zone, and the aquifer can be accomplished. Each treatment process requires only simple modifications of the well configuration. Removal of the contaminants is done primarily through in situ air stripping. The oxygen supplied through the stripping process and additional air injection wells further enhances biodegradation of the contaminants.
The Multifunctional Well is an in situ system for the remediation of hydrocarbon contamination in the unsaturated and saturated zones (Figures 1 - 4). It uses a combination of chemical, physical and biological processes.
The Multifunctional Well consists of a specially configured inner well (e.g. dia = 400 mm) with screened sections which are separated by unscreened casings and packers. Additionally, pumps, a stripping reactor under vacuum, and a free product recovery system, operated under vacuum, are required. An outer well casing which forms the treatment chamber (e.g. dia = 1,000 mm) is arranged coaxially around the inner well and extends from the ground surface down to a depth of approximately 1.0 m. A blower, an off-gas treatment system (e.g. activated carbon filters), a contaminant storage tank, and a vacuum pump are located above ground. Several air injection wells are arranged within the sphere of influence of the Multifunctional Well.
The well screens are located within the unsaturated zone, near the groundwater table or the capillary fringe, and at the bottom of the contaminated aquifer. Contaminated soil vapor can be removed from both upper screen sections, or from each section separately. A reinjection of the air after its decontamination above ground is also possible, thereby creating a vertical air circulation flow. Alternatively, treated soil vapor can be reinjected through additional air injection wells, thereby creating a horizontally directed air flow. Contaminated groundwater is pumped from one section, and after treatment in the stripping reactor, located in the well head, reinjected into one of the upper screens, thereby inducing a vertical groundwater flow.
The circulating flow creates a vertical pressure gradient and forces a vertical movement of the groundwater instead of a simple horizontal flow through highly permeable layers. The flow direction can be reversed depending on the vertical distribution of the contaminants.
Possible Modes of Operation
I. Free Product Recovery (FPR)
Free product floating on the groundwater is removed by applying a vacuum without having to pump groundwater (Bernhardt, 1993,1994/a) Free product moves in the di-rection of the negative pressure gradient (5 - 8 kPa) towards the well in which it accumulates (Figure 1). In order to control the applied vacuum and thus the amount of oil flowing, an adjustable inner tube is arranged inside the well in such a way that the middle screening section is nearly covered by groundwater and the free product floating on top. Because the free product is flowing towards the well continuously, no high air velocities are required for its transport. A vacuum pump produces a vacuum inside a receiver tank and the attached piping which is connected to a double-cased screen containing hydrophobic material (DMF, Bernhardt 1989, Alesi & Rehner 1988). The free product is thus transported to the surface. Because no cone of depression is created a downward movement of free product is prevented.