MULTI-PHASE EXTRACTION WELL DROP TUBE DESIGN FOR LNAPL RECOVERY AT HIGH PERMEABILITY SITES

摘要

The design of a drop tube in a multi-phase extraction (MPE) well can have a significant impact on the effectiveness of a remediation system, especially at sites having high permeability soils. Previous research has presented methodologies for selecting drop tube diameters and slot patterns to minimize pressure losses in an MPE well (Peramaki and Granley, 2001). This research describes the practical implications of drop tube design on the removal of light non-aqueous phase liquids (LNAPL), soil vapor and ground water from an MPE recovery well installed at sites having soils with intrinsic permeabilities in excess of 10-8 cm2. Critical factors in designing a drop tube are: drop tube diameter, the elevation of the drop tube base relative to the LNAPL/water interface, the drop tube-slotting pattern and the presence or absence of a cap on the end of the drop tube. Drop tubes can be designed to produce maximum ground-water flow rates/drawdown (and the corresponding cone of depression), maximum vapor flow rates (and the corresponding vacuum radius of influence), minimum ground water recovery, and/or maximum LNAPL recovery rates. Field experiments were conducted in several wells at two active remediation sites having highly permeable soils (>4x10-7 cm2) and large areas of LNAPL. Numerous drop tube configurations were tested. Conclusions drawn from the field test results indicate that a non-slotted drop tube produces higher LNAPL recovery rates than slotted tubes and that the base of the drop tube should be set at the LNAPL/water interface for maximum sustainable LNAPL recovery (with high ground-water recovery rates). The highest ground-water recovery rates are observed with increasing drop tube submergence, while the highest soil vapor flow rates occur with decreasing drop tube submergence and/or an increase in drop tube slotting. Combining field test results with LNAPL theory, it is postulated that the optimum drop tube configuration for a highly permeable site consists of an unslotted, uncapped drop tube placed within the LNAPL layer at or slightly below the potentiometric surface of the aquifer. It is essential to thoroughly understand the finer points of drop tube design in order to purposefully induce the desired subsurface conditions at a given stage of a project. Proper management of drop tube configuration by making changes in the configuration at appropriate times will maximize remediation and lead to a more rapid clean-up at reduced life-cycle costs.