Current risk-based practices in groundwater remediation often target the removal of floating hydrocarbon layers without the removal of water. Removing just the floating layer, or LNAPL (light non-aqueous phase liquid), is a different challenge from that encountered with previous two-pump, cone of depression methods. Some of the factors that influence LNAPL transport include:
· Density
· Viscosity
· Interfacial Tension
· Relative Permeability
· Saturation and Residual Saturation
LNAPL Recovery
Selection and operation of LNAPL recovery equipment should be based on the LNAPL properties and formation characteristics. It is especially important to address flow rate carefully. High initial levels in a well do not necessarily mean that the well will recover rapidly enough to support high LNAPL pumping rates on a sustained basis. In fact, overpumping can reduce LNAPL saturation levels so much that continuity is broken and water replaces LNAPL in the voids; when this occurs, LNAPL must in turn displace the water before flow can be re-established. However, this can be a very slow process, since the LNAPL hydraulic gradient is likely to be very low. The conclusion is that depending on site conditions, lower pumping rates and/or programmed off periods may enhance overall LNAPL recovery. Due to these factors and the small effective capture zone around an LNAPL-only recovery well, total recovery rates tend to be improved more by using additional wells than by trying to pump at higher
rates*.
LNAPL properties can also change over time. Weathering and biological activity can increase viscosity. Solids and debris in the well can add to these effects, obstructing screens and valves and increasing maintenance requirements.
*See Newell, C.J., S.D. Acree, R.R. Ross, and S.G. Huling, 1995. Light nonaqueous phase liquids, Ground Water Issue, EPA/540/S-95/500, U.S. EPA, R.S. Kerr Environ. Res. Lab., Ada, OK, 21pp.
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Collecting hydrocarbons is easier with the
Ferret System from QED:
-rejects water
-no hydrophobic screen
-now with High Capacity option
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