GROUNDWATER AND LEACHATE: EX-SITU CHEMICAL/PHYSICAL TREATMENT
Ground Water Pumping
A component of many pump-and-treat processes - commonly used ground water remediation technology.
The objective of groundwater pumping is to remove dissolved contaminants from the subsurface. It is also used as a hydraulic barrier to prevent off-site migration of contaminant plumes.
Surfactant Enhanced Recovery
Injecting surfactant micelles or steam into a contaminated aquifer can facilitate the ground water pumping process by increasing the mobility and solubility of the contaminants in the soil.
The surfactant micelles can also facilitate the entrainment of hydrophobic contaminants - allowing removal of multi-phase contaminants. It therefore increases the mass removal of contaminant per pore volume of ground water flushing through the contaminated zone.
- most applicable for contaminated sites with enhanced dense, nonaqueous-phase liquids (DNAPLs).
Drawdown Pumping
Pumping may be accomplished with one or two pumps.
-A single-pump system reduces capital and operating costs and allows simpler control and operation, but produces a stream of mixed water from which NAPL must then be separated.
-The Dual Phase Extraction (DPE)/ free product recovery: for undissolved liquid-phase organics used primarily when a fuel hydrocarbon >8 inches thick is floating on the water table.
Drawdown pumping is effective for NAPL recovery when the aquifer has both:
Note: For best operation, the NAPL thickness should completely cover the pump suction port.
Drawdown pumping is:
Limitations:i) Limitations to all groundwater pumping:
The objective of groundwater pumping is to remove dissolved contaminants from the subsurface. It is also used as a hydraulic barrier to prevent off-site migration of contaminant plumes.
- The well design, pumping system & hence cost are dependent on the physical site characteristics and contaminant type.
- Wells and piezo-meters are used to monitor the contaminant levels to ensure remediation is occurring at an apt rate & verify its effectiveness.
Surfactant Enhanced Recovery
Injecting surfactant micelles or steam into a contaminated aquifer can facilitate the ground water pumping process by increasing the mobility and solubility of the contaminants in the soil.
The surfactant micelles can also facilitate the entrainment of hydrophobic contaminants - allowing removal of multi-phase contaminants. It therefore increases the mass removal of contaminant per pore volume of ground water flushing through the contaminated zone.
- Typical systems use a pump some distance away from the injection point to extract the ground water.
- The extracted ground water is then treated ex situ to separate the injected surfactants from the contaminants and ground water.
- To be cost-effective, the design of the surfactant-enhanced recovery system is critical.
- Once the surfactants have separated from the ground water they can be re-injected into the subsurface.
- Contaminants must be separated from the ground water, then treated prior to discharge of the extracted ground water.
- most applicable for contaminated sites with enhanced dense, nonaqueous-phase liquids (DNAPLs).
Drawdown Pumping
- Drawdown Pumping Wells are often used to pump LNAPL and ground water from recovery wells or trenches. Pumping removes water and lowers the water table near the extraction area to create a cone of depression (cod).
- This produces a gravity head, pushing flow of LNAPL toward the well to increase the thickness of the NAPL layer. In most cases, the greater a cone of depression, the greater the LNAPL recovery rates.
- Following recovery, it can either be disposed of or re-used directly in an operation.
- Systems may be designed to recover only product or recover separate streams of product and water.
Pumping may be accomplished with one or two pumps.
- Single-pump configuration: one pump withdraws both water and NAPL.
- Dual-pump configuration: one pump located below the water table to remove water and a second located in the NAPL layer to recover NAPL.
-A single-pump system reduces capital and operating costs and allows simpler control and operation, but produces a stream of mixed water from which NAPL must then be separated.
-The Dual Phase Extraction (DPE)/ free product recovery: for undissolved liquid-phase organics used primarily when a fuel hydrocarbon >8 inches thick is floating on the water table.
Drawdown pumping is effective for NAPL recovery when the aquifer has both:
- Moderate/high hydraulic conductivity: gives less flow resistance of NAPL into the well.
- A thick layer of low-viscosity NAPL: allows the pumping system to collect a high proportion of NAPL in relation to the amount of ground water.
Note: For best operation, the NAPL thickness should completely cover the pump suction port.
Drawdown pumping is:
- A commercially available technology
- Easily implemented with conventional pumps in wells or trenches
- Low/Moderate cost to install system, but operating cost varies greatly with amount of NAPL recovered.
Limitations:i) Limitations to all groundwater pumping:
- Potentially long time & high cost to achieve remediation goal
- Failure of system would result in plume migration
- Not applicable to contaminants with high residual saturation or high sorption capabilities
- Not applicable to homogeneous aquifers with hydraulic conductivity less than 10-5cm/sec.
- Potential toxic effects of residual surfactants .
- Increased solubility of contaminant may result in off-site migration.
- Generally large volumes of water produced when recovering the free product.
- The production of a cone depression in the water table can trap the fuel in the saturated zone when the water table returns to its original level
.