Two-stage aquifer pumping subject to slow desorption and persistent sources

Area of Science:

• Environmental Engineering
• Hydrogeology
• Remediation Technologies

Background:

• Pump-and-treat is a common groundwater remediation method facing challenges with nonequilibrium desorption, low-permeability zones, and persistent sources like non-aqueous phase liquids (NAPL).
• Traditional single-stage pumping can lead to excessive water extraction, increasing costs and operational burdens.

Purpose of the Study:

• To optimize a two-stage operational scheme (initial high-rate Q1, followed by low-rate Q2) for pump-and-treat remediation.
• To determine the optimal switch time from Q1 to Q2 and the ideal Q2 value.
• To evaluate the strategy's effectiveness in reducing extracted volumes under various contaminant source and aquifer conditions.

Main Methods:

• A two-dimensional groundwater flow and transport model was employed to simulate remediation scenarios.
• Simulations covered finite (NAPL-free) and continuous (NAPL-driven) contaminant sources in homogeneous and zoned aquifers.
• Remediation was simulated over 2000, 6000, and 15,000 days with mass removal and concentration constraints.

Main Results:

• The two-stage strategy achieved equivalent cleanup levels to single-stage pumping while reducing extracted volumes by up to 36% in NAPL-free homogeneous aquifers under nonequilibrium desorption.
• Significant volume savings (comparable to NAPL-free) were achieved for NAPL-driven sources only after source concentration reduction by at least two orders of magnitude.
• In zoned aquifers, the proposed strategy yielded volume savings of up to 24% under NAPL-free and reduced source conditions.

Conclusions:

• The optimized two-stage pumping strategy effectively reduces extracted groundwater volumes, offering a more efficient approach to pump-and-treat remediation.
• The strategy's success is contingent on contaminant source characteristics and aquifer heterogeneity, particularly for persistent NAPL sources.
• This approach provides substantial cost-saving potential by minimizing water handling and treatment volumes.