Do conservative solutes migrate at average pore-water velocity?

Area of Science:

• Environmental Science
• Hydrogeology
• Geochemistry

Background:

• The advective velocity of conservative solutes is often assumed equal to average linear pore-water velocity.
• Direct monitoring reveals discrepancies in heterogeneous media, with solute plumes moving slower than expected.
• This difference may stem from varying pore systems influencing solute transport differently than bulk water flow.

Purpose of the Study:

• To investigate the relationship between advective solute velocity and average pore-water velocity in heterogeneous subsurface environments.
• To determine if and how subsurface heterogeneity affects the speed of solute plume migration.
• To validate laboratory findings against field observations of solute transport.

Main Methods:

• Conducted controlled laboratory tracer tests under one-dimensional and three-dimensional (3D) flow conditions.
• Utilized conservative solutes and dye tracers to monitor plume migration.
• Calculated average pore-water velocity using volumetric flow rates and Darcy's law.
• Compared solute advective velocity with calculated pore-water velocity in homogeneous and heterogeneous models.

Main Results:

• Under one-dimensional flow, solute advective velocity matched average pore-water velocity.
• In a 3D flow system, solutes migrated at approximately 65% of the average pore-water velocity.
• Direct observation confirmed slower solute plume movement in heterogeneous sections compared to homogeneous ones.

Conclusions:

• Subsurface heterogeneity significantly slows the advective velocity of discrete solute plumes relative to average pore-water velocity in 3D flow systems.
• The assumption of equal velocities may not hold true in complex geological settings, impacting contaminant transport models.
• Understanding these velocity differences is crucial for accurate environmental remediation and groundwater management.