Experiments on vertical transverse mixing in a large-scale heterogeneous model aquifer

Area of Science:

• Environmental Science
• Hydrogeology
• Geochemistry

Background:

• Vertical transverse mixing significantly influences the natural attenuation of biodegradable contaminant plumes from continuous sources.
• Understanding mixing in heterogeneous media is vital for accurate environmental fate assessments.

Purpose of the Study:

• To quantify vertical mixing coefficients in heterogeneous porous media.
• To assess the impact of sediment heterogeneity on vertical transverse mixing.
• To compare mixing coefficients from conservative and reactive tracer tests.

Main Methods:

• Conducted conservative and reactive tracer tests in a quasi-two-dimensional sand box (14m long).
• Analyzed digital images of conservative tracer concentration distributions at steady state.
• Utilized inverse modeling of stream-line patterns to correct for plume meandering caused by heterogeneity.
• Visualized reactive plume outlines using pH indicators.

Main Results:

• The vertical dispersion coefficient for conservative tracers was determined to be approximately 4 x 10⁻⁹ m²/s.
• Sediment heterogeneity showed minimal impact on the vertical transverse mixing coefficient with increasing travel distance.
• The transverse dispersion coefficient for the reactive plume was estimated at approximately 3 x 10⁻⁹ m²/s.
• Vertical transverse dispersion coefficients were found to be only slightly larger than pore diffusion coefficients.

Conclusions:

• Vertical transverse mixing is largely independent of sediment heterogeneity in the studied system.
• Reactive plumes may expand considerably if their spread is governed by vertical dispersive mixing.
• Findings are critical for modeling natural attenuation and predicting contaminant plume behavior.