Evaluating non-equilibrium solute transport in small soil columns.

Area of Science:

• Soil Science
• Environmental Chemistry
• Hydrology

Background:

• Soil structure and organic carbon content are key factors influencing water and solute transport.
• Preferential flow paths can lead to rapid and uneven movement of water and contaminants through soil.
• Understanding pesticide leaching is crucial for assessing environmental risk and developing mitigation strategies.

Purpose of the Study:

• To investigate the impact of soil properties on bromide and pesticide (atrazine, isoproturon) leaching.
• To quantify the contribution of preferential flow paths to solute transport and variability.
• To compare the effectiveness of different modeling approaches (CDE, two-region) in describing transport.

Main Methods:

• Conducted displacement studies using undisturbed soil columns from two North German sites with differing soil characteristics.
• Analyzed breakthrough curves (BTCs) using the equilibrium convection-dispersion equation (CDE) and a non-equilibrium two-region/mobile-immobile model.
• Determined transport parameters using the curve fitting program CXTFIT and the time moment method.

Main Results:

• Preferential flow columns (PFCs) significantly contributed to leached fractions (up to 58%) and pesticide variability (over 80% of SD).
• Non-equilibrium models indicated substantial immobile water fractions in PFCs (25-72%), suggesting complex non-equilibrium mechanisms.
• The CXTFIT program provided a better overall reproduction of BTCs compared to the moment method, especially for PFCs.

Conclusions:

• Soil structure and organic carbon content create significant variability in flow paths, impacting pesticide leaching.
• Preferential flow is a dominant process in certain soil types, necessitating advanced modeling approaches.
• The CXTFIT method is more reliable for parameter estimation in complex flow systems than the moment method.