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Combined physical and chemical nonequilibrium transport model for solution conduits
Malcolm S Field1, Feike J Leij2
1U.S. Environmental Protection Agency, National Center for Environmental Assessment (8623P), 1200, Pennsylvania Ave., N.W., Washington D.C. 20460, USA.
Journal of Contaminant Hydrology
|December 3, 2013
Summary
This study introduces a combined physical and chemical nonequilibrium model for solute transport in karst aquifers. The model accurately fits tracer test data, highlighting the importance of both physical and chemical nonequilibrium processes in these complex environments.
Area of Science:
- Hydrogeology
- Environmental Science
- Geochemistry
Background:
- Solute transport in karst aquifers is complex, occurring in solution conduits with rapid, turbulent flow.
- Tracer tests in these systems often show positively-skewed breakthrough curves, indicating physical nonequilibrium.
- Chemical nonequilibrium processes, involving interactions between aqueous and solid phases, are also likely significant but less studied.
Purpose of the Study:
- To develop and apply a combined physical and chemical nonequilibrium (PCNE) model for solute transport in karst aquifers.
- To represent both physical nonequilibrium (mobile/immobile regions) and chemical nonequilibrium (equilibrium/nonequilibrium partitioning) in conduit flow.
- To demonstrate the model's applicability to real-world tracer test data from karst systems.
Main Methods:
- Developed a PCNE model for instantaneous solute release in karst conduits.
- Incorporated mobile and immobile flow regions with first-order mass transfer for physical nonequilibrium.
- Modeled aqueous-solid phase partitioning as equilibrium or first-order processes for chemical nonequilibrium.
Main Results:
- The PCNE model provided exceptionally good fits to three example breakthrough curves from U.S. karst aquifers.
- Transport properties (velocity, dispersion) varied significantly across the studied sites.
- Most solute transport was attributed to the mobile water fraction, influenced by both physical and chemical nonequilibrium.
Conclusions:
- The combined PCNE model is effective for analyzing tracer tests in karst aquifers.
- Physical and chemical nonequilibrium processes interact and are crucial for accurately describing solute transport.
- Understanding these nonequilibrium processes is essential for predicting contaminant movement in karst environments.
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