### The Journey of a Dense Non-Aqueous Phase Liquid (DNAPL) in a Heterogeneous Subsurface Environment#### IntroductionDense Non-Aqueous Phase Li...
Published on by Hossein Ataei Far, Deputy Manager of the Research, Technology Development, and Industry Relations Center at NWWEC
#### Introduction
Dense Non-Aqueous Phase Liquids (DNAPLs) are hazardous contaminants originating from industrial solvents, coal tar, and pesticides. Heavier than water, DNAPLs tend to sink and persist in the subsurface, where their movement in heterogeneous environments with diverse soil and rock structures is complex. Studying DNAPL migration is essential for effective groundwater contamination management.
#### Characteristics of DNAPL Migration
In heterogeneous settings, DNAPL migration is governed by various factors:
1. **Density and Viscosity**: Due to high density, DNAPLs migrate downward, forming dense vertical plumes.
2. **Interfacial Tension**: High tension between DNAPLs and water limits their spread, often leading to small, immobile pools within pore spaces.
3. **Capillary Trapping**: DNAPLs can become trapped above low-permeability layers, like clay, which prevents further downward movement and creates persistent contamination zones.
#### Stages of DNAPL Migration
1. **Downward Migration**: DNAPLs initially migrate downward, but heterogeneity alters this path as DNAPLs encounter different layers.
2. **Lateral Spreading**: On low-permeability layers, DNAPLs spread horizontally, creating wider contaminant plumes.
3. **Pooling and Residual Saturation**: Capillary forces lead to DNAPL droplets being immobilized in pores, forming long-term reservoirs of contamination.
4. **Dissolution**: DNAPLs slowly dissolve, contaminating groundwater over extended periods. Dissolution is prolonged in larger or more dispersed pools.
#### Advances in Understanding DNAPL Migration
Recent research has refined our understanding of DNAPL migration by integrating complex models of physical and chemical heterogeneities. These advanced models capture how varied subsurface features affect plume dispersion, enhancing DNAPL fate prediction and remediation efforts. They are particularly useful for identifying and managing contamination hotspots created by intricate features, such as dead-end zones and clay lenses.
#### Remediation Challenges in Heterogeneous Environments
DNAPL remediation in such settings is difficult because:
- **Limited Accessibility**: DNAPL pools trapped under impermeable layers are hard to access using standard techniques.
- **Long-Term Persistence**: DNAPLs have low solubility and are often resistant to natural attenuation, limiting biodegradation potential.
- **Variable Permeability**: Effective remediation requires comprehensive mapping to identify DNAPL pools accurately.
#### Conclusion
The journey of DNAPLs through heterogeneous subsurface environments is shaped by density, viscosity, interfacial tension, and capillary forces. Subsurface heterogeneity creates challenges that complicate remediation, as DNAPL contamination tends to persist. Recent advances in imaging and modeling are enhancing DNAPL management strategies, offering a more accurate understanding of migration paths and supporting more effective remediation techniques.
#### References
1. Sale, T. C., Parker, B. L., Cherry, J. A., & Parker, R. A. (2008). *Geological and hydrogeological factors affecting DNAPL source zone architecture and remediation*. Environmental Science & Technology, 42(4), 1192-1198.
2. Mercer, J. W., & Cohen, R. M. (1990). *A review of immiscible fluids in the subsurface: Properties, models, characterization, and remediation*. Journal of Contaminant Hydrology, 6(2), 107-163.
3. Pankow, J. F., & Cherry, J. A. (1996). *Dense Chlorinated Solvents and Other DNAPLs in Groundwater: History, Behavior, and Remediation*. Waterloo Press.
4. Guleria, A., Gupta, P. K., Chakma, S., & Yadav, B. K. (2023). *Unraveling the Fate and Transport of DNAPLs in Heterogeneous Aquifer Systems—A Critical Review and Bibliometric Analysis*. Sustainability, 15(10), 8214.
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