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The invisible threat: Why are NAPLs so difficult to detect and remove?
Non-aqueous phase liquids (NAPLs) are organic liquid pollutants that are relatively insoluble in water. Common examples include petroleum products, coal tar, chlorinated solvents and pesticides. With the expansion of pollution removal strategies at the end of the 20th century, the removal of NAPLs has received increasing attention, but their detection and removal still face many challenges. Among various sources, NAPLs can be released into the environment from improper handling of chemicals, leaks from underground storage tanks, septic tank discharges, and infiltration from spills or landfills. The complexity of the movement of NAPLs in subsurface environments makes it difficult to characterize, however, a fundamental understanding of these parameters is critical to selecting appropriate remediation strategies.
Because NAPLs are involved in biological decomposition chains, this allows them to produce intermediate chemicals that pose a particularly acute risk to human health.
Historical background
Prior to 1978, public awareness of groundwater contamination was relatively low. Historically, groundwater has been an important source of water for public water systems, private wells, and agricultural systems. People generally believe that impurities are filtered out of water as it passes through the soil, so there is no concern about underground environmental pollution. In the 1960s, with a large-scale survey of the groundwater contamination literature, awareness of the presence of organic contaminants such as petroleum hydrocarbons began to emerge. With the development of gas chromatography technology in the early 1970s, it became possible to detect groundwater contaminants that were difficult to detect with human senses. This development led to the discovery of extremely harmful NAPLs such as chlorinated solvents and a shift from simple detection of substances to in-depth research on the treatment and removal of NAPLs.
Transport mechanism of NAPLs
The behavior of NAPLs underground is guided by both the composition of the underlying area and the nature of the NAPLs. The underground environment can be divided into two main zones: unsaturated zone (vadose zone) and saturated zone (phreatic zone). When a liquid penetrates into the unsaturated zone under high rainfall conditions, it will penetrate into the saturated zone if the liquid volume is large enough. The behavior of NAPLs is also related to their physical properties. According to the density of NAPLs relative to water, they can be divided into light non-aqueous phase liquids (LNAPLs) and heavy non-aqueous phase liquids (DNAPLs).
LNAPLs tend to float above the water level, while DNAPLs will sink below the water level. The existence of DNAPLs causes lasting harm to groundwater sources and the ecological environment.
The Challenge of Removal Strategies
Relatively small volumes of NAPL can trigger toxic conditions in groundwater, and NAPLs can remain underground for years or even centuries to contaminate groundwater. In addition, the detection of NAPLs is more difficult due to their heterogeneous behavior, so detection strategies must be coordinated when removing NAPLs. In this regard, it is critical to quantify the geographical and phase distribution of NAPLs to help determine the appropriateness of remediation strategies. Of course, the coexistence of effective detection and remediation requires not only specific soil and water level characteristics, but also real-time environmental monitoring.
Through continued research, we may be able to find better ways to deal with these invisible hazards.
Conclusion
How to effectively detect and remove these elusive NAPLs remains an unsolved challenge. Advances in technology may provide new solutions, but continued research and management efforts are key. When we face these problems, can we break through the limitations of existing technology and find an efficient and economical solution?
