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Introduction

Chlorofluorocarbons (CFCs) are synthetic organic compounds that have been widely used in various industrial applications since the 1930s. Their unique chemical properties and atmospheric history make them valuable tools for dating young groundwater, typically ranging from 0 to 60 years old. This article explores the principles, applicable age range, influencing factors, and real-world applications of CFCs dating techniques.

Principles of CFCs Dating

CFCs, such as CFC-11. CFC-12. and CFC-113. were released into the atmosphere in significant quantities beginning in the mid-20th century. These compounds are highly stable and have no natural sources, making them ideal tracers for modern groundwater. The principle of CFCs dating relies on measuring the concentration of these compounds in groundwater and comparing it to historical atmospheric CFCs records. Since CFCs concentrations in the atmosphere have varied over time, the measured concentration in groundwater can help estimate the time since the water last recharged.

Applicable Age Range

CFCs dating is primarily suitable for young groundwater, with an effective age range of 0 to 60 years. This timeframe is determined by the historical atmospheric CFCs concentration records, which began in the 1940s. For groundwater older than 60 years, the CFCs concentration may have reached equilibrium with the atmosphere, making it difficult to distinguish ages accurately. However, when combined with other dating methods like tritium (³H), the applicable age range can be extended.

Table: Comparison of Groundwater Dating Methods

Method	Applicable Age Range	Key Tracers	Advantages	Limitations
CFCs Dating	0-60 years	CFC-11. CFC-12. CFC-113	No natural sources, stable in atmosphere	Limited to young groundwater, affected by local pollution
Tritium Dating	0-50 years	Tritium (³H)	Cost-effective, widely applicable	Affected by nuclear testing, limited age range
Radiocarbon Dating	100-50.000 years	Carbon-14 (¹⁴C)	Suitable for old groundwater	Complex corrections needed, affected by geological processes

Influencing Factors

The accuracy of CFCs dating can be influenced by several factors:

• Hydrogeological Conditions: The thickness of the unsaturated zone and groundwater flow paths can affect CFCs concentrations.
• Local Pollution Sources: Industrial or agricultural activities may introduce CFCs into groundwater, interfering with dating results.
• Redox Conditions: Chemical reactions in the subsurface can alter CFCs concentrations.

Application Cases

CFCs dating has been successfully applied in various hydrogeological studies worldwide. For example:

In the Laizhou Bay coastal aquifers in China, CFCs and tritium were used to study groundwater flow and saltwater intrusion, revealing groundwater ages up to 50 years.

In karst aquifers, CFCs dating helped estimate groundwater residence times and vulnerability to pollution.

Conclusion

CFCs dating is a powerful technique for determining the age of young groundwater, with a primary applicable age range of 0 to 60 years. Its advantages lie in the stability and atmospheric history of CFCs, which provide a clear timeline for modern groundwater recharge. However, the method’s limitations, such as sensitivity to local pollution and hydrogeological conditions, must be considered. By combining CFCs dating with other methods, researchers can enhance the accuracy and applicability of groundwater age assessments, aiding in sustainable water resource management.