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INTRODUCTION
Although groundwater around the world is the most abundant freshwater resource, it is not always available for human consumption as a result of its natural quality or due to contamination. Furthermore, the groundwater residence time may be very long, often thousands of years, especially in deep aquifers or under arid conditions. As a result, the continuous use of these deep, old resources with scarce or no recharge may be unsustainable (Kazemi et al. 2006).
In the important field of hydrogeology and groundwater resources, new scientific, technical, and legal questions are continually being addressed. The resolution of a large number of these problems requires some understanding of the residence time and age of groundwater. In this framework, groundwater age studies require integral and holistic research using different tools (geological, geomorphological, geochemical, stratigraphic, isotopic, etc.). Thus, various pieces of evidence can be obtained that contribute to the better understanding of the regional hydrogeological behavior, an important and vital basis for groundwater management.
The use of stable and radioactive isotopes is of great interest for the development of hydrogeological models, allowing identification of the water origin and age, water mixing from different origins, and water residence time in aquifers. The isotopic techniques require good knowledge of the conceptual hydrogeological model (lithology, hydraulic connections, geochemical processes, etc.) as well as information for the age assessment.
When subsurface flow systems are studied, it is necessary to recognize that they have an inherent complexity (Turnadge and Smerdon 2014). A groundwater sample will generally be the result of mixing that can be attributed to processes such as mechanical dispersion, chemical diffusion, and preferential flow, each of which has the potential to complicate environmental tracer interpretation. Corresponding to the range of mixing that can occur, a broad range of definitions for groundwater age exists; these have been summarized previously by Cook and Böhlke (2000), Kazemi et al. (2006), and most recently by McCallum et al. (2014a), among others. The simplest method by which to estimate the age of a groundwater sample is to use Darcy's law while assuming lateral flow only; this is known as the hydraulic age (Kazemi et al. 2006). Another calculation of groundwater age relates to ages derived from...