2.1 Introduction

There are numerous tracers that have application to hydrogeology. Three groups of tracers are described here:

• Tracers that provide information on the age of groundwater and that are principally used for determining water velocities and aquifer recharge rates. Types of tracers that fit into this group include radioactive tracers (Section 2.3), radiogenic tracers (Section 2.4) and event markers (Section 2.5).
• Tracers that provide information on reaction processes and phase changes (e.g., evaporation, and condensation). Stable isotopes (Section 2.6) are one of the most important tracers in this group.
• Tracers that are used for identifying water sources and groundwater flow and mixing. In addition to the tracers mentioned above, this group of tracers includes noble gases (Section 2.7) and ion concentrations. Ionic tracers (such as chloride) are not discussed.

The following sections summarizes key principles of these different types of environmental tracers. Sampling and measurement techniques are not discussed in this book but can be found in several specialist texts (e.g., Fritz and Fontes, 1980; Clark and Fritz, 1997; Cook and Herczeg, 2000; Clark, 2015). The most commonly used units for measurement for many of the environmental tracers are summarized in Table 1. In general, stable isotope abundances are reported as deviations from a standard, rather than absolute values, whereas tracers used for groundwater dating can be reported in units of radioactivity or concentration.

Table 1 – Different approaches to reporting tracer concentrations in groundwater, together with the most commonly used units for the most widely used tracers. (The examples include chemical elements, with the number to the upper left indicating the number of protons in the nucleus. Rn is Radon, Kr is Krypton, C is Carbon, Ar is Argon, H is Hydrogen (the form with 3 protons is called Tritium and the form with 2 protons is called Deuterium), Cl is chlorine, Sr is Strontium, O is oxygen, CFC is Chlorofluorocarbon, SF₆ is Sulfur Hexafluoride, N₂ is Nitrogen gas, He is Helium. CFC and SF₆ concentrations in water are usually reported as grams or moles of substance per kg of water. However, atmospheric concentrations are reported in parts per trillion, by volume, often abbreviated as ppt or pptv.)