The most direct way to obtain information on groundwater flow is to follow (or trace) the movement of a dissolved substance within groundwater. This approach is most straightforward when a substance is added to the groundwater that was not originally present, and this tracer is added only for a short period of time. Substances like this that are deliberately added for the purpose of studying groundwater movement are referred to as artificial tracers and are not discussed here. However, because groundwater movement is often very slow (sometimes only a few meters per year or less), artificial tracers are of limited use for understanding large groundwater systems. Fortunately, there are several environmental tracers that can provide similar information. Often, these environmental tracers have been added to the groundwater by natural processes operating over long periods of time. Measurement of these tracers can therefore provide information on processes that have operated over large spatial and temporal scales. This is particularly useful for understanding natural flow systems in large basins.
Here, environmental tracers are defined as geogenic (natural to the Earth) or anthropogenic (human created) isotopes, elements or compounds that are widely distributed in the near-surface environment of the earth, such that variations in their abundances can be used to infer environmental processes. Isotopes are a particular category of environmental tracers and are variants of chemical elements that have the same atomic number but differ in their mass number due to different numbers of neutrons in their nucleus. An ideal tracer of water flow is one which is soluble, mobile, relatively unreactive, and easily measured. However, while tracers that behave conservatively (that is, do not cling to subsurface materials nor undergo chemical change) in the environment yield information on water sources or transport processes, tracers that readily undergo chemical reactions can be used to determine hydrochemical conditions in aquifers and reaction pathways. Several environmental tracers also provide information on timescales of subsurface processes. These include radioactive tracers, which decay at a known rate; radiogenic tracers, which are produced and accumulate in the subsurface; and event markers, which are neither produced nor consumed in the subsurface but have a variable and well-known history as to when they entered the groundwater. These tracers can be used to estimate the age of groundwater, which can be used to determine groundwater velocities. Applications of environmental tracers in groundwater therefore include estimation of:
- Mixing between different water sources (Section 3.1);
- Processes and rates of groundwater recharge (Sections 3.2, 3.4 and 3.5);
- Groundwater flow rates (Section 3.3);
- Flow of groundwater to rivers (Section 3.6);
- Groundwater flow through fractures (Section 3.7);
- Palaeoclimate and palaeohydrology (Section 3.8);
- Sources of salinity and other contaminants (Section 3.9); and,
- Calibration of groundwater models (Section 3.10).
The starting point for interpreting environmental tracers is that transport is dominated by advection (transport of a substance by the bulk motion of the groundwater), rather than diffusion and dispersion. This assumption means that as the tracer moves along a flow line water gets progressively older. Within a relatively homogeneous aquifer, it also means that water gets older with depth (Figure 1). There are, of course, several qualifications and exceptions to this general principle, but it provides a good starting point for us to think about the behavior of tracers.