A tracer as used in hydrology is a substance in water that can be used for tracking water movement. Tracers primarily are used to determine the direction and rate of water movement. Knowledge of the rate at which groundwater flows can be useful, for example, in calculating the rate of contaminant movement in an aquifer . Knowledge of the rate at which river-water flows can be used to estimate the timing of floodwater movement in rivers.
Tracers also can be used to determine groundwater age. This estimate, in conjunction with knowledge of the direction of groundwater movement, can facilitate determination of the rate of movement.
Hydrologists make use of both environmental tracers (natural or humanmade compounds present in the environment) and purposely introduced tracers (such as dyes or chloride that are injected into water).
Introduced tracers, such as dyes or chloride, are used to determine the direction and rate at which groundwater moves, the rate at which river-water flows, and mixing characteristics. Hydrologists can inject a tracer into a well (the injection well), and monitor nearby wells for the arrival of the tracer. Arrival of the tracer at a nearby well indicates movement of groundwater from the injection well toward that well, and thus allows determination of direction and rate of groundwater flow. In rivers, measurement of the rate at which a river carries an introduced tracer allows the river's flow rate to be calculated.
Isotopes of oxygen in water molecules are used to determine the source of groundwater and the direction of groundwater movement. The technique is based on the observation that proportions of heavy and light oxygen isotopes in water molecules often are distinct for water from different sources. For this technique to work, the different sources must have distinct isotopic signatures (a distinctive chemical composition), and all the potential sources must be characterized. To determine the direction of groundwater movement, hydrologists can map the movement of water from sources to observation points.
For example, in an agricultural area where irrigation water is derived from rivers, the groundwater underneath this area may contain a mixture of water that originated as precipitation and as irrigation water. By knowing the proportions of heavy and light oxygen isotopes in precipitation and in irrigation water, hydrologists can calculate how much of the groundwater originated from each of these two sources.
Tritium frequently is used as a tracer in groundwater studies. Tritium ( 3 H) is a radioactive isotope of hydrogen. It has both natural and human-made sources. Large quantities of tritium were produced as a result of atmospheric testing of nuclear bombs beginning in the early 1950s. This so-called "bomb tritium" is present in groundwater that entered aquifers after the early 1950s. Thus, the presence of large concentrations of tritium in groundwater indicates the presence of "modern water," that is, water that entered the aquifer after the early 1950s. This information can often be used to determine the direction and rate of groundwater movement.
Stephen R. Hinkle
Clark, Ian, and Peter Fritz. Environmental Isotopes in Hydrogeology. Boca Raton, FL: Lewis Publishers, 1997.
Cook, Peter, and Andrew Herczeg, eds. Environmental Tracers in Subsurface Hydrology. Boston, MA: Kluwer Academic Publishers, 2000.
Rantz, S. E. et al. Measurement and Computation of Streamflow. U.S. Geological Survey Water-Supply Paper 2175, 1982.