Bul 249–Ground-water Recharge and Water Budgets–––pages 22 to 23
Appendix B. Tracers for Recharge Estimation (Allison et al., 1994)
The natural tracers most commonly used in recharge studies are 3H, 14C, 36Cl, 15N, 18O, 2H, 13C, and Cl. Of these, the first three are radioactive, with half-lives of 12.3, 5700, and 301,000 years, respectively. Their concentrations in the hydrologic cycle have been affected greatly by nuclear testing. Both tritium, 3H, and chlorine-36, 36Cl, from atmospheric testing have been used for soil-water tracing and recharge studies. Chlorine-36 has been used increasingly as more analytical facilities have become available. Input concentrations of the other isotopes mentioned above also have changed in time, but across a much longer time scale, due to changes in temperature and rainfall patterns. However, little is known of the temporal changes in the fallout of Cl.
Of the tracers mentioned above, tritium (3H), deuterium (2H), and oxygen-18 (18O) most accurately simulate the movement of water because they form part of the water molecule. In most soils, chlorine-36 and nitrate (NO3) move as the water does, but in some soils with heavier textures, anion exclusion may be a problem, and the tracer may move more rapidly than the water being traced.
Most of the recently developed isotope techniques are aimed at determining the age of water, which in turn permits calculation of ground-water travel time. The recharge rate, R, can then be calculated by R = L e /ta, where e is the effective porosity, L is the distance along the flow path, and ta is the travel time or age of the ground water at the distance L. The three basic types of ground-water dating methods are (1) those methods which rely on input concentrations that have changed in time and are well known, such as the radioactive noble gas krypton-85, and the synthetic organic compounds chlorofluorocarbons (CFCs), used for dating young waters (less than ~40 years old); (2) tracers for which input concentrations have been constant, and decreases in concentration with time occur due to radioactive decay, such as 14C, used for dating waters over the time scales of 200 to 20,000 years; or (3) methods where the input concentrations may have changed with time but can be determined because both parent and daughter isotopes are measured, such as 3H/3He (tritium/tritiogenic helium), which ratio also is used to date young waters (0 to ~50 years).
Mechanisms of tracer infiltration will affect the interpretation of results. Although piston (or plug) flow is often able to explain the behavior of tracers in the field, there is convincing evidence, particularly from humid regions, that water movement along preferred pathways is the rule rather than the exception. Thus, preferential or non-piston-type flow must be dealt with in any comprehensive analysis of recharge. For example, 3H was found much deeper than the recharge rate would imply in native forest, suggesting preferred flow of water along root channels (Allison and Hughes, 1983).
Three techniques have been used for estimating recharge rates from tracer profiles in the unsaturated zone (Allison et al., 1994).
1. From the position of the tracer peak. In this
method, the water in the profile above the peak in tracer concentration
represents the recharge since the time that peak occurred. Any bypass
(preferential) flow will result in recharge being underestimated.
2. From the shape of the tracer profile in the soil.
This is generally more reliable than Method 1 above because information
about flow mechanisms can be obtained. In order to obtain estimates of
mean annual recharge, , a
weighting function that takes into account year-to-year variations of
recharge is needed.
3. From the total amount of tracer, Tt, stored in the profile. This is given by
Tracer methods have a number of attractive attributes (Hendrickx and Walker, 1997). Their movement is governed mainly by the long-term mean soil-water fluxes that lead to recharge. (Many water-balance or soil-water-pressure-based techniques measure fluxes on a much smaller time scale than is needed for recharge estimates.) The use of tracers does not necessitate frequent visits to the field. With tracers, it is possible to estimate smaller fluxes than with other methods. Finally, they are often the only alternative.
The choice of tracer is mainly determined by the time scale of the recharge process (Hendrickx and Walker, 1997). Use of artificial tracers requires that the bulk of the applied tracer has passed through the root zone. The time scale associated with leaching through the root zone is Zr /R, where Zr is the root zone depth, the volumetric water content, and R the recharge rate. For example, in a humid climate (with = 0.1, Zr = 3.3 ft, and R = 3.9 inches/yr), the time scale is one year. However, in an arid climate with a recharge rate of 0.4 inch/yr, the time scale is 10 years. While the former time scale is short enough for the tracer to be applied and the soil sampled in succeeding years, the latter is probably not. However, it would be suitable to use a bomb tracer (i.e. a tracer resulting from nuclear testing).
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Web version August 2004. Original publication date April 2004.