Table of Contents

Introduction

Part I

Appendix A

Appendix B

Appendix C

Part II A and B

Part II C and D

Part III

Part IV

References

Summary

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 /t_a, where _e is the effective porosity, L is the distance along the flow path, and t_a 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, T_t, stored in the profile. This is given by

where T(z) is the tracer concentration of water in the unsaturated zone at a distance z beneath the surface, and (z) is the volumetric water content. For evaporative tracers, such as ³H, mean annual recharge can be estimated by

where T_pi is the tracer concentration of recharge water i years before the present; w_i is the annual recharge weighting factors, and is the tracer decay constant. In this analysis, non-piston flow can be handled because is independent of the distribution of the tracer in the profile.

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 depends on the situation. In most cases, the tracer is used to follow water movement and hence should move with the water. The tracer thus needs to be mobile and soluble; it should not be strongly retarded by the soil or aquifer matrix. Ideally, the tracer should be nonreactive and not transform during transport. Of course, the tracer needs to be easily measured and easily extracted from the soil. If artificial tracers are used, additional constraints need to be satisfied, such as low natural levels of the tracer in the environment, low toxicity, and low radioactivity. For environmental tracers, it is desirable to have large natural variations of tracer concentrations in the landscape. These constraints usually mean that only anions (Cl, Br, ³⁶Cl) or isotopically labeled water molecules (²H, ¹⁸O, and ³H) can be used.

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 Z_r /R, where Z_r is the root zone depth, the volumetric water content, and R the recharge rate. For example, in a humid climate (with = 0.1, Z_r = 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).

Bromide is the most widely used artificial tracer and ³H and ³⁶Cl are the most suitable bomb tracers. However, ³H and ³⁶Cl are too expensive for spatial and temporal variability studies that require many samples. The use of tritium as an artificial tracer is not generally recommended because of concerns about radioactivity and difficulty in application. For such investigations, the use of chloride as an environmental tracer is generally recommended. Employment of multiple tracers can often provide corroborative information needed for correct interpretation.

Because tracers do not measure water flow directly, a number of problems can arise, leading to over- or underestimation of recharge. These problems include secondary (unknown) tracer inputs, mixing, and dual flow mechanisms; such problems only arise if the sources, sinks, and pathways of tracer are not fully understood. Part of the recharge going through preferred pathways (such as root channels or fissures) may invalidate the results of a tracer study.