Using Soil Water Tracers to Estimate Recharge - Part 7

By GR Walker

This booklet covers the soil tracer techniques that have been shown to be useful for Australian situations. These important tracer techniques include those involving surface-applied tracers, historically-applied tracers and environmental soil tracers.

• Language: English
• Publisher: CSIRO PUBLISHING
• Release date: Aug 1, 1998
• ISBN: 9780643106086

Book preview

LIST OF CONTRIBUTORS

Peter Cook

CSIRO Land and Water,

Adelaide Laboratory, Private Bag No. 2 Glen Osmond, SA 5064

Colin Johnston

CSIRO Land and Water,

Perth Laboratory, Private Bag, PO Wembley, WA 6014

Ian Jolly

CSIRO Land and Water,

Canberra Laboratory, GPO Box 1666, Canberra 2600

Munna Sarma

CSIRO Land and Water,

Perth Laboratory, Private Bag, PO Wembley, WA 6014

Peter Thorburn

CSIRO Land and Water,

Adelaide Laboratory, Private Bag No. 2 Glen Osmond, SA 5064

Glen Walker

CSIRO Land and Water,

Canberra Laboratory, GPO Box 1666, Canberra 2600

OVERVIEW

In Australia, not all water-balance methods for estimating recharge are practical. Soil tracer techniques have been shown to be useful for Australian situations. Advantages include less-frequent visits to field sites, estimation of long-term mean water fluxes, and usefulness at low water fluxes. Important tracer techniques include those involving surface-applied tracers (e.g. bromide), historically applied tracers (particularly the ‘bomb’ tracers ³H and ³⁶CI), and environmental soil tracers such as the chloride ion. Because tracers need to be both mobile and conserved, anions and the less common isotopes of the elements occurring in water are generally used.

Of the above techniques, the most commonly used involve environmental tracers, particularly chloride. Methods can be subdivided into steady-state or transient, depending on whether there has been a recent change in land use. The underlying principles of the methods rely on either a balance of the fluxes of chloride within the soil zone or leaching of the chloride due to increases in drainage.

For areas of higher recharge, surface-applied tracers can be used. The vertical movement of these tracers can be used to infer the flux of water. The main difficulty with these methods is the time required for the tracer to move sufficiently deep into the soil profile. ‘Bomb’ tracers provide the equivalent of a surface-applied tracer except that the tracer was applied 30–40 years ago as a result of nuclear testing. These tracers are relatively expensive to use as they require sophisticated analysis methods.

All tracer methods are inverse techniques in that the recharge rate is inferred from the tracer profile rather than being measured directly. The accuracy of the method depends on the sensitivity of the recharge rate to errors of measurement or of model assumptions. It is important to understand the underlying assumptions of the tracer methods and to be aware when errors might invalidate the method.

The main assumptions of the major techniques are presented in this publication, as well as ways in which these assumptions are commonly violated in the field; we also present case studies in which these estimates have been tested in some way. We hope this will enable the reader to better understand whether soil tracer techniques are more suitable than other techniques for his or her field situation and, if so, what tracer technique might be best. We also hope that this will enable the reader to understand what errors are likely to occur and how these may be tested.

Most of the soil tracer techniques rely on one-dimensional vertical flow; some rely on piston flow. Some ignore anion exclusion, and some ignore vegetative recycling of solutes at the soil surface. Preferred-pathway flow will occur if the soil surface is ponded in part of the soil profile and there are large continuous soil pores to some depth. Macropores continuous to water tables of moderate depth have been noted in fractured rock and karstic systems. Otherwise, under normal agricultural use, numbers of macropores connected to the surface tend to decrease substantially below a few metres, and the field situation could still be amenable to some tracer techniques.

In cases where the stratigraphy is not horizontal, there is significant potential for lateral movement to occur, and this would invalidate most techniques. Spatial variability of recharge is an important problem for which there is no easy answer. Soil tracers only offer a point estimate, whereas what is often wanted is an areal estimate. Part of the solution to this problem involves being more specific with the problem definition. Piezometric monitoring will eventually fine tune estimates of recharge for areas where ground water is an important water supply, or where dryland salinity is a problem.

What we generally want from soil tracer methods is a representative value for a given land use on a given soil type. These can then be used to identify areas of high recharge or land practices that lead to high recharge rates. Also, from these estimates, one can relate the recharge rate to factors such as land use, rainfall history and soil type, and hence broadly understand how recharge may vary across the landscape. Forms of remote sensing, such as electromagnetic induction (ground- or aerial-based) or Landsat also provide some scope for indicating the variability of recharge.

The choice of tracer depends on the likely range of recharge rates. For high recharge rates in which the timescale associated with leaching through the root zone is less than one year, an artificial tracer method is the most appropriate. Of the artificial tracers, bromide appears to be the most useful in that it is easily measured, background levels are small, it is not taken up by plants, and it is non-radioactive and non-toxic.

For lower recharge rates in which the time scale associated with leaching through the root zone is less than 10 years, the bomb tracers may be used. However, bomb tracers only partly address the main problems in using environmental chloride methods — that is, spatial variability, lateral movement, and preferred pathway flow. For the Southern Hemisphere, radioactive decay has decreased the usefulness of tritium, leaving ³⁶CI as a likely candidate for a bomb tracer. There are only a handful of unsaturated-zone studies involving ³⁶CI, and a reliable method of estimating fallout is still lacking.

There are certain circumstances where chloride methods fail, even under conditions of one-dimensional flow and no preferred-pathway recharge. For example, in areas in which there is a higher recharge following clearing, it may be difficult to locate a chloride front within a reasonable depth. Also, there may not have been a well-defined chloride front initially. However, one may not be confident that the concentration in the soil water has reached levels corresponding to