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Want to know more?The NCGRT is conducting a wide variety of research on tracers. In November 2012, the NCGRT organised a well-attendedand successful workshop at its UNSW node. The workshop focused on theoretical and technical issues around the use ofheat as a tracer of water ow in groundwater and at interfaces between groundwater and surface-water bodies.

For more information about our projects, visit www.groundwater.com.au

DATA AND MODELLINGTypically, the condence that wecan have in the predictions made bygroundwater models increases as theavailability of quality data increases.

It is often said that we are model richand data poor, meaning that we oftendont have enough data to correctlycalibrate models. Or is it that we arenot data poor, but neglecting other datasources?

Water level data over time is a commondata source that is used to calibratemodels. Field estimates of the propertiesof the aquifer (i.e. the most difcultparameter in groundwater: hydraulic

conductivity) are also observations thatare widely used. The issue with thesedata types is that they tell us exactlywhat is happening at the point ofmeasurement, but they may tell us verylittle about what may be happening onlymetres away.

It is widely understood that using waterlevel data alone can lead to what isknown as non-uniqueness, where awide range of parameter values may beable to reproduce eld data, but youcant be condent in the predictions ofthese models. That is, you could have100 different models that t the data, butwhich one is the right one? They may teld data for the wrong reasons.This iswhere tracer data can be useful.

WHAT ARE TRACERS?In a tracer test, one possibility is for atracer (coloured dye, a chemical orwater with a different temperature) to beinjected into one location (say, a well)and the time taken for the tracer to get to

another location (a stream, for example)is measured. This can tell us about whatis happening between our water levelmeasurements.

Other possible tracers, known asenvironmental tracers, are naturally-occurring chemicals or temperaturedifferences. Environmental tracers cantell us about where groundwater mayhave come from, and when it fell as rainand recharged an aquifer.

Temperature is a widely usedenvironmental tracer; temperaturedata is commonly used to measure theexchange of water between groundwaterand streams and wetlands. Groundwatertemperature typically remains stable,whereas surface-water temperature

changes throughout the day with theheat of the sun. Changes in groundwatertemperature beneath streams cantherefore be used to determine howmuch water is being exchanged betweenthe surface water and groundwater.

Determining the difference betweensurface water and groundwater can beimportant information to help ensure thatwere not counting water twice in watermanagement, which could lead to over-allocation.

HEAT TRACERS AND MODELLINGSometimes, water level data alone isinsufcient to calibrate a groundwatermodel. When observations oftemperature are included, the modelparameters may be able to beconstrained and a unique set ofparameters may be identied. Thismeans we can have more faith in themodels predictions.

Hydraulic conductivity (that is howeasily water can move through a given

This resource introduces tracers, which can be used in groundwater modelling toimprove the reliability of modelling predictions. It follows on from an earlier resourcewhich discusses numerical modelling more generally as well as parameter estimationand uncertainty. It is designed for a general audience.

B a c

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Nico Goldscheider

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material, such as clay or sandstone)has the biggest range of any physicalparameter. By contrast, thermalconductivity (that is, the ability of a givenmaterial to conduct heat) has a verysmall range of parameters. This makestemperature a very useful tracer; if youknow the soil or rock type, you canmake a very good educated guess at itsthermal conductivity.

Heat is also a good tracer because itsfree, unlike other tracers such as dye,and exists naturally in the environment.Natural variations in temperature canthen be used to estimate groundwater

ow rates.

Even where there are not naturalvariances in groundwater temperature, itis possible to injected heated water anduse this to measure the movement ofgroundwater.

WAYS TO USE TRACERSThe use of tracers is an establishedtechnique in groundwater; however, theiruse in modelling is new.

Tracers are often used:

to gain a better understanding ofparameters between observationwells

to locate fractures in fracturedrock aquifers

to obtain understanding of largescale hydraulic conductivity orgroundwater ow rates

in deep bores (hundreds of metres),the shape of a temperatureagainst depth graph can tell us

whether water is moving upwards ordownwards (for example, downwardow may be groundwater rechargefrom rain see Fig. 1)

to determine the exchange betweengroundwater and streams.

HEAT AS A GROUNDWATER TRACER

The use of groundwater tracers to understand groundwater systems(whether that be for decision making, or calibrating groundwater models) iswidely studied widely across the NCGRT.

Research is being undertaken in the following areas:

Programs 1 and 3 are investigating and rening methods to useheat to quantify the exchange between groundwater and surfacewater

Fundamental comparisons between heat transport and (the morewidely studied) solute transport are being performed in laboratorystudies (in Program 1) and with numerical models (in programs 1 and 2)

Program 2 is investigating the interpretation and benets frommeasuring a range of environmental tracers (radioactive carbon, CFCs,tritium, SF6, and more) in complex aquifers.

Environmental tracers are being measured across a number of theSuper Science sites (including by PhD candidates in

Program 3) to generate an understanding of the groundwaterin these areas.

Fig. 2 Temperature anomalies in a boremay suggest fractures. In certain types ofaquifers, fractures may carry much of thegroundwater ow.

Fig. 1 Example of temperature in abore hole to infer recharge of rainfall orgroundwater discharging towards thesurface. The solid line represents theexpected temperature in a bore where novertical ow is present.

Depth(m)

Temperature (C)

TemperatureanomaliessuggestingfracturesDepth

(m)

Temperature (C)

Groundwaterdischarging towardthe surface

Recharge togroundwater(e.g. from rainfall)

CONTACT US Call: 08 8201 2193Fax: 08 8201 7906Email: enquiries@groundwater.com.auWeb: www.groundwater.com.auMail: NCGRT, Flinders University, GPO Box 2100, Adelaide SA 5001