Lowenthal G., Airey P. Practical Applications of Radioactivity and Nuclear Radiations

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9.3 Environmental applications of radioisotopes 291

(d)

(e)

Figure 9.5. (cont.)

involves balancing dredging costs with estimates of the ef®ciency with which

dumped material re-enters the shipping channel. Tracer techniques have been used

to study the rate of movement of dredge spoil at proposed dump sites.

. Sustainable development in the coastal zone: prediction of the impact of a proposed

development on the sand budgets requires good baseline information on bed load

transport.

The results of a typical bed load study are sketched in Figure 9.5(d).

Information was required on the dynamics of sand transport in a channel

connecting a coastal lake system to the sea. Radiotracer was injected at four

locations across the channel. In the northern part of the channel, the net

migration was in one direction; in the southern section, in the opposite

direction. The observed sand migration was the consequence of the current

patterns in the channel during ebb and ¯ood tides. It would have been

dif®cult to obtain such direct information using other methods.

A number of case studies in Latin America and Europe have been reviewed

by Aun and Bandiera in IAEA (1995). On the basis of long experience, the

authors concluded that the bene®ts of tracer technology will only be fully

realised as part of a multi-disciplinary approach to coastal engineering

problems.

Detector calibration

Success in obtaining reliable quantitative information on sand and sediment

movement depends on the quality of the calibration of the monitoring

detector. As with the total count method for measuring ¯ow rates in rivers

and pipelines (Sections 9.3.2 and 8.3.6), the major dif®culty is reproducing, in

the laboratory, the source±detector geometry that occurs in the ®eld.

Calibration of the detector ef®ciency is made in a large water tank partially

Radionuclides to protect the environment292

(f )

Figure 9.5. (cont.)

®lled with sand and designed to simulate conditions at the bottom of the lake

or estuary. A realistic geometry can often be simulated by mounting the

detector on a rig a few centimetres above the surface of the sand (Figure

9.2(b)).

Complications can arise in the ®eld because the radioactive sand released

to the surface is incorporated into the mobile layer as a consequence of

periodic re-suspension and deposition processes associated with bed load

transport. Over extended periods, bioturbation might also be signi®cant. As a

consequence of g ray attenuation, the ef®ciency with which the detector

responds to radioactive sand particles depends on its depth. This effect may

be simulated in the calibration tank by covering the radionuclide source with

successive layers of sediment and taking repeated measurements. When

calculating the calibration factor an assumption has to be made concerning

the distribution of the tracer as a function of depth. The validity of the

assumption should be checked if possible by collecting cores at a number of

locations within the radioactive patch, sectioning the cores and measuring the

activity pro®les directly (Figure 9.5(c)).

Suspended sediment gauges

Nucleonic gauges have been developed for the accurate measurements in real

time of the levels of suspended sediments in rivers and estuaries (Figure

9.5(f )). These data are of interest in understanding issues as diverse as the

stability of dredge spoil grounds and, more generally, the impact on estuaries

of sediments transported to the coastal zone by river systems. Suspended

sediment gauges are designed as gamma ray transmission (Section 7.2.1) or

gamma ray backscatter (Section 7.2.2) gauges. Both types of gauges can

measure sediment loadings equivalent to bulk density changes of less than

1%. With transmission gauges, the sensitive region is the small volume

between the source and the detector. This is a disadvantage compared with

the backscatter gauge, which has a much larger sensitive volume. On the

other hand, the response of the transmission gauge is much less sensitive to

effects due to the proximity to boundaries such as the bed of the estuary or

the water surface. Applications have been made to:

. continuous monitoring of high (say 1 to 100 kg/m

) sedim ent loadings in rivers as a

function of rainfall and at different depths;

. surveying the levels of suspended sediments in reservoirs and harbours as a

function of location and depth; and

. optimising the ef®ciency suction dredging by monitoring the concentration of

dredge spoil in the well of the dredge as a function time.

9.3 Environmental applications of radioisotopes 293

Further information may be obtained from Tazioli and Caillot (IAEA, 1983)

and Aun and Bandeira (IAEA, 1995).

9.4 Applications of naturally occurring radioisotopes

9.4.1 Man-made versus environmental radioisotopes

Naturally occurring, or environmental radioisotopes were introduced in

Section 9.2.2 when they were classi®ed into three categories: cosmogenic

isotopes, fall-out products and primordial isotopes (Table 9.2). Applications

of man-made and environmental isotopes are generally complementary.

Man-made radionuclides are selected from a large number of available

isotopes, are used to label a particular component of a complex system and

are injected at a precise location and according to a precise protocol.

Consequently, very detailed information is obtained on the behaviour of an

environmental system in the vicinity of the injection.

By contrast, environmental isotopes are widely distributed by natural

processes and are used to obtain information over a regional scale. By and

large the data re¯ect the cumulative effect of the environmental process (say,

erosion, siltation, groundwater movement) over time scales related to the half

life of the isotope. Information is normally obtainable over periods equivalent

to four or ®ve half lives. For instance

210

Pb (T

1/2

= 22.3 y) is used to assess

siltation rates over about 100 years, whereas,

C(T

1/2

= 5730 y) and

230

1/2

= 75 400 y) provide information over about 20 000 and 200 000 years

respectively.

9.4.2 Erosion studies

A new use for caesium-137

Good husbandry of fertile soil is central to sustainable development in a

world where populations are expanding. Every effort is being made to

minimise losses due to water and wind induced erosion. Part of this effort

involves the development of reliable measurement techniques. A great deal is

being achieved with satellite imaging. However, there is a need to validate or

interpret the imaging data with independent estimates of erosion patterns at a

catchment scale. One such method involves the use of the environmental

isotope

137

Cs to estimate erosion rates over a whole catchment area.

137

Cs is a

®ssion product which was injected into the upper troposphere and lower

stratosphere during atmospheric nuclear tests. These radionuclides were

Radionuclides to protect the environment294

distributed world-wide by the global atmospheric circulation and, with the

passage of time, diffused to the surface of the earth. There the

137

Cs adsorbed

strongly onto the clay fraction of the soils, and acted as a natural radiotracer

for subsequent erosion and accumulation processes.

Measurable levels of

137

Cs started to appear in soil during the mid-1950s,

reached a peak in the mid-1960s and then declined following the signing of the

Atmospheric Test Ban Treaty in 1964 (Playford et al., 1992). The year by year

variation of the levels of

137

Cs reaching the earth is similar to that of other

`fall-out' products such as tritium and carbon-14 (Figure 9.6(a)) and is known

as the input function. Caesium-137 has a suf®ciently long half life (30.1 y) to

remain useful for the present purpose for many more years to come.

Environmental

137

Cs was ®rst used as a natural tracer to measure the rate

of sedimentation in lakes and reservoirs (Pennington, 1972, Pennington et al.,

1973). The sediment with the

137

Cs attached accumulates in the lake as a

consequence of erosion in the surrounding catchment areas (Figure 9.6(b)). If

there is normal sheet erosion over the catchment, the shape of

137

Cs input

function is preserved in the sediments. It is the shape of the pro®le and not

the radionuclide decay which serves as the basis for dating the accumulation

of the sediments. The ®rst appearance of environmental

137

Cs in the mid-

1950s and the peak in the mid-1960s are both readily identi®able features.

These data may be compared with results from other dating methods and

with the known historical record to gain an improved understanding of

changes in erosion patterns.

Experimentally, the

137

Cs pro®le is measured by

. collecting and sectioning a core obtained from the soil in the region of interest;

. placing each dried and weighed section of the core in a Marinelli beaker over a

high-resolution detector (Figure 9.6(c));

. measuring the countrate in the 662 keV g ray peak; and

. calculating the speci®c activity of the

137

Cs by comparison with results obtained

from a calibrated standard.

Procedures and applications

The measurement, and in particular the prediction, of the rate of erosion of

fertile soils is a key to the development of strategies for the long-term

sustainability of agriculture. The rate of surface erosion depends on a number

of parameters such as the distribution, the amount and the intensity of

rainfall, the slope of the land, the nature and extent of vegetation and the

erodability of the soil. Attempts have been made to develop equations linking

erosion rates to these parameters, and to use the equations to predict erosion

over a wide geographic region (Ritchie and Ritchie in IAEA, 1995). One of

9.4 Applications of naturally occurring radioisotopes 295

the most widely used is the Universal Soil Loss Equation (USLE). However,

it has often been inappropriately applied, both in the United States where it

was developed and around the world (Wischmeier, 1976). An independent

method for the validation of such equations is required.

Carefully planned and executed measurements of the

137

Cs pro®les on the

catchment slopes provide a basis for independent estimates of erosion rates.

The principle is shown schematically in Figure 9.6(d). The greater the erosion

rate, the greater the removal rate of soil and the less the residual

137

Cs in the

soil pro®le. On the other hand, the greater the rate of accumulation of soil in

a catchment, the greater the level of

137

Cs in its pro®le. Activity pro®les

similar to those sketched in the ®gure are not dif®cult to observe. A key for

the reliable interpretation of the resulting data is the identi®cation of a

reference location on the slope where there has been minimal erosion or

sedimentation over the past decades. The identi®cation of the reference site

usually relies on the judgement of local experts.

A complicating factor is that the countrate in the 662 keV peak attributable

137

Cs is often little above background. Experimentally, the background

may be reduced by using a shielded gamma spectrometer with a Compton

suppression capability. Such equipment is, however, specialised and expen-

sive. Mathematical procedures for calculating the speci®c activities of gamma

emitting isotopes at very low levels in natural samples are available, but their

discussion is beyond the scope of this text (Gilmore and Hemmingway, 1995).

An interesting result

The application of

137

Cs to the study of erosion has proven useful to studies

not only on a regional scale but also at the level of an individual farm and in

forested areas. Measurements can contribute to studies of the impact of land

use management on erosion. For instance, a study on a vineyard revealed

that for each bottle of wine produced, approximately one bottle of fertile soil

was lost to the cultivated area. If sustainable development is to be achieved,

an understanding is needed of the total environmental cost of production.

Other techniques

The procedure for using

137

Cs as a dating technique is different from that

used for the majority of radiometric methods. It does not depend on the

9.4 Applications of naturally occurring radioisotopes 297

3 Figure 9.6. Application of environmental

137

Cs to erosion measurements. (a)

The global distribution of

137

Cs (Playford et al., 1992). (b) The dating of sedimen t

accumulation in lakes and reservoirs from

137

Cs levels in drill core. (c) Counting of

sediment samples in a Marinelli beaker; (d) Application of

137

Cs measurements to

catchment erosion.

measurement of radioactive decay but on identifying special features of the

input function such as the onset of the

137

Cs activity (mid-1950s) and the

activity maximum (mid-1960s).

There are also the conventional methods of radiodating using

210

1/2

= 22.2 y),

C(T

1/2

= 5730 y) or

230

Th/

234

U (Figure 1.5), which can serve

to measure sediment accumulation over time scales up to decades, tens of

thousands and hundreds of thousands of years respectively. In these ways,

insights can be obtained on the evolution of estuaries or other geologic

features over recent geological time. Some details are presented in Table 9.2.

9.4.3 Groundwater

Introduction

Adequate resources of fertile soil and clean drinking water are essential for

public health. To complement the discussion of the measurement of erosion

offered in the previous section, attention will now be paid to studies of sub-

surface water. This resource becomes increasingly important in areas where

reserves of good quality surface water have diminished due, for example, to

prolonged dry spells, over-exploitation or contamination. The challenge is to

establish management regimes that will ensure the sustainability of ground-

water yields.

Groundwater resource evaluation

Environmental radioisotopes have proved a useful aid in the investigation of

underground water with a few examples listed in Table 9.1. Applications have

been directed towards:

. identifying the sources of recharge water, i.e. the sources of surface water which

seep unde rground to replenish the basin;

. estimating the extent of mixing of underground water from different sources;

. calculating the age of groundwater samples, i.e. the time which has elapsed since

the water percolated underground;

. deducing the direction and rate of groundwater ¯ow; and

. understanding the processes leading to degradation in groundwater quality.

The environmental isotope approach is here illustrated with reference to a

simpli®ed version of the water table (Figure 9.7). The data obtained for this

purpose are often used to validate groundwater hydraulic models which link

the known water table levels (technically the piezometric surfaces) to the

properties of the water bearing strata (Clark and Fritz, 1997).

Radionuclides to protect the environment298

Two examples will be provided: the location of recharge areas and the

dating of underground water.

Location of recharge areas

Recharge is the process whereby surface water percolates through the soils

and other overlying strata to replenish the underground water. It is therefore

important to ensure that areas of recharge are clearly identi®ed and well

managed. Environmental tritium can make a signi®cant contribution to this

end. As shown schematically in Figure 9.7(a) the direction of groundwater

¯ow is away from recharge areas. The presence of signi®cant levels of tritium

is indicative of groundwater which has in®ltrated through the unsaturated

zone in post-nuclear times (Table 9.2), that is of samples collected from a

bore in or close to the recharge area. It is recalled that most of the tritium in

the environment was derived from atmospheric nuclear testing.

As an aside, it may be noted that small injections of man-made tritium can

be used to obtain detailed information on the transport of water in the

9.4 Applications of naturally occurring radioisotopes 299

Figure 9.7. Application of environmental isotopes to groundwater studies. (a)

Schematic diagram showing underground water percolating through an aquifer and

sampled at various locations for age dating with, say, carbon-14. (b) The use of man-

made tritium to study the rate of recharge of in®ltrating rainwater to an aquifer.

unsaturated zone. Tritium is injected at a constant depth (say 1 m). Cores

within the unsaturated region are collected at time intervals ranging from

about 1 month up to, say, 3 years. With the passage of time, the tracer

maximum is displaced down the pro®le towards the water table. At the same

time, it exhibits dispersion. This behaviour is shown in Figure 9.7(b).

The dating of underground water

The age of a groundwater sample is the time required for the water to migrate

from the area of recharge to the extraction bore. Groundwater migrates away

from the recharge area at a rate which depends on differences in the water

table levels (the so-called piezometric gradient) and the properties of the

water bearing strata. Groundwater ages can either be computed from

hydraulic data, or measured using isotope techniques.

As indicated above, environmental tritium is used to identify water which

has recharged in post-nuclear times. However, modern techniques can be

used to measure tritium to such low levels that groundwater dating up to 100

years is possible. Carbon-14 (T

1/2

= 5730 y) is widely used for the dating of

groundwater samples with residence times ranging from a few hundred up to

30 000 years.

Unlike tritium (

H), carbon is not a component of water but a solute. The

calculation of the groundwater age from the measured activities involves not

only an assessment of the carbon-14 levels at recharge, but also an under-

standing of the source and subsequent behaviour of the dissolved carbonate

in the groundwater system. By dating a large number of samples across a

basin, contours of equal age (isochrones) can be drawn from which the

groundwater ¯ow lines can be deduced.

Chlorine-36 (T

1/2

= 302 000 y) may be used to date water with residence

times in excess of one million years. For instance, the Great Artesian Basin,

Australia, covers about a third of the continent. The major recharge area is in

the east, and the water ¯ows in a generally westerly and southwesterly

direction. The size of the basin is such that groundwater ages reach two

million years near some of the discharge areas. In order to validate chlorine-

36 as a groundwater dating method, an inter-comparison was made between

the isotopic ages, and those calculated from a hydraulic model. Satisfactory

agreement was found.

The atom ratio

Cl/Cl depends not only on the age and the hydraulic

properties of the groundwater, but also on the extent to which it has dissolved

sub-surface chloride. Chlorine-36 techniques are being widely used to study

salination and the degradation of groundwater quality.

Radionuclides to protect the environment300