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Groundwater investigation techniques 187

depend on its suitability in terms of detectability, (1973), tracer tests can be used in mapping and
toxicity, relative cost and ease of use (Table 5.9) and characterizing karst conduit networks. By comparing
the choice of tracer test method will be determined discharges and masses of tracer at the entrance to and
by the hydrogeological properties to be measured exit from a karst system, it is possible to classify the
(Table 5.10). conduit network into one of five topological types
The chlorofluorocarbons CFC-11 (trichlorofluo- (Fig. 5.38).
romethane), CFC-12 (dichlorofluoromethane) and The scale of investigation of tracer tests can range
CFC-113 (trichlorotrifluoroethane) have received from laboratory experiments to the site scale (tens
attention recently as groundwater tracers with useful or hundreds of metres) and to the regional scale (kilo-
reviews of their source, distribution in groundwaters metres) in karst aquifers (Box 5.3). When compared
and applications in tracing and age dating modern with the scale of influence of other field methods for
groundwaters provided by Plummer and Busenberg determining aquifer properties, Niemann and Rovey
(2000) and Höhener et al. (2003). CFCs are synthetic, (2000) noted, in an area of glacial outwash deposits
halogenated, volatile organic compounds that were near Des Moines, Iowa, that the hydraulic conductiv-
manufactured from 1930 for use as aerosol propel- ity values of outwash determined from pumping tests
lants and refrigerants until banned by the Montreal by curve matching techniques (see Section 5.8.2) can
Protocol in 1996. In general, CFC dating is most likely be an order of magnitude larger than values found
to be successful in rural settings, with shallow water from a tracer test using the conservative solute
tables, where the groundwater is aerobic, and not chloride. This discrepancy may be caused by the dif-
impacted by local contaminant sources such as septic ferent scale and dimensionality of the two test meth-
tanks or industrial applications (Plummer & Busenberg ods (the cones of depression for the pumping tests
2000). As discussed in Section 4.4.2, age dating tech- were about 30–130 m while the tracer tests related to
niques have limitations, with greater confidence in a zone of influence of less than 30 m) with dispersion
deriving apparent age obtained when multiple dating of the tracer within the glacial outwash preventing
techniques are applied to the same sample, for ex- the conservative solute from flowing exclusively
3
3
ample CFCs and H/ He dating (Section 4.4.5). within smaller, high permeability paths which have
Some of the most commonly used tracers in a strong influence on the groundwater flow and
groundwater studies are the fluorescent dyes fluo- hydraulic conductivity measured by pumping tests.
rescein (uranine) and rhodamine WT, the optical Hence, the results from Des Moines suggest that the
brightner photine CU and bacteriophage. The detec- velocity of a conservative solute plume in an inter-
tion and assay of fluorescent dyes is made by illum- granular aquifer may be overestimated if values of
inating the test solution at an appropriate narrow hydraulic conductivity derived from pumping tests
band of wavelengths (the excitation wavelength) and were used in calculations.
measuring the amount of fluorescent light emitted At field and regional scales, tracer tests can be
at a corresponding longer wavelength band (the carried out with or without wells or boreholes and
emission wavelength). Representative excitation and can be performed under natural hydraulic gradient
emission spectra are given in Table 5.11 for eight conditions (see Box 6.3) or under forced gradient
fluorescent dyes. Measurements are made using a (pumping) conditions (for examples see Niemann &
fluorometer and standards of known concentration Rovey 2000). The advantages and limitations of this
used to derive a calibration curve relating fluores- range of tests and detailed protocols for conducting
cence in arbitrary units to concentration, typically two of the most useful tests, the single borehole dilu-
−1
measured in µgL (Fig. 5.22a). tion method and the convergent radial flow tracer
Smart and Laidlaw (1977) and Atkinson and Smart test, are discussed by Ward et al. (1998). Both these
(1981) evaluated the suitability of different fluores- tests can provide measurements of aquifer properties.
cent dyes for water tracing and note their extensive Relative to pumping tests, the advantages of the
application in Britain, particularly in the investigation single borehole dilution method conducted under
of karstic limestones (Box 5.3). From the topological a natural hydraulic gradient are the low cost of
work of Brown and Ford (1971) and Atkinson et al. materials and equipment and the simplicity of the