Geology Reference
In-Depth Information
Table 13 A Few Tracers Used in Karst Hydrogeology.
Product
Color
Analysis
methods
Detection
threshold
Fixes to
activated
charcoal
Drawbacks
fl uorescein
green
fl uorescence
a few μg·L -1
yes
can be mistaken for
natural fl uorescence
and vice-versa
rhodamines
red
fl uorescence
a few μg·L -1
yes
adsorbs easily
eosin
pink
fl uorescence
a few μg·L -1
yes
spectral signature
is similar to that of
fl uorescein
duasyn
blue
fl uorescence
a few μg·L -1
yes
not sold yet
commercially in Europe
naphthol
no color
fl uorescence
a few μg·L -1
somewhat can be mistaken for
natural fl uorescence
and vice-versa
K iodide
no color
ionic
chromatography
10 μg·L -1
no
costly analysis
lithium
chloride
no color
atomic
absorption
10 μg·L -1
no
costly analysis
Na Cl
no color
conductiviy
1 g·L -1
no
requires large quantities
radioactive
substances
no color
number of
particles
_
no
delicate procedure
bacteria
no color
microscope
_
no
delicate procedure
bacteriophage
viruses
no color
microscope
_
deteriorate in pumping
colored
spores
variable
microscope
_
no
labor-intensive detection
When illuminated by a monochromatic beam of light at a specifi c
wavelength in order to excite them, these substances emit a specifi c emission
radiation.
The substance is not directly proportioned; instead the intensity of the
fl uorescing signal is measured. There is therefore the risk of confusion with
pollutants or with naturally present substances in the water. A number of
organic substances are fl uorescent, with a maximum of around 250 nm
(humic and fulvic acids). The dosing of tracers should therefore always
be verifi ed with qualitative research, a high concentration being capable
of indicating only a natural fl uorescence peak (Figure 85). Values that are
too low are therefore not reliable, and a number of older tracer tests, once
deemed positive, are now being questioned.
Qualitative monitoring of tracers is done with excitation spectra.
Generally, natural fluorescence shows peaks with very elongated
wavelengths (many fl uorescent substances), whereas injected tracers show
narrow, intense peaks. Nevertheless, a weak peak can in some cases be
masked by a high background of natural fl uorescence.
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