A compact field fluorometer and its application to dye tracing in karst environments
A compact field fluorometer and its application to dye tracing in karst environments
Amaël Poulain 0 1 2
Gaëtan Rochez 0 1 2
Jean-Pierre Van Roy 0 1 2
Lorraine Dewaide 0 1 2
Vincent Hallet 0 1 2
Geert De Sadelaer 0 1 2
0 Department of Physics, University of Namur , Rue de Bruxelles No. 61, B-5000 Namur , Belgium
1 Department of Geology, University of Namur , Rue de Bruxelles No. 61, B-5000 Namur , Belgium
2 Royal Meteorological Institute of Belgium , Avenue Circulaire No. 3, B-1180 Bruxelles , Belgium
Dye tracing is a classic technique in hydrogeology to investigate surface-water or groundwater flow characteristics, and it is useful for many applications including natural or industrial issues. The Fluo-Green field fluorometer has been successfully tested in a karst environment and is specifically suitable for in-cave karst water monitoring. Karst research often uses dyes to obtain information about groundwater flow in unexplored cave passages. The compact device, alternatively named Fluo-G, meets the requirements of cave media: small (10 × 16 × 21 cm), lightweight (0.75 kg without ballast) and simple in conception. It is easy for cavers to set up and handle compared to other sampling methods. The fluorometer records uranine, turbidity and temperature with a user-defined time-step (1 min - 1 day). Very low energy consumption allows 9,000 measurements with six AA batteries. The device was calibrated and tested in the laboratory and in field conditions in Belgian karst systems. Results are in good fit with other sampling methods: in-situ fluorometers and automatic water sampling plus laboratory analysis. Recording high quality data (breakthrough curves) in karst with in-cave monitoring is valuable to improve knowledge of karst systems. Many hydrological and hydrogeological applications can benefit from such a low-cost and compact device, and finding the best compromise between resources and quality data is essential. Several improvements are possible but preliminary field tests are very promising.
Tracer tests; Field technique; Fluorometer; Groundwater monitoring; Karst
Many scientific fields use tracer tests to understand the flow
organization and characteristics of water. This tool can answer
multiple questions for both surface water and groundwater
studies (Käss 1998). The fields of environmental management and
engineering can also benefit from dye tracing to answer multiple
questions about flow connections, catchment area, aquifer
vulnerability, pollution and construction leakages. In the karst
environment, they represent a powerful tool for the determination of
an unexplored system configuration (Goldscheider et al. 2008).
Hydraulic connections, catchment area or transit time can be
easily obtained from dye tracing (Meiman et al. 2001; Lauber
et al. 2014). In this context, fluorescent dyes tend to approach the
Bideal tracer^ as they are reasonably conservative, safe,
inexpensive and highly detectable (Benischke et al. 2007). Uranine is for
now the most commonly used tracer because of these
The objectives and context of the dye tracing in karst areas
are two key elements to determine the right sampling
approach (Smart 2005). Qualitative and semi-quantitative
methods like visual detection and activated charcoal answer
geographical issues (connections, catchments, travel time)
with limited material resources. Charcoal bags are broadly
used because they are cheap and easy to implement but are
subject to contamination. They are mainly valuable for
preliminary tests and inaccessible sites (Goldscheider et al.
Quantitative methodologies give more valuable
information because frequent water sampling allows the
measurement of dye concentration in time. These data are
instructive regarding the hydrogeological issue and for
cave exploration. Automatic water samplers optimize the
sampling task and improve the temporal resolution of
data. Nevertheless, sampling, handling and analyzing water
samples remains a significant task, even more with
multiple sampling stations.
More recently, automatic field fluorometers have given the
opportunity to make in-situ measurements of fluorescent dyes
in water (Schnegg 2002). The temporal resolution, precision
and accuracy are significantly enhanced. Furthermore, those
devices reduce the need of frequent handling in the field due to
automatic recording over an extended period of time. Another
advantage of this in-situ measurement is the absence of
transportation and analysis of bottled samples.
As discussed by Smart (2005), the dye tracing and specifically
the sampling design has to be defined regarding the objectives and
the scientific, socio-economic and logistic context. The first
objective for cavers and karst researchers is geographical: flow routes,
karst catchments, travel times. In addition, hydrogeological
objectives can bring valuable data for cave research: velocity, dispersion,
storage, conduits characteristics and possible retardation (Dewaide
et al. 2016). In this context, cave researchers aim to run with
limited technical, financial and personnel resources (Smart et al.
1998). Error tolerance and data resolution will depend upon the
audience and the objectives but primarily it must be cost-effective.
In-situ fluorometers tend to be the most valuable technique in
terms of quality data relative to costs.
This paper presents the Fluo-Green, a recently developed
fluorometer for hydrological investigations. The device can be
used in variable environments but it has been specifically
designed and tested for karst. In-cave monitoring can bring more
detailed insights into the internal structure of karst aquifers
(Goldscheider et al. 2008); nevertheless, cave access is often
difficult and monitoring operations are few and under-exploited
(Lauber et al. 2014).
The best compromise has to be found between quality data
(precision and accuracy) and costs regarding the objectives
and requirements of the research issue. The question of the
benefit of increasing data quality compared to the customer
needs has to be asked. For Bsimple^ geographical and
hydrogeological issues, experimenter’s expectations can be
lowered with simplified material.
The Fluo-Green device, or Fluo-G, was designed by G. De
Sadelaer in 2015. Laboratory calibrations and field tests were
successfully conducted in 2015 and 2016 at the University of
Namur in cave systems of Belgium. Comparisons with other
sampling methods in variable sampling environments allowed
the evaluation of the field performances of this new device.
Description of the Fluo-G
The Fluo-G fluorometer is a two-in-one device with in-situ
fluorescence measurement and data logger in the same box
(Fig. 1). The device is compact (10 × 16 × 21 cm) and extra
light (0.75 kg without ballast). The measurement is made
through a transparent casing with an excitation-detection unit.
The excitation unit comprises a set of two LEDs, and
wavelengths are selected for uranine excitation (470 nanometers
[nm], 6,300 millicandela [mcd]) and turbidity measurement
(625 nm, 6,500 mcd). The Fluo-G is designed to measure
uranine only, since it is the most common fluorescent dye in
karst hydrogeology and cave exploration. Other fluorescent
dyes could easily be measured by choosing another LED
wavelength for the excitation unit and applying the adequate
calibration procedure. The detection unit is right next to the
excitation unit and has a RGB (red-green-blue) sensor
perpendicular to the light source. Detection and correction of
turbidity and a water temperature probe are also integrated.
Excitation and detection sequences are controlled by an
Arduino coupled to a data logger shield with a microSD
memory card. A real-time clock is used to wake up the fluorometer
and run the measurement at a defined time-step. A minimum
time step of 1 min allows to record high quality data for further
interpretation. The energy consumption is 0.2 mAh per
measurement and the device is turned off between measurements. Six
AA batteries (1,800 mAh) give 30 days of lifetime with a
measurement time-step of 5 min. Before each sequence of excitation,
the detection unit makes one dark measurement to allow a
possible daylight correction. Main characteristics of the Fluo-G are
given in the Table 1. A comparison is made with the GGUN
FL30 of Albillia Switzerland since it is a standard commercial
instrument for in-situ dye-tracing monitoring.
As mentioned in the Table 1, the main advantages of
FluoG are the compact size and the low energy consumption
allowing extended monitoring. Disadvantages are a small
number of detectable dyes (GGUN FL30 has more channels)
and a smaller maximal depth of use. The latter is limited by the
tightness of the transparent casing, which is one of the main
possible deficiencies of the Fluo-G and has to be improved in
the future. GGUN-FL30 also has detection threshold and
resolution that are somewhat better than the Fluo-G. Advantages
and disadvantages of the Fluo-G compared with other
monitoring methods will be discussed later.
Data acquisition is automatically started at connection of
the battery. No external connections are needed during normal
use of the fluorometer. Raw measurement data are stored on
the internal microSD memory card as a text file. The data can
be read into spreadsheet software to apply calibration
parameters and get the results in ppb or μg/liter.
Table 1 Properties of the
FluoGreen and comparison with the
GGUN FL30 (Schnegg 2002)
Standard calibration procedure
Calibration of the Fluo-G is recommended before every
experiment to guarantee data quality. The calibration procedure
evaluates the response of the sensor with respect to the water
of the monitored site. The background fluorescence can also
be removed to avoid misinterpretation and get the real
concentration of fluorescent dye.
This calibration is applicable when using a single color dye
tracer. Although it is possible to use other tracers, the device
was calibrated for uranine. Uranine has a very low minimal
detectable signal, so the sensitivity is high compared to other
tracers (Smart et al. 1998). Due to cost compromises, no extra
filters were installed in the optical detection path, which will
lead to a supplementary offset signal (see ‘Correction for stray
Figure 2 shows the relationship between the concentration
of the tracer (u) and the G channel output signal of the detector
that has been fitted with a second order polynomial. The R2 is
0.999 so the model fits the experimental data quite perfectly.
The general equation for the calibration is:
where the following terms and definitions apply:
10 × 16 × 21 cm
Tested at −4 m
Fig. 2 Calibration curve for the Fluo-Green showing the relationship
between uranine concentration (u) and ADU counts of the detection unit
Concentration of the uranine dye
The corrected ADU value detected by the green
channel (see ‘Correction for stray light’ section;
ADU: analogue digital units, the result of an A/D
conversion, also called arbitrary digital units)
Constants derived after calibration curve fitting.
Here the minimum resolution of the sensor is
0.09 ppb/count. A routine calibration can be
limited to a three-point calibration at 1, 10 and
100 ppb and an offset measurement at 0 ppb.
Correction for stray light
Stray light from surrounding light and the excitation LED
enters the detector directly, via reflections or by water
turbidity. This interference will produce a variable (high) offset
signal at the output of the detector. It is necessary to correct this
interference in order to obtain reliable data.
A classical filter fluorometer uses filters with a high optical
density to avoid any offset signal. The Fluo-G setup has a
limited optical density, and a reference channel is used to
correct the green detector output signal (ADU). Due to the
quasi-identical optical path, stray light is detected identically
by all three detector channels (red, green and blue) with
exception of a gain difference and a very small zero offset. This
property makes a good offset correction possible. The
compensation will lead to a corrected G channel with a zero error
of ±1 count. Correction of surrounding light (daylight or
artificial lights) is done by a dark background measurement with
all LED off (no excitation). The green detector output (ADU)
measured with no excitation is subtracted from the signal
measured when the excitation is on. The correction amplitude
of artificial lighting is visible on Fig. 5.
Light from the excitation LED will be scattered to the
emission detector (offset). This offset is already corrected
by the method already described.
Excitation and emission light will be absorbed (gain). The
influence of turbidity on the measured uranine
concentration has to be determined during the turbidity calibration,
and a correction is applicable.
Turbidity is measured in a wavelength that is not
influenced by tracer excitation/emission spectra; here a red LED
at 625 nm is used.
Rather than laboratory tests and calibrations, field tests were
needed to validate the fluorometer. Precision and accuracy are
important parameters to be tested in the cave environment to
evaluate their consistency with the objectives and context of
the tracing (Smart 2005). The Fluo-G has been tested in the
framework of hydrogeological studies in 2015 and 2016 at the
University of Namur (Belgium). Four experiments were
conducted into different karst systems of southern Belgium
(Fig. 3). Different sampling environments were selected:
A surface karst resurgence with daylight interference
An underground river with a pebble riverbed, high water
current and artificial lighting
An underground river with a mud riverbed and very low
Drip-water from a stalactite
The Fluo-G was coupled to additional sampling methods in
order to compare monitoring results. An automatic water
sampler and laboratory spectrofluorometer was used for test No. 1.
A GGUN FL30 fluorometer (Schnegg 2002) was used for test
No. 2. All fluorometers and the spectrofluorometer were
calibrated using the site blank water and the same uranine.
The Haquin karst system (Lustin) was investigated during the
first test (Fig. 3b). It is a 3-km-long sinkhole-resurgence
system in Frasnian limestones of southern Belgium. The
resurgence was equipped with an automated water sampler with a
1-h time step during 48 h. Water samples were analyzed with
Fig. 3 Maps of the study sites. a
Location of the test sites within
the southern Belgium limestones.
b Location of test Nos. 1 and 3 in
the Frasnian limestones of the
Meuse valley. c Location of test
Nos. 2 and 4 in the Givetian
limstones of the Lomme Valley. d
Cross-section in the Rochefort
cave showing the surface-to-cave
dye tracing (No. 4) in the Givetian
an Agilent spectrofluorometer at the University of Namur.
Three Fluo-G were installed in the resurgence with a 5-min
The three Fluo-G breakthrough curves show similar results
after correction with calibration formulas (Fig. 4). In-situ
measurements are also in good fit with results from the laboratory
spectrofluorometer. The first arrival-of-tracer events are
similar and the maximum concentration difference is 0.2–0.8 ppb.
Those results show the ability of the Fluo-G to give reliable
data in terms of precision (the three devices give the same
results) and accuracy (as the spectrofluorometer value is
expected to be the most accurate measurement).
Fig. 4 Breakthrough curves for test No. 1. Comparison between
spectrofluorometer laboratory analysis on water samples (green dots)
and three Fluo-G in-situ fluorometers (FLUOG#1, 2, 3). The influence
of daylight on the Fluo-G measurement is visible
An important daylight noise is observable in all the
breakthrough curves. The dark measurement allows a partial
correction and the BTC is easily distinguishable from the noise;
nevertheless, a daylight coverage should be implemented for
surface measurements. Additional smoothing of the data can
easily remove residual daylight noise.
Test No. 2: underground river with pebble riverbed, high
water current and artificial lighting
The second field test was conducted in the Givetian
limestones of the Lomme karst system (Fig. 3c). The Fluo-G and
a GGUN-FL30 field fluorometers were installed in the
Rochefort cave underground river (50 L/s). This river has high
current and a pebble riverbed. Uranine was injected in a small
sinkhole 1 km upstream and the GGUN FL30 fluorometer
(Schnegg 2002) was used for comparison as it is a reference
in modern submersible fluorometers (Goldscheider et al.
Figure 5 shows the strong similitude between the GGUN
FL30 and the Fluo-G BTCs. Basic features of the restitution
are the same: time of first arrival (24 h), peak time (33 h) and
maximum concentration (difference of 0.7 ppb). Total
restitution difference is 3.3% between the first arrival and t = 100 h.
Stray light induced by artificial lighting in the cave is visible
on the uncorrected Fluo-G breakthrough curve (Fig. 5a). The
dark measurement allows for the removal of this interference
as illustrated in Fig. 5b. A signal noise of ± 0.05 ppb is visible
on the Fluo-G BTC. This noise is less visible on the GGUN
curve; however, this variability does not affect the
interpretation of the results.
Fig. 5 Breakthrough curves for test No. 2. Comparison between in-situ
fluorometers Fluo-G and GGUN FL30. a Fluo-G data without stray light
correction. b Fluo-G data with stray light correction (via dark
The third test was made in the Tailfer karst system (Fig. 3b),
which is similar to the Haquin system (test No. 1) in terms of
geologic context and size. The Fluo-G has been installed in the
small underground river of the Alexandre cave. This river has a
muddy riverbed and almost no visible water current. High water
turbidity was observed during the measurement. The BTC of this
experiment is displayed in Fig. 6.
The shape of the BTC is characteristic of a karst stream
dominated by advection and dispersion. The dye restitution
is clearly visible despite the very low tracer concentration. The
maximum concentration is 0.8 ppb with a background
fluorescence signal ranging from 0 to 0.1 ppb. The result of this
test shows the ability of the Fluo-G to detect small
concentrations of uranine despite the basic design of the excitation/
detection unit. This low restitution threshold allows one to
use less tracer which is both a financial advantage and also
avoids visual contamination. Figure 6 also shows the
measurement error with the Fluo-G, ranging from 0.06 to 0.09 ppb.
Although this could become an issue to detect small changes
in fluorescence signal, it can be easily overcome by using
more dye during injection. For this case, the error/peak
concentration ratio is 1/8 and allows a precise determination of
Fig. 6 Breakthrough curve (BTC) for test No. 3. Result shows a typical
BTC of an advective karst system despite a tracer concentration lower
than 1 ppb
Test No. 4: drip-water monitoring for vadose zone tracing
A vadose zone tracing test was performed in the Rochefort
cave of the Lomme karst system (Fig. 3c,d). Uranine was
injected at the surface and the monitoring (drip-rate, uranine)
was made under a perennial stalactite (5–25 L/h), 50 m below.
The Fluo-G was submerged into a small bath (5 L) collecting
Figure 7 shows the parameters recorded with the Fluo-G in
the stalactite drip-water: battery voltage, water temperature
and uranine concentration. Additional parameters such as
surface rainfall, surface temperature and stalactite drip-rate are
also displayed in order to properly interpret the Fluo-G results.
The Fluo-G successfully records uranine concentration and
water temperature for 30 days; an additional battery can be used,
allowing 2 months of measurement with a 5-min time-step.
Extended lifetime is due to the very low energy consumption
of Fluo-G and is crucial for long-term experiments or remote
study sites (Poulain et al. 2015). The small size of the device
allows one to make the measurements in a small bath, which
tends to avoid concentration buffering for drip-water monitoring.
The temperature signal recorded by the fluorometer shows a
resolution of 0.06 °C, which is precise enough to allow a good
correlation with surface air temperature in this case. Field
temperature measurement with the GGUN shows a resolution of
0.01 °C, which can be useful in case of low variability signals.
Advantages and disadvantages of the Fluo-G
This new device has the same kinds of advantages as any other
field fluorometer: extended autonomy, data resolution, no
water bottle to handle, no contamination or tracer degradation, no
freezing sensitivity and reduced dye tracer mass to inject.
Basically, the direct measurement of the tracer concentration
Fig. 7 Surface and cave
parameters recorded during test
No. 4. Fluo-G parameters are
displayed on the left axis: battery
voltage (Volts), drip-water
temperature (°C) and uranine
concentration (ppb). Other
parameters are displayed on the
right axis: surface rainfall (mm/
h), surface air temperature (°C)
and stalactite drip-rate (L/h)
simplifies the handling on the field as described by Schnegg and
Doerfliger (1997) for the GGUN FL-30. For both kinds of
devices, the acquisition, calibration and display of data are easy.
As mentioned previously, the Fluo-G has been especially
designed regarding the specific objectives and context of karst
tracing with geographical/hydrogeological issues. Some advantages
arise given its conception. The size and portability make it easy to
handle by only one person even in difficult caves. It can be
turned-on at the surface and carried in a small bag. The other
kinds of devices are often heavy and/or oversized for most of the
caves and are better used in karst springs or easily accessible
karst systems. The simplified design guarantees a low energy
consumption and allows an extended autonomy with small
batteries, which can be useful for long-term tracing in remote areas.
Finally, the production costs tend to be minimal which should
represent a major advantage for potential users.
The Fluo-G is under constant development and
improvements are necessary despite the first good results. The
capabilities of this version are intentionally limited compared to
currently available commercial solutions. It was designed for
uranine only since it is the most used dye by speleologists and
karst hydrogeologists. It does not have the capabilities to
measure simultaneous dyes like the GGUN FL30. Another
disadvantage of the Fluo-G is the data collection. Since it is a 2-in-1
system, the device must be out of the water for data reading;
however, a simple handling allows a quick reading of the data
and the rebooting of the system with minimum data losses.
Another possible issue is the sealing of the box in case of
extreme flood events.
The Fluo-G field fluorometer is a new kind of compact field
fluorometer for hydrogeological use. The intent was to create
a simple, compact and easy-to-use device. Standard
components were required to build a low-cost and simple product.
The fluorometer has been tested in a karst environment but
can also be used in other hydrological environments that
require dye-tracing methodology.
Karst systems and caves offer great opportunities for scientific
research and exploration to understand both the functioning and
organization of groundwater. In-cave dye tracing can give
valuable additional information about the internal structure and
hydraulic functioning of karst aquifers (Lauber et al 2014);
however, specific conditions inside caves require particular dispositions,
which is especially the case for tracer test techniques.
Besides the classic advantages of automatic field
fluorometers compared to charcoal bags and water sampling, this
device is small, extra-light and energy efficient. The handling is
very simple for calibration, data acquisition and results
display. The Fluo-G does not have all the capabilities of similar
devices used for fluorescence measurement. The purpose was
to design a simple and low-cost product for cavers and karst
researchers. Uranine, turbidity and temperature are the three
parameters that can be measured.
The fluorometer has been successfully tested in the
laboratory and in the field and the results are in good fit with other
methods in terms of dye concentration, data resolution,
precision and accuracy. While the main goal was to get quality data
in line with the specific issues in karst research, the next step is
to ensure the durability of the device regarding the aggressive
karst environment. The Fluo-G has so far been tested in
different case studies in Belgium. Applications to a larger
number of environments, also with different field settings and
specific issues will help to highlight capabilities and limitations of
the device and will suggest further improvements.
Acknowledgments The development of the Fluo-Green was made
possible through the financial support of the Belgium National Fund for
Scientific Research (FRS-FNRS) in the framework of the KARAG
Project (www.karag.be). The authors would also like to acknowledge
N. Goldscheider and an anonymous reviewer for their valuable
comments, which helped to improve this manuscript.
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