A compact field fluorometer and its application to dye tracing in karst environments
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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.
KeywordsTracer tests Field technique Fluorometer Groundwater monitoring Karst
Un fluorimètre de terrain compact et son application pour les essais de traçage en environnements karstiques
L’essai de traçage est une technique classique en hydrogéologie pour étudier les caractéristiques des écoulements de surface ou souterrains, et il est utile pour de nombreuses applications notamment pour des problématiques environnementales ou industrielles. Le fluorimètre de terrain Fluo-Green a été testé avec succès en milieu karstique et il est spécifiquement adapté pour le monitoring des eaux en grotte. La recherche en environnement karstique utilise souvent des traceurs pour obtenir des informations sur les écoulements d’eau souterraine dans des zones non-explorées. Le dispositif compact, aussi appelé Fluo-G, répond aux exigences du milieu souterrain : compact (10 × 16 × 21 cm), léger (0.75 kg sans lest) et de conception simple. Il est facile pour les spéléologues de le mettre en place et de le manipuler par rapport à d’autres méthodes d’échantillonnage. Le fluorimètre mesure l’uranine, la turbidité et la température avec un intervalle de temps défini par l’utilisateur (1 min – 1 jour). Sa très faible comsommation en énergie permet 9,000 mesures avec six piles AA. L’appareil a été calibré et testé en laboratoire et sur le terrain dans des systèmes karstiques belges. Les résultats sont en adéquation avec d’autres méthodes d’échantillonnage : fluorimètres in-situ et prélèvement automatique d’échantillons d’eau suivi d’analyses au laboratoire. L’enregistrement de données de haute qualité (courbes de restitution) en milieu karstique avec un monitoring en grotte est précieux pour améliorer la connaissance des systèmes karstiques. De nombreuses applications hydrologiques et hydrogéologiques peuvent bénéficier d’un tel équipement compact et peu coûteux. De plus, trouver le meilleur compromis entre les ressources disponibles et une bonne qualité des données est un élément essentiel. Plusieurs améliorations sont possibles mais les essais préliminaires sur le terrain sont très prometteurs.
Un fluorímetro de campo compacto y su aplicación al seguimiento de colorantes en ambientes kársticos
El trazado de colorantes es una técnica clásica en hidrogeología para investigar las características del flujo del agua superficial o del agua subterránea, y es útil para muchas aplicaciones incluyendo cuestiones naturales o industriales. El fluorímetro de campo Fluo-Green se probó con éxito en un ambiente kárstico y es específicamente adecuado para el monitoreo de agua kárstica en cavernas. La investigación del karst utiliza a menudo colorantes para obtener la información sobre flujo del agua subterránea en canales inexplorados de las cavernas. El dispositivo compacto, denominado alternativamente Fluo-G, cumple los requisitos de los medios de cavernas: pequeño (10 × 16 × 21 cm), liviano (0.75 kg sin lastre) y de concepción sencilla. Es fácil para los espeleólogos para establecer y manejar en comparación con otros métodos de muestreo. El fluorímetro registra uranina, turbidez y temperatura con un paso de tiempo definido por el usuario (1 min – 1 día). El consumo de energía muy bajo permite 9,000 mediciones con seis pilas AA. El dispositivo se calibró y probó en el laboratorio y en condiciones de campo en sistemas kársticos belgas. Los resultados se ajustan bien con otros métodos de muestreo: fluorímetros in situ y muestreo automático de agua más análisis de laboratorio. La grabación de datos de alta calidad (curvas de avance) en el karst con monitoreo en cavernas es valiosa para mejorar el conocimiento de los sistemas kársticos. Muchas aplicaciones hidrológicas e hidrogeológicas pueden beneficiarse con un dispositivo de bajo costo y compacto, y es esencial encontrar el mejor compromiso entre los recursos y los datos de calidad. Varias mejoras son posibles, pero las pruebas de campo preliminares son muy prometedoras.
染色示踪是水文地质中调查地表水流或地下水流特征传统的技术,对于包括天然或工业问题的许多应用非常有益。在岩溶环境中成功测试了荧光绿野外荧光计,特别适合洞穴内岩溶水的监测。岩溶研究经常使用染色以获取未勘察过的洞穴通道中的地下水流信息。被称为Fluo-G的这个紧凑的装置能够满足洞穴介质的需要:小巧(10 × 16 × 21 cm)、重量轻(无承载物时为0.75 kg)、概念简单。与其他采样方法相比,布设和操作非常容易。荧光计采用自定义时步(1分钟到天)记录荧光素钠、混浊度及温度。能量消耗非常低,六节AA电池能进行9,000次测量。在实验室和比利时岩溶系统的野外条件下对装置进行了校正和测试。结果与其他采样方法符合良好:原位荧光计和自动水采样加上实验室分析。岩溶采用洞穴内监测记录高质量数据(突破曲线)对于增进对岩溶系统的认识非常有用。许多水文和水文地质上的应用可以从如此廉价和紧凑的装置中获得益处,找到资源和质量数据之间的最佳妥协至关重要。若干地方的改进是可能的,但初步的野外试验结果显示非常有前途。
Um fluorímetro compacto de campo e sua aplicação com traçadores de corante em ambientes cársticos
Teste de traçador é uma técnica clássica em hidrogeologia para investigar águas superficiais ou características do fluxo das águas subterrâneas, e é útil para muitas aplicações incluindo problemas naturais e industriais. O fluorímetro de campo ‘Fluo-Green’ foi testado com sucesso em um ambiente cárstico e é adequado especificamente para monitoramento de águas em cavernas. Pesquisas em carste geralmente usam corantes para obter informações sobre o fluxo das águas subterrâneas em passagens de cavernas inexploradas. O equipamento compacto, alternativamente chamado de ‘Fluo-G’, atende aos requerimentos do ambiente de cavernas: pequeno (10 × 16 × 21 cm), leve (0.75 kg sem balastro) e de simples concepção. É de fácil instalação e operação para exploradores de cavernas comparado com outros métodos de amostragem. O fluorímetro registra uranina, turbidez e temperatura com espaçamento de tempo definido pelo usuário (1 min – 1 dia). Com pouco consumo de energia, permite 9,s000 registros com seis baterias AA. O dispositivo foi calibrado e testado em laboratório e em condições de campo em sistemas cársticos belgas. Os resultados estão em bom acordo com outros métodos de amostragem: fluorímetros in-situ e amostragem automática mais análise em laboratório. A obtenção de dados de alta qualidade (curvas de identificação) através de monitoramento em cavernas é valiosa para melhorar o conhecimento de sistema cársticos. Muitas aplicações hidrológicas e hidrogeológicas podem se beneficiar de tal equipamento compacto e de baixo custo, e a procura pelo acordo entre recursos e qualidade de dados é essencial. Muitas melhoras são possíveis, porém os testes de campo preliminares são promissores.
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 “ideal 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 characteristics.
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. 2008).
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 “simple” 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
Properties of the Fluo-Green and comparison with the GGUN FL30 (Schnegg 2002)
10 × 16 × 21 cm
Probe : 16 × 17 cm
Logger : 29 × 25 × 19 cm
Probe : 7.3 kg
Logger : 7.2 kg
Cable : 1 kg/10 m
2 × 1.0 kg
2 (470 and 625 nm)
Uranine and turbidity
4 (370, 470, 525 and 660 nm)
3 tracers + turbidity
1 min – 1 day
2 s (1 channel) – 15 min
Duration of measurement sequence
Total measurement = 9.4 s
77 mA (measurement)
0 mA (stand by)
50 mA (measurement)
1.5 mA (stand by)
Detection resolution (uranine)
3,000 ppb (theoretical)
Temperature probe resolution (field measurement)
Max depth of use
Tested at −4 m
As mentioned in the Table 1, the main advantages of Fluo-G 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.
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 light’ section).
- u (ppb)
Concentration of the uranine dye
- G (counts)
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)
- C1, C2, C3
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.
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 water current
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.
Test No. 1: surface karst resurgence with daylight interference
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 an Agilent spectrofluorometer at the University of Namur. Three Fluo-G were installed in the resurgence with a 5-min time-step configuration.
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. 2008).
Test No. 3: underground river with mud riverbed and low water current
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 BTC characteristics.
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 drip-water.
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 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.
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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