Tracing in brief
Artificial tracing is a fundamental method of investigation and control for the management and conservation of water resources and environmental protection. It is based on the injection into the water environment – the natural, but also urban, and even industrial environment – of markers whose fluorescence properties or saline character, for example, enable the pathways of waters and of their dissolved and particulate pollutants to be traced, their residence and transit times to be determined, etc. The questions which can be answered by tracing techniques are: Where does the water flow? Where does it come from? Is there a hydraulic connection between two points? How do substances (including pollutants) disseminate in the water? The use of artificial tracers is often the only way to prove beyond doubt an actual hydrogeological phenomenon. Some experts compare tracer tests in hydrogeology to X-rays in medicine (Käss 1998).
For quality monitoring and sustainable management of water and environmental resources, the main applications of tracer techniques are:
- Characterization of natural hydrogeological systems, estimation of their water resources,
- Delimitation of feeding areas and protection perimeters of catchment works for drinking water supply,
- Evaluation of the impact of direct discharges from wastewater treatment plants in the natural environment (in accordance with the French decree of 22 June 2007 on the collection, transport and treatment of wastewater),
- Studies prior to the realization of construction wetlands for wastewater treatment (EPNAC 2013),
- Studies of the dissemination of existing or potential groundwater pollution,
- The design of depollution wells and antipollution barriers,
- The design of injection-based in situ remediation of polluted groundwaters,
- Geotechnical studies (seepage, building leakage ...),
- The circulation of water and pollution in plants and industrial areas.
History and Prospects
Tracing techniques have their origin in karst hydrogeology (Mangin et al. 1976). Since then, artificial tracing has been used to highlight the relationships between swallow holes and stream losses and sources in karst aquifers (Plagnes 1997). The use of tracers in studies of groundwaters marks the beginning of modern research on groundwater flow in karst formations (Atkinson and Smart 1979). This method has become particularly relevant due to growing issues of vulnerability and pollution of water resources, especially as higher water needs have led to increasingly exploiting karst resources (Bakalowicz 1977). Currently two-thirds of the drinking water distributed in France comes from groundwater or captured sources, with karst aquifers representing about one third.
Despite these strengths, tracing is still little known, under-used, and when it is, it is often used improperly, including by designers and managers who face problems of water resources or of protection of water quality. Major efforts are still required to establish protected areas for drinking water supply, firstly by adapting legislation to the specific case of the transport of pollutants in the karst, and secondly by finding the most suitable methodology to extrapolate the test results and include them in the vulnerability assessment (Callier et al. 2005, Muet et al. 2005, Jozja 2008).
Tracing techniques have long been developed as an independent specialty, which is why it is necessary for inexperienced users to contact experts before performing tracing tests. Nowadays, precise answers are expected in the field of pollutant transport, which generates a marked increase in the requirements for the planning, execution and interpretation of tracing tests.
Tracing also has a great potential for R&D development to improve its performance and expand its applications throughout the field of water and environmental management. The main barriers limiting its applications are an insufficient understanding of the behavior of tracers in the environment (degradation over time due to environmental factors, adsorption-interactions with rocks and suspended solids in the water, etc.). From the methodological point of view, a series of improvements (taking into account the background fluorescence of natural waters, discrimination of interferences of colloidal suspended solids, etc.) will together achieve an improved performance of the method. The development of in situ detection techniques or new types of tracers is also an avenue of research that is being considered in this field.
Atkinson T.C. Smart P.L. 1979. Traceurs artificiels en hydrogéologie. Bulletin du BRGM, 2ème série, section III, n° 3, p. 365-380.
Bakalowicz M. 1977. Etude de degré d’organisation des écoulements souterrains dans les aquifères carbonatés par une méthode hydrogéochimique nouvelle. C. R. Acad. Sci. Paris, 284-D, p. 2463-2466. .
Callier L. Chartier R. Courtois N. 2005. Surveillance des eaux souterraines au droit des installations classées en milieu karstique. Application de l’art. 65 du décret du 2 février 1998. Rapport Final. BRGM/RP-54 596 –FR, 38 p.
EPNAC (Groupe de travail national sur l'Evaluation des Procédés Nouveaux d'Assainissement des petites et moyennes Collectivités) 2013. Contenu des études préalables à la réalisation d'une Zone de Rejet Végétalisée. IRSTEA, mars 2013, 16 p.
Jozja N. 2008. Importance de la composante analytique dans la fiabilité de l’interprétation d’un traçage. Actes de la Journée technique du Comité Français de l’Association Internationale des Hydrogéologues « Hydrogéologie et karst au travers des travaux de Michel Lepiller », Orléans, 16-17 mai 2008, p. 207-218.
Käss W. 1998. Tracing Technique in Geohydrology. Taylor & Francis, 585 p.
Lepiller M. 2006a. Val d’Orléans. Aquifères et eaux souterraines en France. Ouvrage collectif sous la direction de Roux J.C. Tome 1, BRGM Editions, p. 200-214.
Lepiller M. 2006b. Gâtinais de l’est. Aquifères et eaux souterraines en France. Ouvrage collectif sous la direction de Roux J.C. Tome 1, BRGM Editions, p. 291-299.
Mangin A. Molinari J. Paloc H. 1976. Les traceurs en hydrogéologie karstique. Leur apport à la connaissance des réservoirs aquifères calcaires. La Houille Blanche, N° 3-4, p. 261-267
Muet P. Vier E. Cadhillac L. Marchet P. 2005. Procédures de protection des captages d’alimentation en eau potable en milieux karstique en France. Bilan et préconisation. Cahiers de l’Association Scientifique Européenne pour l’Eau et la Santé, Volume 11, Fascicule 1, p. 41-47.
Plagnes V. 1997. Structure et fonctionnement des aquifères karstiques. Caractérisation par la géochimie des eaux. Thèse de l’Université de Montpellier II. Document BRGM 294, 375 p.
Roux J.C. (dir.) 2006. Aquifères et eaux souterraines en France, Tome 1, BRGM Editions, 479 p.
Waduge A. Cohen E. Divine C. 2013. Tracer testing strategies for effective design and implementation of in situ groundwater remediation. Journal of Environmental Science and Engineering A, 2, p. 759-769.