SOURCE: CETRAHE, BRGM, 2019.
Artificial Tracing in Hydrogeology: Best Practices.
Information System for Groundwater Management in the Centre-Val de Loire, published online in January 2019.
BEFORE CONDUCTING A TRACING, PRELIMINARY STEPS NEED TO BE PLANNED :
The first step is to clearly define the objectives of the tracing: tracing for the recognition of underground flows, simulation of pollutant transfer, aquifer characterization tests with the determination of hydrodispersive parameters (flow velocity, kinematic porosity, dispersivity), etc. This step is very important as the strategic choices that will be adopted later will be a compromise between the objectives and the cost.
The second step involves gathering as much existing information as possible, as well as documentation on previous tracings (see dedicated article on regional inventory). The collected information should include all geographical, topographical, geological, hydrogeological, and anthropic data (water uses, wells, etc.). As for previous tracings, even if they do not meet today's evaluation criteria with satisfactory reliability, they will be rich in information and very useful for avoiding certain pitfalls.
The third step is the site reconnaissance where the tracing will be carried out. It involves locating potential injection points (direct access or through an unsaturated zone, absorption capacity, possibilities for charging and overflow, need for flushing, accessibility especially for vehicles transporting water for flushing, etc.) and potential discharge points (wells, uncaptured sources, surface water outlets, operation, accessibility, possible flow measurement, etc.). At the end of this visit, it is important to assess the feasibility of implementing various monitoring devices (manual sampling, automatic sampler installation, fluorimeter installation, activated carbon detector attachment, influence of pumping regimes, influence of chlorination, etc.) and to anticipate hydrological conditions that may be different (and variable) during the test.
AFTER ADDRESSING THESE STEPS, THE SIZING OF THE TRACING CAN BE CARRIED OUT.
Single tracing or multi-tracing? Multi-tracing involves simultaneously injecting different tracers at multiple injection points. It allows answering multiple questions at once, reducing costs, and saving a considerable amount of time. However, it requires a judicious choice of the tracers used, which should be sufficiently conservative in the context and without presenting analytical interferences among them.
The choice of tracer(s) is particularly important for the sizing of multi-tracings, as it determines the final result based on its performance and also influences other strategic choices (injection quantity and monitoring types). A good understanding of the physicochemical properties of the tracers, as well as their behavior in the relevant environment(s), helps better adapt the tracer to the geological, physical, and hydrological context.
The quantity of tracer to be injected is always a delicate question.Several formulas exist, but they assume a prior knowledge of the medium and the representing parameters. Ideally, having conducted a previous tracing in a similar context would provide the best guidance. The software TRAC (free) in its "Simulation" section allows making estimations by selecting the analytical solution adapted to the hydrogeological context, which best corresponds to the tracer's transit in the chosen tracing system.
In practice, the quantity is estimated based on expert judgment, considering the hydrogeological context. Between empiricism, intuition, and experience, to resolve the question, two determining elements must be considered: the dilution that the tracer is expected to undergo, often approximated using distance, the tracer's analytical performance, and the monitoring methods.
TRACING CANNOT PROVIDE INFORMATION ABOUT THE ENTIRE HYDROLOGICAL OR HYDROGEOLOGICAL SYSTEM. THE RESULTS ONLY APPLY TO THE TESTED PART. TO EXTRAPOLATE TO ANOTHER PART OF THE AQUIFER, THE HOMOGENEITY OF THE MEDIUM MUST BE CERTAIN.
Best practices involve providing prior information to authorities (regional departments, law enforcement, etc.) and residents (local government) before carrying out the tracing operation. This helps to avoid concerns and alerts related to water discoloration, especially in the case of fluorescent or dye tracers. Before any injection, it is necessary to take water samples as control samples. If the protocol includes the use of activated carbon detectors, it is also important to plan for the immersion of "control" fluocapteurs at an appropriate frequency. For reconnaissance tracings, conducting them during periods of high water levels generally provides more favorable conditions due to faster flows, preferably targeting a recession period. It is recommended to perform simulation tracings under contrasting hydrological conditions (low and high water levels) as the results obtained can vary significantly.
MODE OF MONITORING AND ANALYSIS
During a tracing operation, the analytical component is of great importance. A reliable interpretation can only be formulated from results based on rigorously controlled measurements and analytical reasoning.
THE MODE OF MONITORING AND ANALYSIS DEPENDS ON SEVERAL FACTORS:
• Type of monitored water point(s): spring, well, borehole, river, etc.;
• Possibilities for equipment installation: available space, safety, power supply, access, etc.;
• Available budget.
The most reliable method of monitoring and analysis is water sampling with laboratory analysis. Laboratory equipment today allows for the detection of substances in very low concentrations. For fluorescent tracers, laboratory spectrofluorimeters (direct measurement of fluorescence) can achieve very low detection limits, on the order of 0.001 µg/L for uranine. Spectral analysis conducted by a spectrofluorimeter is an essential diagnostic tool for detecting and interpreting a tracing, especially since injection quantities are becoming increasingly small to remain below the visibility threshold at recovery points.
PECTROFLUORIMETER (SOURCE: CETRAHE)
Field instruments allowing in-situ measurements also contribute to improved tracer monitoring. Increasingly sophisticated instruments are available: field fluorimeters, specific electrodes, sensitive conductivity meters, etc. For fluorescent tracers, the use of a field fluorimeter can be very useful.These devices are easy to use and provide nearly real-time results, even in the case of multi-tracing. However, it is advised to avoid using them as the sole monitoring device, especially for multi-tracing. Natural variations in water fluorescence, as well as interferences between tracers, can be mistakenly interpreted as recoveries. It is therefore recommended to couple this monitoring with sampling, whether automatic or manual, to verify the presence or absence of the tracer through spectral analysis in the laboratory.
Regarding activated carbon detectors (fluocapteurs) occasionally used for fluorescent tracers, they should be used as a last resort when field conditions do not allow for any other detection method.They can also be used as a secondary means of detection to expand spatial monitoring in the context of reconnaissance tracings for secondary points. However, caution should be exercised in interpreting the results that follow. Among common tracers, monitoring with a fluocapteur can only be considered for tracers like uranine or eosin, with certain precautions (see technical note n°1 from CETRAHE). Red tracers (such as Rhodamines) cannot be monitored using this method, as activated carbon has shown an inability to retain them under laboratory conditions at concentrations in water below 30 µg/L.
The fluocapteur method is also unsuitable for fluorescent tracers that emit in the blue range (sodium naphthionate, Amino.G. acid, Tinopal).Finally, ionic tracers (salts) can be accurately measured by various devices (ion chromatography, spectrophotometry, atomic absorption spectroscopy, etc.). However, the natural presence of these ions in water interferes with their detection at low concentrations, despite the performance of the equipment used. The dosing of the injected quantity must therefore be carefully studied, so that it is sufficiently high to be detected at monitoring points, and moderate enough not to disrupt water usages (water supplies, natural environments).
DATA EXPLOITATION AND INTERPRETATION
The results of a tracing are illustrated by the tracer recovery curve, providing the evolution of concentrations over time at the recovery point. Mastering the flow rates at the recovery point allows for the calculation of a recovery balance (restored mass and recovery percentage), and the Distribution of Residence Times (DRT), which describes the tracer's transit through the tracing system.The DRT corresponds to the probability density function that gives the probability that a tracer molecule will reside in the system.It is, in fact, the distribution curve of the tracer cloud. When the injection can be approximated as a "Dirac" impulse (i.e., a brief injection), the DRT gives the impulse response of the tracing system for the hydrological conditions it is in at the time of tracing (Lepiller M. & Mondain P-H, 1986). From the DRT, various parameters describing the tracer's transit can be calculated, such as the mean residence time and the apparent velocity. The interpretation of results differs based on the objective. For reconnaissance tracings, the main objective is to accurately determine the affiliation of an injection point to the karstic system's recharge area. For quantitative tracings (simulation), it is important to accurately describe the tracer's transit characteristics, as well as the hydrodispersive parameters for tracings in porous media. Analytical tools to assist in parameter estimation exist. The TRAC software, in "Interpretation" mode for tracings, allows for the interpretation of a tracing using various analytical solutions by adjusting solution parameters and comparing with observed data.
DO NOT CONFUSE INSTRUMENTAL DETECTION THRESHOLDS WITH REAL DETECTION LIMITS, WHICH STRONGLY DEPEND ON THE NATURAL WATER BACKGROUND NOISE LEVEL AND VARY ACCORDING TO THE TRACER.
Finally, following the tracing operation and interpretation of results, the operator is prompted to enter the information into the Tracings Database application. This is a national-level database dedicated to data management. (cf. dedicated article on declaration and banking)