Hydrological tracing

Artificial tracers in hydrology

The use of artificial tracers in hydrogeology is a very ancient technique. It is an effective tool for managing and preserving water resources and protecting the environment.

The increasing demand for hydrological assessments requiring the use of artificial tracers is motivated by two main factors: the challenge of managing water resources and the growth in pollutant shipments.

This new technique means that the fluorescent and saline properties of tracers can be employed to detect the point of origin of water, the location of run-off and hydraulic connections, and to reveal the risks of pollutants spreading.

The results obtained can provide definitive answers to all these problems, helping empower economic and social actors regarding their environmental impact.

Overview of the different tracers

Below, a reminder of the different possible applications and their objectives:

*Applications*	*Goals*
*Abstraction of groundwater/spring water*	Delimitation of a supply catch basin
	Check that a determined point is found in the catch basin
	Study of the respective inputs of different waters
*Protection areas of under-ground waters*	Check of an hydraulic connection between the injection and catch point
	Study of transit times and flow velocity
	Sizing of protection zones
*Supply areas of run-offs*	Delimitation of feeding areas
	Check that a given location is found in the feeding area of capture
	Knowledge of underground inflows
*Risks assessment / Incidents simulation*	Check of a hydraulic connection and an installation/ a capture site
	Assessment of the operational timing
	Monitoring of implantation of the supervision point
	Estimate of the size of the impacts during risk assessments
	Simulation of the incidental effect causing infiltration of dangerous liquids in underground waters
*Contaminated Sites / Dubious areas*	Study of seepage runoff
*Contaminated Sites / Dubious areas*	Verification of the right place of sampling points downstream from a site
*Landfills*	Verification of outflows under existing landfills
	Monitoring of seepage paths : meteoric waters flow towards foreseen drainages
	Monitoring of waterproofing of cover layers
	Assessment of sites for new landfills : e.g. verification that a site is away from a supply catch basin
	Verification of the right spot of the surveillance point
*Interaction surface waters / ground waters*	Detection and localization of seepage or water flows exfiltration sections
	Detection of outflows under water streams
	Identification of drain places of closed lakes
*Detection of parasite waters*	Identification et quantification of the arrival to a seepage catch coming from a water stream
*Detection of parasite waters*	Verification of the arrival to a capture of close rainwater infiltrations
*Drilling/Piezometer*	Control of the representativeness of underground water samples through tagging the drilling fluid
*Drilling/Piezometer*	Control of leak-proof caps dividing various levels of catchment
*Expert assessment in case of damages*	Verification of the point of entry of waters seeping into a building
*Determination of aquifer parameters / Modelling of ground-water flows*	Determination hydraulic parameters such as the outflow velocity and dispersion coefficient
	Calculation of the storing volume from empty volumes allowing outflow
	Adapt and validate outflow models and mass transport in solution form
	Verifications of outflows directions predicted with respect to observed directions

In particular in karstic environment, artificial tracing remains essential to mark out hydrogeological systems and assess the transit characteristics from the Residence Time Distribution (RTD). Although karstic and cracked environments concentrate the larger variety of tracing applications, the latter are not limited to this type of environment:

*Aquifers Types*	*Ways for tracers’ application*
*Karstic Aquifers*	Regional recognition of underground outflows
	Development of conceptual models of karstic systems
	Delimitation of catchment basins from karstic sources
	Highlight of the flux of karstic waters
*Aquifers in loose rocks*	Regional studies of underground outflows
	Delimitation of catchment basins
	Experimental simulation of the spread of pollutants
	Determination of aquifer’s parameters
	Delimitation of input areas of polluting material
*Flows in aquifer slopes and in cracked rocks*	Highlight of outflows in bedrocks
	Studies of aquifer structures (cracks, rifts, furrow etc.)
	Highlight of preferential flows in water slopes (water veins…)