1. A cost-benefit approach to tracer testing
Categorisation of Systems, Purposes, Processes & Tracers
Examples of specific questions
Critical protocols, QA & QC
Cost / Benefit

2. Systems Categorisation
Waste water treatment / distribution systems
Drinking water treatment / distribution systems
Gas / Steam distribution systems
Non-environmental or industrial systems:
Surface water
Groundwater
Hydrologic Systems:
Associated Environmental
& Geologic Systems:
Contaminated land
Oil field applications
Gas phase tracing (atmospheric / soil)

3. Categorisation of Purpose
Typical Tracer Test Objectives
Groundwater connections
Seepage velocities
Hydraulic conductivity
Kinematic porosity
Groundwater flow mechanism
Contaminant behaviour
Establish flow horizons
Characterise recharge
Hierarchy of Tracer Test analysis
Connectivity
Travel time
Statistical (mean, variance, moments)
Transfer function (coefficients of transfer model)
Analytical solutions (few parameters)
Numerical solutions (few to many parameters)
Constrain predictions
Typical groundwater tracer test objectives are listed on the left. On the right hand side is reproduced a table from the BGS/EA collaborative tracer test manual (Ward et al. 1998), which is the standard UK reference work.
The table lists seven levels of hierarchy of analysis, culminating in 'constraining predictions'. I argue that as one requires some form of conceptualisation throughout, that constraint prediction (hypothesis testing) should form a part of analysis at every stage.
This point is also discussed further below.
Ward et al

4. Process Categorisation
Mixing
Dilution
Advection / Dispersion
Diffusion
Chemical reaction
Ion exclusion
Adsorption
Absorption
Leakage
Surface water / groundwater interaction
Degradation
Decay
Note scale dependency of some processes eg:
Dispersion (spatial scale)
Diffusion (temporal scale)
Processes are presented ahead of particular types of tracer as tracers of various forms are typically used to evaluate the relative extent and importance of these processes.
WRT scale – it is important to be aware that the same tracer may respond differently at different scales, and thereby elucidate different processes. EG diffusion is a time-dependent phenomenon, and depending on the temporal scale of transport (the experiment) may provide information about fracture flow only (short times), fracture and matrix porosity (medium times) or bulk porosity (long times).

5. Tracer Categorisation
Ambient tracers
Geochemistry:
Physico – Chemical parameters
Major ion chemistry
Trace element chemistry
Anthropochemistry:
Contaminant spills / leaks
Atmospheric gases
Artificial tracers
Temperature
Micro-organisms
Particles
Dyes
Ions
Gases
Fluorocarbons
Stable isotopes
Radioisotopes
Ambient tracers – those that are present in the environment already.
Artificial tracers – those we might introduce to stress a system or produce a specific effect.
Bottom graphic – increase in atmospheric concentration of CFCs and SF6 throughout 20th Century.

6. Tracer Categorisation
Ambient tracers
Geochemistry:
Physico – Chemical parameters
Major ion chemistry
Trace element chemistry
Anthropochemistry:
Contaminant spills / leaks
Atmospheric gases
Artificial tracers
Temperature
Micro-organisms
Particles
Dyes
Ions
Gases
Fluorocarbons
Stable isotopes
Radioisotopes
LOTS OF CROSSOVER!
Note that many of the tracers will elucidate common controlling processes. From a cost-benefit point of view, as many groundwater investigations will as a matter of course acquire, for example, major ion or trace element chemistry, information regarding processes of interest may already be held by the investigative team.

7. Examples of specific questions
How do I discriminate between processes responsible for residual contaminant release?
How long will contamination persist above a given concentration?
What is the proportion of fracture / matrix flow?
Is this SPZ realistic?
How does the permeability vary spatially?
Are these two points connected?
What is the groundwater velocity?
What should be the immediate response to a groundwater contamination incident ?

8. Know your question
How do I discriminate between processes responsible for residual contaminant release?
How long will contamination persist above a given concentration?
What is the proportion of fracture / matrix flow?
Is this SPZ realistic?
How does the permeability vary spatially?
Are these two points connected?
What is the groundwater velocity?
What should my immediate response to a groundwater contamination incident be?
Varying levels of conceptualisation
Questions should be appropriate for one's existing level of conceptual understanding
Tracer TEST – test hypotheses (one is usually required to estimate parameters in order to conduct a test successfully – iterative processes)
From the list of specific questions it is obvious that some are considerably more complex than others. Thus, there is a level of system conceptualisation required prior to asking any particular question.
From an artificial tracer test point of view, even the most basic 'point – to – point' experiment to demonstrate connection requires one to consider how much dilution might be expected (in order to quantify amount of tracer to inject) and to estimate travel times – in order to be sampling at the right time, as well as assuming that a connection exists – ie flowpath assumptions. Thus, 'prediction constraint' or hypothesis testing should be present at whatever level of tracer test one employs.
The graphs at bottom show changes in groundwater chemistry with distance (0 – 10,000m) from a sining river. These illustrate particular processes of dissolution (increases in HCO3 & EC) and ion exchange (increase in the Mg/Ca ratio). Simple chemistry.

9. Boundary conditions
An 'artificial' tracer test should be very well defined
Well-constrained input functions
Can be conducted in conjunction with pumping tests or other controls on the flow field
Generally well-understood properties of tracers (e.g. diffusion co-efficients)
Preparatory column experiments can be used to establish aquifer-specific sorption isotherms if necessary
Wide variety of contaminant analogues – tailored to your requirements
All facilitated by adequate preparation
WRT Artificial tracer tests, one of the principal advantages is the tight control available over the boundary conditions of the experiment, particularly the input function – in terms of concentrations/amounts, shape of input pulse, length of input time.
On occasion the flow field can also be controlled to a greater or lesser degree (eg from laboratory column experiments to pumping wells) and this enables more precise analysis as well as potentially examining different responses under changing flow conditions.
The diagram at bottom is a simple pipe network model of a karst aquifer showing principal flows in a particular system.
To be able to constrain one's boundary conditions, one needs to have excellent protocols for QA/QC – see next slide
Basic karst / fracture network model where:
Qout = Q1 + Q2 + Qaq1 + Qaq2
Qlost = 0.5 Qin

10. Protocols, Quality Assurance & Quality Control
Strict observance of protocols is required for successful tracer testing
Quality Assurance: Sampling and analytical protocols
Quality Control: Methods for indication / quantification of error (duplicates, trip blanks, field blanks, decontamination test blanks)
Quantification of error (QC) necessitates & informs protocols (QA) (iterative process)
Sources of error may be several and non-obvious
Familiarity with analytical methods and laboratory processes is important.
Good QA/QC provides the necessary assurance to both regulators and clients, and practitioners with robust results
Protocols are critical for successful tracer tests, not only in order to provide the analyst/modeller with closely constrained boundaries and thus a better handle on model uncertainty, but critically to provide the regulator and/or client with assurance that the experiment will be conducted with all due regard for the safety of the hydrologic system in question.

11. Summary
Define your objective within the context of your conceptual understanding
Be aware of the processes likely to contribute to system behaviour
Recognise the possibility of already having / intending to collect information that reflects the processes you are interested in
Artificial tracer test: tightly constrained boundary conditions
Be aware of critical protocols and QA / QC requirements
More on Cost / Benefit soon...
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12. Breakthrough curves testing a hypothesised dipole flow field between a sinking stream and a pumping well. The linear part of the breakthrough curve 'tail' reflects the dominant process controlling contaminant release from the aquifer. Although the dipole model fitted the tailing precisely at the pumping well, it also fitted precisely the curve from a well elsewhere within the flow field, thereby indicating that the dipole odel (which should only have fitted at the pumping well) was not a good model of the system. Dual porosity behaviour and regional groundwater gradients were considered to have greater influence on the ongoing release of proxy contaminant (ie tracer).