Source Tracing Techniques [Mar 2008]
Water movement and relative impact of pollution sources on water quality
The Environment Agency (EA) defines tracers as follows: “Tracers are substances which may be used to deduce the direction, destination and velocity of water or other substances”. Source tracing is a technique that uses tracers to track the movement of water to determine the relative contribution of various sources on its composition at specific points. For example, a tracer might be used to determine if faecal pollution from an effluent discharge pipe is passing out to sea or returning to the coast and thereby impacting water quality on the nearby beaches.
The technique is particularly useful where the quality of a water catchment is being impacted by a large number of diffuse non-point sources such as the runoff from cattle farms and septic tanks leaking into numerous streams. It is often very difficult to know which of these sources is having the greatest impact on water quality and hence how best to take corrective action to improve water quality. Using a tracer to clarify how much contaminated water is reaching the main catchment from each source can pinpoint the cause of contamination and help manage the situation.
Typical applications of water flow tracing include:
- Identifying and quantifying sources of faecal or chemical pollution
- Provide base data for the development and validation of catchment area input models
- Localising leak areas in sewer systems
- Establishing connectivity in portable water supplies
- Establishing time of residence in reservoirs and in other water systems
- Tracing the flow of groundwater in aquifers
- Validated flow models of contaminated groundwater
- Establish the fate of run-off from fields
- Tracing the dispersion of chemical sprays such as pesticides
- Developing ocean flow models
Choice of tracers
The choice of tracer will depend largely on the type of study and the required properties of the tracer. The main properties affecting the choice of tracer are:
Ability to mimic transport of the target substance:
The tracer should be carried in the bulk of the water and not sediment out or react with materials in the environment. If modelling faecal contamination it should be ideally transported in a similar manner to bacteria or viral particles.
Sensitivity of detection:
The tracer should be easy to detect against the background of the environment.
Persistence:
The tracer should be reasonably inert and stable. The stability of the tracer over the course of the study should be known.
Safety and approval for release:
It is normally necessary to seek approval from relevant authorities before releasing a tracer. The tracer should not cause harm to human health, aquatic organisms or the environment.
Float tracing method
One of the simplest ways to monitor water flow is the float tracing method in which buoyant objects such as ping pong balls or say a buoy with radar reflector are released into the body of water and visually tracked to see where the object goes.
A well known example of this occurred when 28,800 plastic toy animals (“Floatees”) were accidentally released from a cargo container into the mid-Pacific in 1992. Hundreds of the plastic bath toys came ashore, eight to nine months later 2,200 miles away on beaches near Sitka, Alaska, providing useful information on ocean currents. Beachcombers are still finding them washing up on various beaches all over the world. Some of the problems associated with using floating objects as tracers in this way are that the objects may move quite differently to the water body being investigated, only a limited number of objects can be used providing low sensitivity, they will only track surface flows and cannot be used to track circulation at depth or groundwater flows, and they can cause undesirable visible pollution in the water.
Soluble chemicals and inorganic ions
Soluble chemicals such as salt will move with the bulk water and have a long history of use as tracers. For example, in 1872, a connection between the sinkhole into which sewage from a farmhouse was discharged and a spring used for drinking water that had been implicated in a typhoid epidemic in north-west Switzerland was demonstrated by injecting salt into the sinkhole at the farmhouse. The salt was subsequently detected in the spring, confirming sewage from the sinkhole was flowing into the stream. Today, inorganic ions including dichromate, iodide, chloride, bromide, lithium and nitrate are used in tracer studies. They are highly water soluble and inherently stable although prone to ionic exchange or binding to clays and other materials. They can typically be detected by standard analytical techniques such as atomic adsorption spectrophotometry. The ion should be chosen with care as the limit of detection for these studies is frequently limited by the background of ions present in the environment. The ions can be toxic to aquatic life and limits are normally set on the maximum concentrations permitted in fresh and salt water.
Radionucleotides
Radionucleotides such as 24Na, 51Cr, 198Au, 82Br, 86Rb, 110Ag and 131I make highly sensitive tracers that can be easily detected by a scintillation counter. However, the release of radiotracers into the environment is strictly controlled under the 1960 Radioactive Substances Act in the UK and their use tends to be limited by difficulties in obtaining approval for use.
Fluorescent dyes
Probably the most common type of soluble water tracer used is fluorescent dyes. Fluorescein (a green fluorescent dye) was first used as a water tracer in 1877 to trace the sinking portions of the upper Danube river. Although there are now a large number of dyes available, only a few are considered safe for environmental release. Initially these dyes were used qualitatively, being released into the water at a source location then visually following the plume of coloured water to see where it would go.
It was later discovered that these dyes would adsorb onto charcoal suspended in mesh bags in the current and could later be removed from the charcoal with an alcohol based solvent. This allowed samples to be collected at multiple points over a period of time, even if the dye could not be seen visibly in the water.
The technique became quantitative with the development of a device called the spectrofluorimeter which provides a precise measurement of the concentration of the fluorescent dye in the sample. Most of the fluorescent dyes have low toxicity.
The EA has recommended maximum concentrations for tracer studies based on the maximum concentration at which the dye is expected to have no detrimental effect on the environment. Another limitation of dye based tracers is that the concentrations should generally be kept below the limit of visible detection as it is generally considered unacceptable to noticeably colour the water - very few people want to drink or swim in bright red water. Although these dyes will break down by photolysis some residue is likely to persist in the environment for several years and may interfere with subsequent tracing studies.
Small microscopic beads coated in fluorescent dyes can be used as tracers that can be seen as bright points of light against a dark background using a fluorescent flow cytometry. They are commercially available in a variety of sizes between 0.2μm to 5μm in diameter and colours. These beads make excellent mimics of microbial cells for use in tracer studies. They are highly stable and once added to an environment may persist for a very long time. As with the fluorescent dyes the limited number of colours available can limit the number of different source points used in a study.
Live tracers
Microbial cells and spores such as Bacillus globigii, Serratia spp. and Saccharomyces cerevisia (yeast) have been used as live tracers. The microbial cells can be grown to high densities around 1011 cells per litre and can be detected using plate counts on selective media. The tracer organism must be chosen with care to ensure it is not pathogenic and will not have a negative impact on the environment. As the cells are living organisms they may become non-viable (die off) or grow and multiply under favourable conditions changing the number of cells detected. The cells can also become trapped, adsorbed to solids or eaten by filter feeders. Additionally, indigenous organisms in the sample able to grow on the selective media can frequently interfere with detection giving a relatively high background, which decreases the limit of detection.
Bacteriophage
Small virus particles known as bacteriophage have been used as tracers. For example, bacteriophage ΦHSIC-1 has been used to track the flow of sewage from septic tanks in Key Largo, Florida to adjacent canal waters. In the UK, bacteriophage tracers have been used to investigate water movement at Colliford lake, investigate a sewer leak in St. Agnes and determine the travel time in the river Dart.
Bacteriophage make excellent models for the pathogenic viruses in sewage as they are similar in size and structure so would be expected to be transported in a similar manner. Like the viruses from sewage they are likely to become trapped or adsorbed to solids. Bacteriophage will only infect specific strains of bacteria, and so should not pose a risk to human health and they can be detected quantitatively. Although there are several types of bacteriophage suitable for tracer studies, they may be transported differently so are generally not used in studies where different tracers are required.
DNA
DNA can be used to mark materials with an extraordinarily high level of sensitivity and specificity. The high sensitivity of DNA tracers come from the fact that it is possible to detect a single specific sequence of DNA using a technique known as Polymerase Chain Reaction (PCR). Initially, PCR could only be used qualitatively to prove a given DNA sequence was present. However, a modification of the technique known as Quantitative PCR (qPCR) now makes it possible to work out precisely how many copies of the DNA sequence were present in the sample.
Naked DNA is very susceptible to degradation from microbial enzymes and UV light, so environmental DNA tracers need to be embedded in something that will protect them from degradation in the environment. One of the biggest advantages of this type of system is that unlike fluorescent dyes where only a few different coloured dyes are available there are essentially an unlimited number of different tracer sequences. Unique tracers can be added at a number of different points in a catchment system at the same time or at the same point with short delays between each addition allowing a much more flexible modelling system.
In summary
Tracing techniques provide powerful tools for modelling water movement and establishing the relative impact of different pollution sources on water quality. A variety of tracer types can be selected depending on the objectives of the study and sensitivity of the environment. Modern DNA based tracer particles make particularly effective environmental tracers because they are biocompatible, non persistent, can be detected at extremely low concentrations, and there are practically an unlimited number of unique tracer sequences that can be used.
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