Framework

Artificial Tracers

Artificial tracer tests can be used to evaluate the extent to which aquifers interact with surface water features, providing information on groundwater flow paths, travel times, velocities, dispersion, flow rates and the degree of hydraulic connection (Flury and Wai, 2003;Otz et al, 2003). These tests involve the introduction of a tracer material or chemical and subsequent monitoring of its movement. This differs from environmental tracer methods which rely on the measurement and interpretation of background concentrations of the chemical constituents of water, such as major ions, stable or radioactive isotopes.

Tracer Types

In artificial tracer tests, a substance is introduced and monitored to track the movement of water. The movement of the tracer should match that of the water flow regime, so it should not be affected by sorption onto geological material, changes in chemistry (such as pH or salinity), or degradation by physical or biological processes. The tracer should not affect the water flow regime, by changing fluid density or viscosity. The tracer should have low background levels and be able to be measured simply and cheaply to low detection levels. As tracer tests involve the introduction of a substance into the environment, the tracer should have low toxicological or pathogenic impacts. The tracer should be stable for the duration of the test but not be retained as residual material in the longer term.

Dyes have a long tradition of use as tracers, commencing with tracing the source of typhus epidemics in Europe ( des Carrieres, 1883). Sulforhodamine B, Rhodamine B, Sodium Fluorescein and Rhodamine WT are popular due to low cost, easy detection to low limits with a fluorometer, and the potential for visualisation (Flury and Wai, 2003). Major ions such as chloride and bromide have been used as they behave conservatively and rarely sorb onto geological material. A lithium chloride solute tracer was used to estimate seepage fluxes in a headwater stream (Harvey et al, 1996). Organic compounds such as ethanol, benzoate and fluorinated benzoates have been proposed as tracers (Malcolm et al, 1980; Bowman and Gibbens, 1992). However, retardation and degradation is an issue in low pH conditions and with the presence of abundant clay, iron oxides or organic material (McCarthy et al, 2000; Jaynes, 1994). Flourescent polyaromatic sulfonates have been trialled in geothermal groundwater studies due to their resistance to thermal decay (Rose et al, 1998). The use of isotopes as artificial tracers tends to be limited due to radiation risks and the complexity of chemical analysis. Short lived isotopes such as selenate (75S) as well as deuterium (2H) are considered the most useful (Flury and Wai, 2003). In some studies, a particular pollutant of water is investigated, requiring the tracer to follow the fate of the pollutant rather than water flow. This is the case when non-pathogenic microorganisms are used to trace the transport of human pathogens. Solid and colloidal particles such as clubmoss spores have been used in studies in karstic aquifers, but analysis requires filtering and microscopic examination (Drew, 1968). The use of nanotechnology in hydrogeochemical studies, especially the application of chemical-specific nano-scale tracers developed by the biomedical industry has been suggested (Divine and McDonnell, 2005).

Tracer Methods

Various strategies can be used to undertake an artificial tracer test. The constant injection rate technique is commonly employed in the field to ensure complete mixing of the injected tracer. A dye such as Rhodamine WT is released upstream of the study reach (see Figure 1). Using medical devices for controlling intravenous fluid injection, the dye injection can be started several hours before the beginning of the investigation. Water samples are collected regularly at different locations and the dye concentration analysed with a fluorometer. In this way, very low concentrations can be measured far downstream. The samples are taken from the centre of the channel at each location. By analysing the dye concentration in the sample, the amount of water required to dilute the injected dye solution to the sample concentration can be determined. The amount of water determined includes the amount of stream flow passing the injection site, plus any groundwater contributions the stream has received between the injection and sample sites.

Figure 1: Dye tracer technique for assessing groundwater and surface water interaction in the field (Otz et al, 2003)

A tracer injection trial can provide valuable insights into the rate and direction of groundwater movement near a surface water feature (Dahm & Valett, 1996). This involves establishing monitoring sites both within the aquifer and along the stream. This network can include:

1. Existing boreholes or piezometers in the vicinity of the stream;
2. Sampling pits dug near the stream and accessing the shallow watertable (these should be refilled after the experiment);
3. Any springs or seepage areas evident in the area;
4. Specific sites to monitor the stream itself;
5. Constructing minipiezometers both within and adjacent to the stream bed.

A tracer is then injected into a centrally located pit or piezometer and the time recorded. Each of the groundwater and stream monitoring sites is subsequently sampled to measure if the tracer is detected, and the time when this occurs. This is used with the distance between the injection site and the monitoring site to calculate groundwater velocity and direction.

An even simpler approach is to inject a small bolus of dye about 5-15cm within the stream sediment using a syringe with a long cannula, or via a minipiezometer (Grimm & Fisher, 1984). The time of injection and also the time and position that the dye first appears at the sediment surface are recorded. The distance between the injection point and emergence point is used to calculate groundwater velocity and direction. If the dye does not emerge, the injection site is excavated or the minipiezometer checked for the status of the dye bolus. If the dye has disappeared this suggests that lateral or downward seepage conditions prevail, rather than upward.

In a point dilution test, the tracer is added to a piezometer and the subsequent rate of dilution of the tracer within the piezometer is monitored. This data is used to estimate the groundwater velocity at the piezometer (Halevy et al, 1967; Gaspar 1987). Electrical conductivity measurements were used to record the dilution of a KCl tracer added to piezometers at three contrasting Australian riparian and estuarine sites (Lamontagne et al, 2002).Displacement of the tracer by its improper release into the piezometer or due to the subsequent recirculation process was found to be the main technical difficulty.

Advantages

1. Application and monitoring of the tracer can be designed and implemented in a controlled way. The amount of tracer introduced is known, allowing quantification of aquifer parameters and fluid transport properties.
2. Specific processes in particular hydrogeological settings can be investigated by using appropriate tracers. The method is particularly useful in characterising groundwater flow in highly variable aquifers (such as fractured rock or karsts) and in solute transport studies (such as contaminants and nutrients). Specific tracers can be used to track pollutants such as human pathogens, where the movement and fate of these pollutants may not match water flow.
3. Tracers can be used to assess the significance of local geological features (such as faults, clay layers or cave systems) on stream-aquifer connectivity.
4. Seepage can be assessed either qualitatively (such as visual inspection of the presence of dye or the use of charcoal-based detectors) or quantitatively (such as modelling of time-concentration breakthrough curves to derive travel time characteristics).
5. Tracers can provide direct evidence for the movement of water from one point to another, which is easily understood by the public, regulatory agencies or the courts (Mull et al, 1988).

Disadvantages

1. Requires careful planning and design with some pre-test knowledge of hydrogeology. Unanticipated short travel times can lead to high tracer concentrations being released to watercourses and potentially into public water supplies. This was the case when a borefield tracer test in 2003 unintentionally dyed red the water supply for about a million people (SS Papadopulos and Associates, 2004). Part of planning includes meeting any regulatory controls related to the release of chemicals into the environment for public health or ecosystem protection.
2. The performance of the tracer in matching water movement can vary with the hydrogeological setting. Dyes can have complex chemical interactions which tend to be pH-dependent or can be selectively sorbed with geological material. Sometimes, the dye mixture viscosity can be dramatically affected by variations in the ambient temperature, complicating the determination of flow rates.
3. Tracer tests can have overheads in terms of cost and time, particularly when investigating longer or slower groundwater flow paths.

Case Studies

Piora aquifer, Switzerland

Relevant Links

Dyetracing.com
Turner Designs Practical Guide to Flow Measurement
US Geological Survey Rhodamine WT Reader

References

Dahm CM and Valett HM, 1996. Hyporheic Zones. In: Hauer FR, Lamberti GA (eds). Methods in Stream Ecology. Academic Press, San Diego: 107-119.

Des Carrieres D, 1883. Etiologies de l'epidimie typhoide quia eclate a Auxerre en septembre 1882. Bull. Mem. Soc. Hosp. Paris, Ser. 2,9:277-284.

Divine CE, McDonnell JJ, 2005. The future of applied tracers in hydrogeology. Hydrogeology Journal 13:255-258.

Drew BP, 1968. A review of the available methods for tracing underground waters. Proc. Br. Speleol. Assoc. 6:1-19.

Flurey M, Wai NN, 2003. Dyes as tracers for vadose zone hydrology [PDF 3.3MB]. Reviews of Geophysics 41,1:2-27.

Grimm NB and Fisher SG, 1984. Exchange between surface and interstitial water: implications for stream metabolism and nutrient cycling. Hydrobiologia 111: 219-228.

Halevy E, Moser H, Zellhofer O, Zuber A, 1967. Borehole dilution techniques: a critical review. Proceedings, 1966 Symposium of the International Atomic Energy Agency, 531-564.

Harvey JW, Wagner BJ, Bencala KE, 1996. Evaluating the reliability of the stream tracer approach to characterise stream-subsurface water exchange. Water Resources Research 32(8):2441-2451.

Jaynes DB, 1994. Evaluation of fluorobenzoate tracers in surface soils. Ground Water 32, 532-538.

Lamontagne S, Dighton J, Ullman W, 2002. Estimation of groundwater velocity in riparian zones using point dilution tests [PDF 220KB]. Technical Report 14/02. CSIRO Land and Water.

Malcolm RL, Aiken GR, Thurman EM, Avery PA, 1980. Hydrophilic organic solutes as tracers in groundwater recharge studies. In: Baker RA (ed) Contaminants and Sediments, Butterworth-Heinemann, Woburn, Mass. 71-88.

McCarthy KA, McFarland WD, Wilkinson JM and White LD, 1992. The dynamic relationship between ground water and the Columbia River: using deuterium and oxygen-18 as tracers. Journal of Hydrology 135, 1-12.

Mull DS, Liebermann TD, Smoot JL, Woosley LH, 1988. Application of dye-tracing techniques for determining solute transport characteristics of groundwater in karst terranes [PDF 882KB]. Report EPA904/688-001, US Environmental Protection Agency.

Otz MH, Otz HK, Otz I, Siegel DI. 2003. Surface water/groundwater interaction in the Piora Aquifer, Switzerland: Evidence from dye tracing tests. Hydrogeology Journal 11: 228-239.

Rose PE, Benoit WR, Adams MC, 1998. Tracer testing at Dixie Valley, Nevada using Pyrene Tetrasulfonate, Amino G and Fluoroscein. Transactions Geothermal Resource Council 22:583-587.

Saenger N, Lenk M, Hydraulic head and tracer experiments - two techniques to examine the hydraulic exchange through a riffle. http://wabau.kww.bauing.tu-darmstadt.de/~saenger/techniq.htm

S S Papadopulos and Associates, 2004. Evaluation of tracer tests conducted at the Northwest Wellfield, Miami-Dade County, Florida.