A conservative tracer of water pathways has the physical, chemical or biological characteristics (Section 8.1) that allow it to move at the same velocity as the water, but not allow it to bind (e.g. sorb) on to any surrounding media (i.e. organics, soil, regolith, rock or river bed). Tracers include sodium chloride, bromide, fluorescent dyes (e.g. Rhodamine WT) and the hydrogen- and oxygen-isotopes of water. Tagging subsurface flow with the isotopes of water has the advantage that they are the water itself, but the disadvantage that the analysis using a mass spectrometer (Section 8.4) is expensive and fractionation can occur during sampling (Kendall and McDonnell, 1998; McGuire and McDonnell, 2007). Common salt is often used as a tracer for dilution gauging of small rivers and streams (Section 7.5) and has the advantage that it can be traced in situ (or non-invasively) using electrical resistivity methods (Section 6.1.7; Osiensky and Donaldson, 1995; Vanderborght etal., 2005) or by using electrical conductivity probes within piezometers (Section 8.5). Additionally, tracers can be sampled from within piezometers or extracted using suction lysimeters (also called vacuum samplers; Section 8.3.2: Nielsen and Nielsen, 2006). At natural exposures of the soil-bedrock interface, tracers may be collected from the same location as the rate of water exfiltration is measured (Tromp-van Meerveld et al., 2007).
Tracers can be artificially added to the subsurface system using a surface irrigation system, line-source injection (drip or spray), via a trench or via piezometers and observation wells.
An example injection and sampling array for tracer tests on a hillslope section is shown in Fig. 6.9. Within this study Chappell and Sherlock (2005) applied a NaCl tracer as a line-source spray at the start of individual rainstorms. Tracer was then
extracted from the subsurface using vacuum samplers at four depths (Section 8.3.2) and the electrical conductivity measured using an electrical conductivity probe. The NaCl concentration was then determined using a site-specific calibration. Gold-coated resistance cells (Coleman, 1946) were used to monitor the electrical conductivity of the subsurface water in situ. Tensiometers (Section 6.2.3) were also installed within this array and the data combined with unsaturated hydraulic conductivity data (Section 5.2) to calculate Darcian velocities (Chapter 14).
Subsurface flow tracing can be used to determine a mean pore-water velocity (vpore) and thence the Darcy velocity (^Dafcy) as derived by the Darcy-Richards equation, qDarcy vporen where n is the porosity (Section 5.4). Derivation of the mean pore-water velocity from a tracer plume does, however, require the measurement of the local longitudinal dispersion coefficient and inversion of the advection-dispersion equation (Section 5.5; Chapter 14).
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