D6 Hydrological Tracers and Isotopes

Topic

Hydrological tracers and isotopes make it possible to determine the origin of water observed in a river or aquifer, to separate the various components that make up a flood hydrograph, and to measure how long that water has resided underground before re-emerging.

Stable water isotopes—specifically the proportions of heavy oxygen and heavy hydrogen (deuterium) present in the water—vary systematically based on the temperature at which the water condensed and the path taken by the air mass that transported it; it is precisely this systematic variation that results in rainwater, snowmelt, and groundwater each possessing a unique isotopic signature.

When two water sources with distinct isotopic signatures mix, the resulting isotopic composition falls somewhere between the two, following a two- or three-source mixing equation as applicable; this intermediate position allows for the estimation of the fraction of mixed water derived from each original source, thereby enabling the separation of the aforementioned hydrograph components.

Meanwhile, the mean residence time of groundwater is estimated using radioactive tracers: tritium—a radioactive form of hydrogen with a half-life of approximately twelve years and four months—allows for the dating of water recharged between 1950 and the present, given that any water recharged prior to that date has lost almost all its original tritium through radioactive decay.

Artificial tracers—such as chlorides, dyes, or sulfur hexafluoride—enable the direct measurement of both flow velocity and dispersivity within an aquifer; this is achieved by injecting the tracer at one point and subsequently measuring the time it takes to appear—and at what concentration—at a downstream location. The cosmogenic isotope beryllium-10, which slowly accumulates on the soil surface, also reflects the soil's erosion rate: the higher the erosion, the lower the concentration of this isotope that manages to accumulate in the exposed soil, because the erosion process itself—described elsewhere in this series—washes the soil away before the isotope has time to build up. Thus, from the origin of the water discussed at the beginning of this document to the erosion rate addressed at the end—and encompassing the isotopic signature of each water source, mixing equations, and tritium dating—hydrological tracers and isotopes allow processes occurring entirely underground to leave behind a chemical signature that can be measured at the surface.

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