Techniques

Environmental Tracers

Environmental tracers are present in the Earth's atmosphere and are used to estimate recharge rates and evaluate system response to land use/land cover change and paleoclimate.

Chloride

Chloride is the most widely used environmental tracer and originates in precipitation and dry fallout and is transported into the subsurface with infiltrating water. Chloride concentrations in unsaturated zone pore water are inversely related to recharge: high chloride concentrations indicate low recharge rates because chloride accumulates in the subsurface as a result of evapotranspiration whereas low chloride concentrations indicate high recharge rates because chloride is flushed through the subsurface. Water fluxes and ages of pore water can be estimated using the chloride mass balance (CMB) approach in which the mass of Cl into the system (precipitation and dry fallout) times the Cl concentration in precipitation and dry fallout is balanced by the mass out of the system (recharge) times the Cl concentration in recharge water in the unsaturated zone. The age of the pore water can be estimated by dividing the cumulative mass of Cl from the surface to the depth of interest by the Cl input to the system. Uncertainties in recharge estimates have been analyzed in detail. Results of the CMB approach indicates that there is no recharge in interstream settings in the Chihuahuan Desert or in interplaya settings in the High Plains. Recharge is focused beneath streams and playas in these settings. Spatial variability in chloride data can be used to evaluate relationship between soil water flux and geomorphic setting in semiarid regions.

References

Scanlon, B. R., 1991, Evaluation of moisture flux from chloride data in desert soils: Journal of Hydrology, v. 128, p. 137-156. [PDF]

Scanlon, B. R., 2000, Uncertainties in estimating water fluxes and residence times using environmental tracers in an arid unsaturated zone: Water Resources Research, v. 36, no. 2, pp. 395-409. [PDF]

Chlorine-36 and Tritium

The subsurface distribution of tracers resulting from atmospheric nuclear bomb testing can be used to determine the rate of water movement during the past 50 yr. Examples of bomb pulse tracers include tritium and chlorine-36. Tritium concentrations increased from 10 to = 2,000 TU during atmospheric nuclear testing that began in 1952 and peaked in 1963 to 1964. Tritium has a half life of 12.34 yr. Bomb pulse chlorine-36 resulted from neutron activation of chlorine-35 in seawater during nuclear testing in the Pacific in the mid 1950s. Chlorine-36 has a half life of 301,000 yr. In many semiarid regions, these tracers are restricted to the root zone, which indicates very little water movement during the past 50 yr. Such information is important for siting waste disposal systems.

References

Scanlon, B. R, 1992, Evaluation of liquid and vapor flow in desert soils based on chlorine-36 and tritium tracers and nonisothermal flow simulations: Water Resources Research, v. 28, no. 1, pp. 285-297. [PDF]

Scanlon, B. R., Kubik, P. W., Sharma, P., Richter, B. C., and Gove, H. E., 1990, Bomb Chlorine-36 analysis in the characterization of unsaturated flow at a proposed radioactive waste disposal facility, Chihuahuan Desert, Texas: Nucl. Inst. Meth. Phys. Res. B52:489-492.

Oxygen (18O) and Hydrogen (2H or Deuterium)

Stable isotopes of oxygen (18O) and hydrogen (2H or deuterium) have been used to evaluate unsaturated flow. Because of coupling between fractionation of oxygen and hydrogen, a plot of stable isotopes of water samples from precipitation, rivers, and lakes throughout the world follows a straight line called the global meteoric water line (Craig, 1961). Local meteoric water lines have been established from precipitation at severa locationsl. Although the isotopic composition of individual rain events is highly variable, the annual mean values are fairly constant (Gat, 1981). The isotopic composition of pore water in the unsaturated zone is generally either depleted or enriched relative to the mean isotopic composition of rainwater. Enriched values of the stable isotopes indicate evaporation from surface water prior to infiltration or from pore water in the unsaturated zone. Slopes of 2H versus 18O are ~ 5 for surface water evaporation but can decrease to 2 for evaporation of pore water in the unsaturated zone. Stable isotope profiles from Eagle Flat basin in West Texas parallel the local meteoric water line beneath areas of ponded water (playa and fissure) indicating no evaporation. In contrast, stable isotope profiles in interdrainage settings show enrichment of 18O relative to 2H with a slope of ~ 3. This low slope is consistent with evaporation of pore water in the unsaturated zone, similar to slopes determined by Allison (1982) for evaporation from dry soils. These profiles were used to estimate evaporation rates of up to 0.5 mm/yr.

Reference
Scanlon, B. R., Goldsmith, R. S., and Langford, R. P., 2000, Relationship between arid geomorphic settings and unsaturated zone flow: case study, Chihuahuan Desert, Texas: University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 261, 133 p. [to order]

March 2006