Stable isotopes applied as water tracers in column and field studies

The stable isotopes deuterium (2H, D) and oxygen 18 (18O) were applied in water for use as tracers in column experiments and in two field studies. Their performance was compared against uranine and was used to characterize saturated and unsaturated water movement and depths of plant water uptake. Deuterium and 18O are completely soluble and chemically and biologically stable. They are not subject to radioactive decay like tritium, nor photodegradation and sorption processes, like uranine. The column studies were conducted under saturated conditions; they explored variations of (i) flow rates (1.4-3.5 ml min−1), (ii) column lengths (0.5, 1.0, 1.5 m), and (iii) tracer concentrations (0.07, 0.14, 0.28 ml of a 99.8% D2O solution). A one dimensional dispersion model was used to generate parameters that allowed us to compare the tracers. The column experiments showed higher mean transport velocities and smaller dispersion coefficients for deuterium in comparison to uranine. The first field study, on a rain dominated floodplain, found unsaturated flow rates of 0.03-0.04 m day−1. The second field study examined snowmelt infiltration on a loess soil and found unsaturated flow velocities of 0.002-0.004 m day−1 over a six month period. Plant samples taken from the soil plots during late spring and summer reflect decreasing soil water deuterium concentrations and indicate depths of plant water uptake. Stable isotopes of water proved to be useful as a tracer in all studies and offer a suite of new possibilities in the field of biogeosciences because of the ability to directly label water molecules and to analyze small sample aliquots.