Application of environmental isotope tracers to modeling in hydrology

Abstract

Advances in nuclear science and computer development have made practicable chemical-transport models employing the environmental isotopes found in natural waters. The heavy isotopes of hydrogen, deuterium (stable) and tritium (radioactive), forming part of the water molecule, are the most conservative tracers of water known. The heavy stable oxygen isotope, 18O, circulates in tandem with deuterium in the hydrologic cycle. Tritium fallout varies seasonally and annually in a systematic fashion, and due to its property of radioactive decay is highly useful for dating time of recharge of soil moisture and groundwater. 14C is valuable for tracing old groundwater owing to its relatively long half-life of 5568 yr., but detailed knowledge of the hydrochemistry of the system is required for successful modeling. Deuterium and 18O are of principal value because they reflect the temperature at which precipitation condensed. Thus, they show pronounced seasonal variability and also the effects of altitude, latitude, distance from the sea, amount of precipitation, history of evaporation, and long-period climatic cycles. Principal areas of application of environmental isotopes to hydrologic modeling have been in soil-moisture transport, modeling of mixing and evapotranspiration in large lakes and swamps, glacial-ice movement, groundwater flow, and transport of contaminants in groundwater.