18O/16O ratio as a tool in studies of water movement in the soil

Full text: The measurement of the 18O/16O ratio is a tool in studies of water movement in the soil column and its way down to the groundwater. Water is a tracer of low cost, which is available in large amounts. Water of a slightly depleted or enriched 18O/16O ratio should be used. For the practical application of the method the fractionation of isotopes at the soil surface must be known. Under the climatic conditions of Juelich, no fractionation is observed between the mean δ-values of precipitation, soil water and groundwater. Especially at the soil surface no enrichment of 18O was observed, except under extremely dry conditions. The annual minimum of the 18O/16O ratio in spring can be used as a natural tracer of the water movement in the soil. The 18O/16O ratios of air water vapour (observed daily since 1981) and local precipitation (monthly means since 1974) are close to the equilibrium liquid/gas phase. These equilibrium conditions are responsible for the lack of 18O enrichment at the soil surface. A first experimental study was made with a grass-covered lysimeter which had been filled with 'parabraunerde' (loess type) and left in the field for two years. At the beginning of the observation period an amount of 7 L/m218O-enriched water (+60 per mille, artificially enriched by distillation) was applied to the surface. Two days later the same amount of water of normal local isotopic content (-7.6 per mille) was added. Cores were taken regularly. It was found that the label moved down the soil column very quickly. In the plant water the higher δ18O values were immediately apparent. On the next day the plant water rapidly went down to a lower δ-value, this being the result of the mean isotopic composition in soil water and in air water vapour. The distribution of 18O in the soil depends on the root activity. The transport of water in the roots lowers the amount of tracer and contributes to the mixing process within the soil column. Tracer studies must take into account the effect of vertical water transport by the roots, which is upwards. The water in the plant xylem has the mean isotopic composition of the root zone in the soil. (author)