Transport behavior displayed by water isotopes and potential implications for assessment of catchment properties

Abstract

Measurements of water isotopes are used routinely to estimate water transit time distributions and aquifer storage thickness in catchments. Water isotopes (e.g., D2O/H218O) are generally considered to behave identically to water molecules (H2O); they are thus often considered fully representative of water movement and preferred over inert chemical tracers for catchment assessment purposes. However, laboratory-scale measurements presented here show that water isotopes exhibit transport behavior that is essentially identical to that of inert chemical tracers. The very act of tagging water molecules, implicit in the measurement of any water isotope, yields measurements representative of movement as a chemical tracer. The resulting measurements are then interpreted quantitatively, focusing on a comparative assessment of apparent mean water and mean tracer velocities, and the applicability of Fickian and non-Fickian (anomalous) transport models. For both water isotopes and inert chemical tracers, the measured mean tracer velocity is not necessarily equal to the apparent mean water velocity. It is thus critical to recognize this inequality when estimating catchment properties. For example, accounting for anomalous transport of water isotopes can significantly reduce overall estimates of aquifer storage thickness over an entire watershed.