Water isotopes exhibit anomalous transport: implications for assessment of catchment properties

To estimate water transit time distributions and aquifer storage thickness in catchments, measurements of water isotopes are used routinely. Water isotopes (e.g., D₂O/H₂¹⁸O) are generally considered to behave identically to water molecules (H₂O); they are thus 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 anomalous transport behavior that is essentially identical to that of inert chemical tracers. For both water isotopes and inert chemical tracers, subject to anomalous transport, the measured mean tracer velocity – of both water isotopes and inert chemical tracers – is not equal to the mean water velocity (Darcy velocity). The often-manifested inequality between apparent mean water velocity and estimated mean tracer velocity must therefore be recognized when estimating catchment properties. For example, accounting for anomalous transport of water isotopes can significantly reduce estimates of aquifer storage thickness over an entire watershed.