Borehole time domain reflectometry in layered sandstone: Impact of measurement technique on vadose zone process identification

The hydraulic behaviour of the vadose zone of a layered sandstone aquifer has been investigated using borehole-based Time Domain Reflectometry (TDR). Both a commercially available portable packer TDR system (TRIME-B3L Borehole Packer Probe) and specially designed borehole-emplaced TDR probes were used to monitor seasonal fluctuations in water content in the vadose zone of a layered sandstone over 1 year under natural rainfall loading. The data show that the vadose zone contains seasonal perched water tables that form when downward percolating water reaches layers of fine grained sandstone and siltstone and causes local saturation. The formation of perched water tables is likely to lead to lateral flow bypassing the less permeable, finer layers. This contrasts with behaviour inferred from previous studies of the same aquifer that used borehole radar and resistivity, which suggested its vadose zone behaviour was characterized by uniform downwards migration of wetting fronts. To investigate the impact of measurement technique on observed response, the TDR data reported here were used to produce simulated zero offset profile (ZOP) borehole radar responses. This simulation confirmed the limited ability of ZOP borehole radar to detect key vadose zone processes, because the phenomenon of critical refraction minimizes the sensitivity of the results to high water content layers. The ability of the resistivity method to detect perched water table responses is also limited, because of the relatively large sampling volume of the technique. The study illustrates that inappropriate technique selection results in hydrological process mis-identification, with serious consequences for the usefulness of data in hydrological modeling.