Quantifying braided river loss to groundwater using Active Distributed Temperature Sensing

<p>Globally, braided river systems are a major recharge mechanism for alluvial aquifer systems providing a significant contribution to groundwater, yet this process of surface water &#8211; groundwater (SW - GW) interaction is a gap in hydrological research. River leakage from braided rivers is the main source of groundwater recharge in the Canterbury Plains of New Zealand (Coluccio & Morgan, 2019). This study investigated surface water &#8211; groundwater interaction in the Waikirikiri Selwyn River, in the South Island of New Zealand, using Active-Distributed Temperature Sensing (A-DTS) and estimated groundwater recharge to the alluvial aquifer system, as outlined in Banks et al. (2022).</p> <p>The field study site is within an ephemeral losing reach of the river and contains two active channels. Braided rivers are dynamic, high-energy environments; therefore, the fibre-optic cables were installed beneath the ground to protect this infrastructure from regular flood events. Novel horizontal Directional Drilling was used to construct two, 100 m long drillholes at a depth of approximately 5 m below ground level and perpendicular to the river channel. The drillholes were completed with a hybrid fibre optic cable containing four multi-mode fibres and copper conductors. Additionally, a vertical A-DTS installation was constructed to 30 m depth adjacent to the river channel and horizontal drillhole.&#160;</p> <p>A series of twelve back-to-back A-DTS surveys on the horizontal and vertical A-DTS installations were conducted over 48-hrs. River stage and flow during the survey period was constant, hence steady-state groundwater recharge conditions were assumed. The localised temperature variations along the cables indicated spatial variation of preferential groundwater recharge pathways. Groundwater velocities were derived using both analytical and numerical solutions and preliminary results indicate vertical groundwater velocities exceeding 10 m/d. By calculating groundwater velocities it is possible to quantify groundwater recharge from braided rivers with high spatial and temporal resolution, which can aid in understanding the recharge process and the relationship between river stage height and groundwater recharge rates.</p> <p>&#160;</p> <p>References</p> <p>Banks, E. W., et al. (2022). "Active distributed temperature sensing to assess surface water&#8211;groundwater interaction and river loss in braided river systems." Journal of Hydrology <strong>615</strong>: 128667.</p> <p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;</p> <p>Coluccio, K. and L. K. Morgan (2019). "A review of methods for measuring groundwater-surface water exchange in braided rivers." Hydrology and Earth System Sciences <strong>23</strong>: 4397-4417.</p>