Limits of heat as a tracer to quantify transient lateral river‐aquifer exchanges

Abstract The application of heat as a tracer for assessing river‐aquifer exchanges has been mainly limited to vertical flow through the riverbed. Lateral river‐aquifer exchanges become more important than vertical riverbed exchanges if the river is deeply incised into an aquifer. Few studies have examined lateral river‐aquifer exchanges and the ability of heat to constrain such exchanges. This study aims to perform a robust assessment of the limits of heat as a tracer to quantify lateral river‐aquifer exchanges. The study is based on a section of the Meuse River in Belgium, a river predominantly gaining in the studied area and becoming intermittently losing in the winter time. A calibrated transect model shows that river temperature can affect groundwater temperature up to 9 m into the aquifer. An accompanying synthetic modeling investigation using Monte Carlo simulation shows that heat data for distances between 4 and 9 m from the river can reduce the uncertainty of river‐aquifer exchanges for conditions similar to those of the transect model. The ability of heat to reduce the river‐aquifer exchange uncertainty improves with distance from the river because of the reduction in the number of acceptable model realizations. The optimal distance is 8 m from the river where the groundwater temperature is no longer affected by the river temperature. The synthetic modeling also indicates that heat alone cannot constrain river‐aquifer exchanges better than the commonly used hydraulic head. However, when combined with hydraulic head, heat can significantly reduce the uncertainty of river‐aquifer lateral exchanges under gaining conditions.