Temporal and spatial variations of shallow subsurface temperature as a record of lateral variations in groundwater flow

In the present paper it is shown how profiles consisting of closely spaced (≤10 m) temperature measurements at shallow depth, obtained at several instances during one season, provide a detailed record of lateral variations in vertical groundwater flow. This is illustrated by a field study around the Peel Boundary Fault zone that cuts through the unconsolidated, siliciclastic deposits that occur in the southeastern part of Netherlands. This regionally important fault forms at many locations a strong barrier to horizontal groundwater flow and therefore induces complex groundwater flow patterns. Temperature anomalies (over 2°C) are observed over short distances. These anomalies reverse over the season. Numerical modeling of coupled groundwater flow and heat transport demonstrates how the temporal and spatial variations of subsurface temperature are the result of the interaction between seasonal fluctuations in surface temperature and spatial variations in groundwater flow. In addition to the horizontal profiles, temperature-depth profiles obtained in groundwater observation wells were used to constrain the larger-scale characteristics of the groundwater flow system. In order to simulate the observed geothermal patterns it appeared to be essential to account for the long-term changes in surface temperature. Although groundwater temperature data are commonly used to constrain groundwater flow fields on regional scale or to calculate vertical groundwater velocities at point locations beneath small streams, the present study is one of the first to integrate these different scales and incorporate the impact of recent climate change.