Revealing the temporal dynamics of subsurface temperature in a wetland using time-lapse geophysics

Abstract

Summary There is growing recognition of the need to understand the subsurface hydrological mechanisms within wetlands, given their potentially significant role in the hydrological and biogeochemical function of catchments, as well as their effects on the wetland habitat itself. Conventional subsurface hydrological sampling and monitoring techniques are often limited in such environments because of the invasive nature of such approaches and the sensitivity of the environment. In this study we use ground penetrating radar (GPR) and electrical resistivity tomography (ERT) to characterise the stratigraphy at a small riparian wetland site. Then, through time-lapse ERT measurements over a 12 month period, we demonstrate how changes in resistivity may provide additional value about localised recharge. We compare direct measurements of water table depth and pore fluid electrical conductivity and temperature, with subsurface resistivity derived from the ERT measurements and reveal that temporal fluctuations in temperature of the subsurface dominates the change in resistivity over the monitored period. From this we use the ERT images of resistivity to estimate spatio-temporal changes in subsurface temperature and thus infer localised zones of groundwater recharge due to suppressed seasonal variation in temperature. Although we focus here on one 2-D vertical profile through the wetland, the results highlight the potential value of non-invasive time-lapse geoelectrical surveys for mapping 3-D thermal patterns within a wetland environment.