Estimating horizontal velocity in shallow aquifers from temperature perturbations in observation boreholes responding to pumping

Abstract

The practice of measuring groundwater temperature is growing rapidly and has already demonstrated to be a valuable source of information for estimating soil hydraulic parameters, groundwater–surface water exchanges, and for identifying preferential flow in both fractured and sedimentary aquifers. In the zone of seasonal thermal dynamics of unconfined sedimentary aquifers, temperature measurements in observation wells are found to exhibit a particular perturbation when the water column responds to pumping wells located in the proximity. This paper presents a description of the mechanism underlying this thermal perturbation, validating it by means of three-dimensional numerical modeling, and explores its potential as a source of information. In particular, it presents a framework enabling to estimate in-well water horizontal velocities from the time required for these temperature perturbations to reach a peak. The proposed technique is based on simple data post-processing and does not require additional experimental endeavors beyond the deployment of temperature and pressure sensors. Furthermore, through a global sensitivity analysis based on Monte Carlo numerical simulations, we show that in-well heat propagation begins to show sensitivity to the thermal properties of the well screen, the filter pack and the natural porous medium at thermal Peclet numbers lower than approximately 3.7.