High-resolution quantification of groundwater flux using a heat tracer: laboratory sandbox tests

Abstract

Groundwater flux is the most critical factor controlling contaminant transport in aquifers. High-resolution information about groundwater flux and its variability is essential to properly assessing and remediating contamination sites. Recently, we developed a new thermal method that has shown considerable promise for obtaining such information in an efficient fashion. This new approach is based on the previously proven method of using a heat tracer to track groundwater movement and the development of fiber optic distributed temperature sensing (FO-DTS) technology for high-resolution temperature measurement (cable wrapping). Results of an initial field application indicated that heat-induced temperature profiles provided new insights into subsurface flow variations. However, the relation between the thermal profiles and groundwater flux is only qualitative; a quantitative analysis is highly desirable in order to obtain a more definitive relationship between the heating-induced temperature increase and groundwater flux. In this work, we constructed a sandbox to simulate a sand aquifer and performed a series of heat tracer tests under different flow rates. By analyzing the temperature responses among different tests, we developed a quantitative temperature-flux relationship, which can be used for the new thermal approach to directly predict groundwater flux under field conditions. A new method implementing a borehole liner is introduced to separate the two main heat transport mechanisms of advection and thermal conduction by preventing flow from entering the well during heat tests. This method has shown the ability to diagnose if thermal conduction is homogenous or heterogeneous within the tested domain.