Limitation of using heat as a groundwater tracer to define aquifer properties: experiment in a large tank model

A large tank (4 × 8 × 1.4 m) filled with loamy sediments and equipped with 26 standard piezometers and 20 multilevel temperature μloggers is used to carry out a thermal monitoring test and investigate the limits of heat as tracer to define aquifer parameters. A constant temperature test was conducted by originating a heat plume using a groundwater heat exchanger and a constant head was used to create a steady state flux. Temperature was monitored continuously during 40 days by data μloggers. To estimate aquifer parameters, a heat transport model was constructed using MT3DMS. Although the boundary conditions are well known, there is still a bias between computed and measured temperatures. Results show that in fine-grained sediments, which are thermal diffusion dominated, it is difficult to precisely distinguish and quantify diffusivity and dispersivity components without also considering solute tracer tests. In this type of alluvial sediments, using heat as groundwater tracer to define aquifer properties can be problematic, since heat transport is relatively insensitive to the longitudinal dispersivity, a relevant parameter for solute transport modeling.