Experimental analysis of heat as a tracer for the quantification of thermal dispersion and water flow in sand

Heat has been used as a tracer to quantify water flow through sediments. Two open questions were determined from a literature review on heat transport modelling. Firstly, there is disagreement regarding the significance of thermal dispersivity for Darcy related velocities. Secondly, the reliability of using temperature time series for the quantification of vertical flow velocities through sediment has never been tested experimentally. A hydraulics experiment was designed and constructed to investigate these questions. The experiment enables the induction of steady-state flow velocities through any type of sediment. This facilitates the investigation of tracer transport under controlled hydraulic and temperature conditions. A PC based electronic system for the generation of heat and multi-channel measurement of temperature, electric conductivity and pressure was built. The experiment was filled with coarse (2 mm) well sorted sand and fully saturated with water. The capability of the hydraulic set-up was tested with colour dye. The sensor network enabled monitoring of heat and solute tracer movement in separate runs, but under identical conditions. Thermal dispersion as a function of typical groundwater flow velocities (q) was quantified (Re<2.5). The results clarify the disagreement in the earth sciences, and a model for Darcy flows is proposed based on the experimental data. Velocity quantification with thermal time series and 1D analytical models was tested for a flow range of 0<q<25 m/d. Velocities independently derived from and heat and solute transport agree for q