Heat as a hydraulic tracer for horizontal subsurface flow constructed wetlands

Hydraulic tracer studies for horizontal subsurface flow constructed wetlands require addition of a conservative chemical tracer to track the flow path of wastewater inside subsurface media. However since one would wish to use fully conservative tracers disposal problems of the wetland effluent are created. In this study the use of heat as a hydraulic tracer was explored. Heat is considered an environmentally friendly alternative to chemical tracers as the post-study wetland and effluent temperature equilibrates to ambient conditions. Nevertheless the non-conservative behaviour of heat creates a distorted response curve at the outlet from which the hydraulic performance indices cannot be easily computed. In this paper we were able to develop a mapping methodology accepting a heat tracer response curve as an input and is converted to a conservative chemical tracer response curve by establishing a mathematical relationship between heat and conservative solute hydrodynamic dispersion. The methodology was tested by conducting a dual heat-chemical tracer study on a laboratory-scale unplanted subsurface flow constructed wetland and the predicted chemical tracer response was compared with the actual experimental chemical tracer response data. The predicted response curve adequately matched the experimental response curve supported by the fact that there was a 5% and 6% relative difference in Peclet number and mean of the RTD respectively. The outcome of this study is that it is possible to use the proposed mapping methodology in conjunction with a heat tracer to quantify hydraulic behaviour of subsurface flow constructed wetlands without having to use a conservative chemical tracer.