Heat tracing as a tool for locating and quantifying hydrological hot spots and hot moments that impact surface water and groundwater quality

Abstract:

Hot spots and hot moments of biogeochemical transformations in stream ecosystems are often driven by rapid water exchange across the streambed interface. Few field methods are available for quantifying variability of hydrologic exchange rates across the streambed interface through space and time at high resolution. Advances in heat tracing provide opportunities for improved assessment of the paired spatial and temporal structure of heterogeneity in water flux and chemistry in the hyporheic zone. Here, we present a synthesis of heat transport monitoring and modeling studies aimed at improving spatial and temporal characterization of water exchange across the bed interface. Hot spots of water and solute exchange at the bed interface are quantified in the field at the reach scale by integrating high-resolution streambed temperature maps with point measurements of water flux inferred from 1D temperature profiles. The effectiveness and potential errors of this methodology are explored through numerical groundwater flow and heat transport modeling. Hot moments of water and solute exchange are quantified in the field using high-resolution distributed temperature sensing, paired with 1D heat transport modeling and detailed water quality profiles. The effectiveness and potential errors of quantifying temporal variability in water flux using heat tracing are explored through controlled laboratory experiments. Our results demonstrate the enormous potential for using heat tracing to quantify spatial and temporal changes in water flux across the bed interface at high resolution. The methods presented take advantage of inexpensive temperature sensors and user-friendly modeling methods, such as VFLUX, making heat tracing a good option for field practitioners interested in observing spatial and temporal heterogeneity of water flux at the bed interface.