ESTIMATING GROUNDWATER GAIN AND LOSS USING SYNOPTIC ENVIRONMENTAL TRACER SAMPLING (Invited Presentation)

In this talk, we explore the use of synoptic surveys with environmental tracers and stream discharge to quantify stream loss, gain and net exchange. Estimating integrated stream loss to the subsurface over long reaches is challenging. Current measurement methods are largely point-based, are mostly limited to hydraulic gradient and/or temperature profile time series and are difficult to upscale. Synoptic sampling for environmental tracers and discharge is a powerful method for detecting and quantifying groundwater discharge (gain) to streams over 1-100 km length scales, and has been used to identify, quantify and characterize groundwater discharge to streams across a variety of settings globally. Stream water loss in isolation does not affect the concentration of tracers in the stream, making tracer-based methods more sensitive to groundwater discharge. Thus, tracer-based methods have traditionally only been used to quantify groundwater gain and not loss. However, stream loss is a fundamental term in the stream water mass balance, which is coupled to the stream transport equation. In addition, stream discharge measurements provide constraint on net exchange over the reach, which can be coupled to the tracer-based discharge estimates to provide information on loss. Here we model stream transport with the coupled transport and stream mass balance equation, which includes loss, to explore the effect of stream loss on tracer concentration. We discuss the theory, quantify the sensitivity of tracer signals to stream loss and identify conditions where stream loss can be estimated by tracers. Finally, we apply this method to field data, to estimate integrated loss, gain and net exchange. Improved methods to quantify integrated loss and net exchange over larger spatial scales would improve both stream and groundwater budgets, as well as knowledge of the interaction between stream and alluvial groundwater systems.