Citation:

Briggs, M., S. Buckley, A. Bagtzoglou, D Werkema, AND J. Lane. Actively Heated High-Resolution Fiber-Optic Distributed Temperature Sensing to Quantify Flow Dynamics in Zones of Strong Groundwater Upwelling. WATER RESOURCES RESEARCH. American Geophysical Union, Washington, DC, 52(7):5179-5194, (2016).

Impact/Purpose:

Areas of groundwater (GW) upwelling in streams impact surface water chemistry and ecology by providing nutrients and moderating temperature extremes [Hayashi and Rosenberry, 2002; Hancock et al., 2005]. The transport of nutrients via GW discharge into streams influences the diversity and abundance of macrophytes [Eglin et al., 1997; Snyder et al., 2013], and supports algal communities [Brunke and Gonser, 1997]. In summer, when GW is colder than surface water (SW), areas of upwelling can moderate SW temperatures through strong advective and conductive heat exchange, and provide thermal refugia zones for cold-water species [Mathews and Berg, 1997; Ebersole et al., 2003; Briggs et al., 2013; Caissie et al., 2014; Kurylyk et al., 2014b]. Colder water will maintain a greater oxygen concentration and lower animal metabolism; therefore, areas of GW upwelling in summer may enhance the survivability of thermally stressed stream species [Nielsen et al., 1994; Galbraith et al., 2012].

Description:

Zones of strong groundwater upwelling to streams enhance thermal stability and moderate thermal extremes, which is particularly important to aquatic ecosystems in a warming climate. Passive thermal tracer methods used to quantify vertical upwelling rates rely on downward conduction of surface temperature signals. However, moderate to high groundwater flux rates (> -1.5 md-1) restrict propagation of diurnal temperature signals, and therefore the applicability of several passive thermal methods. Active streambed heating from within high-resolution fiber-optic temperature sensors (A-HRTS) has the potential to define multidimensional fluid flux patterns below the extinction depth of surface thermal signals, allowing better quantification and separation of local and regional groundwater discharge. This method is demonstrated at a stream with strong upward vertical flux in Mashpee, Massachusetts, USA. Nine A-HRTS were emplaced vertically into the streambed in a grid with ~ 0.40 m lateral spacing. Long-term (8-9 hr) heating events were performed to confirm the dominance of vertical flow to the 0.6 m depth, well below the extinction of ambient diurnal signals. To quantify vertical flux, short-term heating events (28 min) were performed at each A-HRTS, and heat pulse decay over vertical profiles was numerically modeled in radial 2-D using SUTRA; modeled flux values are similar to those obtained with seepage meters, Darcy methods, and analytical modeling of shallow diurnal signals. Additionally, repeatable differential heating patterns along the vertical sensors may indicate sediment layering and hyporheic exchange superimposed on regional groundwater discharge.

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