Streambed temperature mapping, hydraulic testing using minipiezometers, and geochemical analyses of interstitial water of the streambed were used to delineate the pattern of ground water discharge in a sandy streambed and to develop a flux-based conceptual model for ground water/surface water interactions. A new and simple empirical method was used to relate fluxes obtained from minipiezometer data to streambed temperatures. The relationship allowed flux to be calculated at locations where only streambed temperature measurements were made. Slug testing and potentiomanometer measurements at 34 piezometers indicated ground water discharge ranged from 0.03 to 446 L/m2/day (and possibly as high as 7060 L/m2/day) along a 60 m long by 11 to 14 m wide reach of river. Complex but similar plan-view patterns of flux were calculated for both summer and winter using hundreds of streambed temperatures measured on a 1 by 2 m grid. The reach was dominated by ground water discharge and 5% to 7% of the area accounted for approximately 20% to 24% of the total discharge. < 12% of the total area consisted of recharge zones or no-discharge zones. A conceptual model for ground water/surface water interactions consisting of five different behaviors was developed based on the magnitude and direction of flux across the surface of the streambed. The behaviors include short-circuit discharge (e.g., high-flow springs), high discharge (e.g., preferential flowpaths), low to moderate discharge, no discharge (e.g., horizontal hyporheic or ground water flow), and recharge. Geological variations at depth played a key role in determining which type of flow behavior occurred in the streambed.

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Abstract:Thermal stream loadings from both natural and anthropogenic sources have significant relevance with respect to ecosystem health and water resources management, particularly in the context of future climate change. In recent years, there has been an increase in field-based research directed towards characterizing thermal energy transport exchange processes that occur at the surface water/groundwater interface of streams. In spite of this effort, relatively little work has been performed to simulate these exchanges and elucidate their roles in mediating surface water temperatures and to simultaneously take into account all the pertinent hydrological, meteorological and surface/variably-saturated subsurface processes. To address this issue, HydroGeoSphere, a fully-integrated surface/subsurface flow and transport model, was enhanced to include fully integrated thermal energy transport. HydroGeoSphere can simulate water flow, evapotranspiration, and advective-dispersive heat and solute transport over the 2D land surface and water flow and heat and solute transport in the 3D subsurface under variably saturated conditions. In this work, the new thermal capabilities of HydroGeoSphere are tested and verified by comparing HydroGeoSphere simulation results to those from a previous subsurface thermal groundwater injection study and also by simulating an example of atmospheric thermal energy exchange. High-resolution 3D numerical simulations of a well-characterized reach of the Pine River in Ontario, Canada are also presented to demonstrate thermal energy transport in an atmosphere-groundwater-surface water system. The HydroGeoSphere simulation successfully matched the spatial variations in the thermal patterns observed in the riverbed, the surface water and the groundwater. The computational framework can be used to provide quantitative guidance towards establishing the conditions needed to maintain a healthy ecosystem.

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