Using Emerging Methods to Investigate Stream and Groundwater Interaction at Multiple Spatial Scales.

摘要

Fundamentally, streams represent physical conduits of water across gradients, yet a more holistic definition reveals stream corridors support a mosaic of living communities in a blend of surface and ground waters. The physical and biogeochemical patterns these dynamic systems support affect natural habitat and water quality, directly impacting the human experience. Our understanding of stream and groundwater interactions is at a time of rapid expansion due to an increase in environmental awareness, accountability, and emerging techniques which can be used to decipher underlying controls and develop predictive relationships. Water temperature has been used as a qualitative environmental tracer during the forging of this country from Lewis and Clarks pioneering spatial explorations to Thoreau's revolutionary scientific investigations; yet only very recent modeling and technological advancements have allowed us to apply these principles in a more distributed quantitative fashion. The resulting description of physical flow dynamics can be combined with innovative biogeochemical assessments to determine the fundamental linkages between inert and living processes along the stream corridor.;The magnitude and spatial distribution of groundwater inflows to streams is a known control on stream water quality. These inflows can be recognized and evaluated through a variety of methods, each with its own sensitivity and basic requirements. One such method is using the temperature differential between surface and groundwaters to both locate and quantify groundwater inputs. The emerging method of fiber-optic distributed temperature sensing (DTS) uses the temperature dependent backscatter of light along fiber-optic cables to determine temperature at high spatial and temporal resolution, essential creating continuous thermometers that may be applied to aquatic systems over a broad range of spatial scales. My initial investigations involved a quantitative comparison of heat tracing with DTS to existing methods of evaluating groundwater inflows (dye dilution gauging, differential gauging, and geochemical end-member mixing) along Nine Mile Creek in Syracuse, New York, USA. I found that DTS heat tracing generated comparable quantitative estimates of groundwater discharge to the stream, and provided the finest spatial characterization of these inflows of all methods tested.;The "hyporheic zone" describes where stream water temporarily enters the sub-surface, which is known to be biogeochemically reactive, before potentially mixing with shallow groundwaters and returning to the stream. This flux across the streambed interface has driven much recent research, but the intrinsic spatial and temporal variability have proven a challenge to define. I modified DTS optical fibers to improve spatial resolution from 1.0 to 0.014 meter so the propagation of diurnal temperature patterns into the steambed could be recorded and applied to one-dimensional conduction-advection-dispersion models to determine the vertical component of hyporheic flux. I installed these custom high-resolution fiber-optic temperature sensors within the streambed above two beaver dams in Lander, Wyoming, USA for five weeks as stream discharge dropped by 45%. The resulting rich datasets revealed flux was organized by streambed morphology with strong, deep flux at glides and near-dam bars, and weak, shallow flux at pools and bars set farther upstream. Additionally, these morphologic units showed contrasting temporal trends in flux penetration and magnitude.;One benefit of such refined descriptions of the physical hyporheic system is that they can be directly compared to ambient biogeochemical data collected in coincident vertical profiles to evaluate the physical controls on streambed chemistry and nutrient cycling. I collected pore water at multiple depths, once a week, and analyzed these samples for several conservative and redox-sensitive solutes. The results revealed strong correlation between vertical flux magnitude and the degree to which hyporheic water was "oxic-stream-like" or "anoxic-reduced". Residence time along hyporheic flowpaths was found to be a dominant control on redox condition, a relationship that held for both spatial and temporal flux patterns. This data set was augmented by an injection of the new biologically sensitive resazurin environmental tracer which showed that hyporheic flowpaths had much greater rates of aerobic reactivity compared to the net streamflow, but this signal was indistinguishable at the reach scale.;The cumulative result of the past three years of stream research using emerging ideas and methods is an improved understanding of these intricate and fascinating biomes. I hope this knowledge will serve to improve the management of, and appreciation for, the veins of our shared landscape.