H055-02
From soil to stream: reading subsurface structure from stream chemistry
Wednesday, 9 December 2020: 04:04
Virtual
Bryn Stewart1, Hang Wen2, James B Shanley3, David Norris4, Thomas Adler5, Adrian Adam Harpold6, James W Kirchner7, Julia N Perdrial5, Donna M Rizzo8, Gary Sterle6, Kristen Underwood8 and Li Li9, (1)Pennsylvania State University Main Campus, University Park, PA, United States, (2)Pennsylvania State University, State College, PA, United States, (3)USGS, Montpelier, VT, United States, (4)Centre for Ecology and Hydrology, Bangor, United Kingdom, (5)University of Vermont, Department of Geology, Burlington, VT, United States, (6)University of Nevada Reno, Department of Natural Resources and Environmental Science, Reno, NV, United States, (7)ETH Swiss Federal Institute of Technology Zurich, Department of Environmental Systems Science, Zurich, Switzerland, (8)University of Vermont, Department of Civil and Environmental Engineering, Burlington, VT, United States, (9)Pennsylvania State University Main Campus, Department of Civil and Environmental Engineering, University Park, PA, United States
Abstract:
Concentration‐discharge (C‐Q) relationships of solute export record the response of earth surface processes to changing hydrometeorological conditions. Contrasting C‐Q relationships have been pervasively observed, yet a mechanistic framework that can interpret diverse patterns remains elusive. This work tests the hypothesis that chemical contrasts between shallow and deep waters determine C‐Q patterns. We use data from two intensively measured sites, the Sleepers River Research Watershed in Vermont, US and the Plynlimon watershed in Wales, UK, to assess 1) how the vertical distribution of subsurface chemistry controls observed C-Q patterns in stream water, and 2) if we can use stream water chemistry to infer subsurface chemical gradients.
We test the hypothesis by comparing independently measured solute concentrations in soil water and groundwater with solute concentrations inferred from stream chemistry at high and low flow conditions for over 30 solutes across the two sites. The comparison indicates that the soil and groundwater concentration gradients determine solutes’ export behavior. For example, solutes prevalent in groundwater, including Ca, Mg, Na, and Si, exhibit dilution (concentrations decrease as discharge increases). Solutes that dominate in shallow soils, such as dissolved organic carbon (DOC), exhibit flushing (concentrations increase as discharge increases). Solutes with similar concentrations in the shallow soils and deep groundwater, such as Cl, show chemostatic behavior (concentrations do not change as discharge increases). At Sleepers River, the sulfate (SO4) concentration is higher in groundwater compared to soil water, and it exhibits dilution. At Plynlimon, however, the relatively similar SO4 concentrations in soil water and groundwater lead to a chemostatic pattern. Such contrasting patterns reveal the importance of solute distribution in conjunction with hydrological flow paths. Stream chemistry generally resembles groundwater chemistry under low flow conditions and soil water chemistry under high flow conditions. Thus, stream chemistry and C-Q patterns reflect the catchment’s subsurface composition and stratification, as water flows through different depths and carries the unique chemical signature of different zones to the stream.
