Quantifying Spatial Patterns of Hyporheic Exchange in a Floodplain Meander System via High-Resolution Hydrogeologic, Geochemical and Geophysical Observations

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The spatial and temporal patterns of hyporheic exchange impact riverine ecosystems through regulation of stream temperature and water chemistry. In gravel-bedded meandering streams, these patterns are tied to the hydraulic architecture of the floodplain aquifer, which is a product of the geomorphic history of the channel. In this study, we investigate the connection between floodplain stratigraphy and hyporheic exchange in an intensively instrumented reach of the East Fork Jemez River located in the Valles Caldera National Preserve. We employ a suite of hydrogeologic, geochemical, and geophysical methods to document the stratigraphy of the floodplain meander system and quantify groundwater fluxes to the stream channel. We map the floodplain aquifer and characterize subsurface hydraulic properties of the floodplain using a dense network of piezometers across a 340 m long, 2 m wide reach that includes four meanders. Grain size analysis, slug tests, and low-field borehole nuclear magnetic resonance (NMR) measurements were conducted in a subset of augerholes and reveal distinct gravel- and clay-rich units. Slug test hydraulic conductivities varied from 1x10-8 to 1x10-4 m/s, while NMR-derived estimates of hydraulic conductivity ranged from 1x10-11 to 1x10-8 m/s. Slug test and NMR-derived hydraulic conductivity estimates showed general correspondence, with the largest deviation occurring in gravel-rich layers. NMR logging, which provides information about the saturated pore size, revealed variability in the extent to which gravel layers included fine particles. Our observations suggest large variability in the extent to which gravel-rich layers are accompanied by fine particles. Electrical resistivity surveys and borehole NMR across a single meander provided evidence for continuous clay (conductive) layers across meander bends. However, geochemical analyses of groundwater in the network of wells indicate large variability in chemical composition (major cation and anion) and dissolved oxygen, over small spatial scales. Our multi-method mapping of a floodplain meander system demonstrates the utility of high resolution spatial characterization to constraining hyporheic fluxes.

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Presented at the American Geophysical Union Annual Meeting on December 11, 2018 in Washington DC