Implications of groundwater-surface water connectivity for nitrogen transformations in the hyporheic zone
Abstract
We present the preliminary results of a major multidisciplinary research project examining nutrient attenuation in the hyporheic zone (HZ). The data are derived from a densely instrumented river reach in the Eden catchment, Cumbria, UK. Our setup allows vertical, lateral and longitudinal flow paths and flux through the HZ to be measured from a network of over 100 piezometers. We combine this with analysis of the spatial variation in the biogeochemistry of the HZ through porewater sampling at 10, 20, 30, 50 and 100cm depth below the riverbed to give the distribution of redox-sensitive species. These data are augmented further using isotope (in situ dentrification), fine resolution profiling using DET passive sensing and geophysical measures. One of the goals of our project, reported here, is to test whether the variation in redox-sensitive chemistries (NO3, CH4, Fe and Mn) is correlated with the variation in riverbed permeability and degree of hydrological connectivity between groundwater and surface water. Initial results suggest that the saturated hydraulic conductivity, measured using slug tests at depths of 20, 50 and 100 cm below the riverbed, did not vary significantly across the river reach. However, vertical hydraulic gradients (VHG%) ranged from 1.7 to 19% between the subsurface and surface water, suggesting significant spatial variability in upwelling. The river appears to be characterised by two distinct hydrologic zones: an upstream zone with high upwelling potential and a downstream zone of low potential. Riffles in the upstream zone of the river showed higher VHG% than either the riffles or pools of the downstream zone. These data suggest that not only adjacent riffle-pool sequences but also larger reach scale geomorphic features need to be considered to understand the flow dynamics. In mapping these results onto the biogeochemistry, there appears to be substantial variation in the distribution of redox-sensitive chemical species, with zones of lower NO3 concentrations having correspondingly higher concentrations of dissolved CH4, Fe and Mn. Groundwater-surface water mixing alone may be insufficient to explain the nutrient distribution patterns observed. At the river reach scale, the HZ may comprise spatially distributed zones of varying redox with the potential for reactive N removal and/or production. Unraveling the drivers and consequences of this spatial patterning will be explored.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.H24C..05H
- Keywords:
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- 0470 BIOGEOSCIENCES / Nutrients and nutrient cycling;
- 1813 HYDROLOGY / Eco-hydrology;
- 1829 HYDROLOGY / Groundwater hydrology;
- 1830 HYDROLOGY / Groundwater/surface water interaction