Linking local riverbed flow patterns and pore-water chemistry to hydrogeologic and geomorphic features across scales

Ibrahim, Tristan and Thornton, Steve and Surridge, Ben and Wainwright, John (2009) Linking local riverbed flow patterns and pore-water chemistry to hydrogeologic and geomorphic features across scales. In: AGU Research Abstracts :. American Geophysical Union.

Abstract

The groundwater-surface water interface (GSI) is a critical environmental hotspot, a key area influencing the fate of carbon, nutrients and contaminants of surface and subsurface origin, and a zone of ecological importance. Policy seeking to mitigate issues relating to dissolved contaminants and to improve stream health, increasingly recognizes its significance, particularly in the context of integrated management of streams and aquifers. Techniques assessing riverbed flow and solute patterns are often limited to the local scale. When related to the multi-scale pattern of hydrogeologic and geomorphic features controlling stream, hyporheic and groundwater fluxes, they can improve larger scale predictions of flow and solute behaviour at the GSI. This study develops a conceptual model of riverbed flow and solute patterns, and tests it in a 4th order stream in the UK. It assesses the interaction between large scale subsurface flowpaths, driven by the distribution of bedrock outcrops, and the expansion and closure of alluvial deposits, and small-scale hyporheic flowpaths, driven by riffle-pool sequences. It uses two networks of riverbed mini-piezometers and multi-level samplers: network 1, across fifteen sites in a 7.2 km length of river in unconstrained (open alluvial valley), asymmetric (bedrock outcropping on one bank) and constrained (bedrock on both banks) contexts; and network 2, across six riffle-pool sequences in a 350-m reach, at the transition between asymmetric/unconstrained and constrained contexts. Subsurface flowpaths and stream-water infiltration were deduced by relating vertical exchange fluxes to stream and pore-water patterns of conservative natural tracers. Biogeochemical processes were highlighted using reactive natural tracers. At network 2, measurements of surface water profiles and riverbed coring were also undertaken, and dissolved metal concentrations in the first 15 cm of sediments assessed using gel probes. Network 1 was sampled twice. Monthly head measurements and seasonal pore-water sampling were undertaken at network 2. Results from network 1 showed the combined influence of valley type and riffle-pool sequences on riverbed flow and solute patterns. In asymmetric and unconstrained contexts, stream water infiltrated deeply in the riverbed in riffles. In pools, a vertical stratification of hydrochemically distinct flowpaths, often disconnected hydrologically from the stream, was observed. Biogeochemical reactions were more developed in pools. In constrained context, riverbed pore-water was dominated by upwelling subsurface flowpaths displaying a low vertical variability of hydrochemistry. Biogeochemical reactions were not apparent, and stream water infiltration was limited to shallow depth in riffle context. The replicability of these patterns with regards to local conditions (groundwater discharge, surface water profile and riverbed permeability) will be discussed at network 2. Finally, implications of the conceptual model for the near-bed ecosystem will be illustrated by assessing the bioavailability of metals in the shallow riverbed. This work will help to better design monitoring strategies aiming to consider the control of the GSI on solute fate across scales.