Attenuation capacity of hyporheic and riverbed sediments and its relationships to fluvial and geological environments

Introduction

This project is investigating the geochemical characteristics of riverbed sediments that affect natural attenuation of pollutants during migration between aquifers and rivers. The project will establish the importance of geological and geomorphologic variability on pollutant attenuation capacity.

Objectives and hypotheses

The hyporheic zone is the interface between groundwater and surface waters, which often exhibits chemical and hydraulic gradients, and in which there is exchange and mixing of stream and groundwaters. Relative to adjacent aquifers it is often considered to be organic carbon-rich and microbially active and has been observed to provide significant attenuation capacity under certain conditions.

This project will investigate the geochemistry of hyporheic sediments in a range of fluvial and hydrogeologic settings to investigate the relationships between geological terrain, fluvial morphology and river stage. Focussing on clay mineralogy, cation exchange capacity, organic carbon content and Fe/Mn oxides, the study will focus on the intrinsic attenuation capacity of hyporheic sediments to a range of pollutants, in order to establish the attenuation capacity of the hyporheic zone in selected UK river systems.

Previous work has investigated the geochemical properties of a wide range of UK aquifers that control their intrinsic attenuation capacity, and this study with extend that work into the hyporheic zone. Investigations will establish the variability and control on hyporheic attenuation capacity as a function of reach-scale variation in geomorphology (riffle, bar, pool etc) and hydrology (influent/effluent), kilometre-scale variation in bedrock lithology/mineralogy and catchment-scale effects of river stage.

The hypothesis is that the attenuation capacity of hyporheic sediments is greater than adjacent aquifer sediments and that it varies with morphological and geological conditions within a catchment, and that the hyporheic attenuation capacity can be predicted by reference to fluvial morphology. We will test this by geochemical and hydrochemical analyses of a wide range of hyporheic sediments and porewaters and by laboratory attenuation studies.

Approach

The research is focused on two rivers with similar bedrock geology. The River Tern (Shropshire) is a relative low-energy lowland river that crosses sandstones of the Permo-Triassic Sherwood Sandstone Group. The River Leith (a tributary of the River Eden) passes over similar geology, but is an upland system and its higher flow velocities result in a coarser grained bedforms. Sediment cores through the fluvial sequence will be taken at numerous points within the catchment (circa 50 in each) at a range of scales to identify the magnitude of hyporheic attenuation capacity (and its relations to fluvial form and geological conditions), and to determine local, reach and catchment scale variability in the intrinsic attenuation capacity of the sediments. A smaller number of aquifer and drift samples will be collected in each catchment to allow assessment of the relative attenuation capacity of aquifer, drift and riverbed sediments in each catchment. The geochemical data will be combined with results from other studies in the Environment Agency´s hyporheic zone research programme to develop conceptual models of HZ flow and attenuation in sandstone catchments.