Tristan Ibrahim

Interaction between aquifers and streams in highly heterogeneous environments

Introduction
This project aims to assess the hydrological and hydrochemical heterogeneity of interaction between aquifers and streams in a context of highly heterogeneous aquifers and discuss the ecological implications of this process.

Summary
The Water Framework Directive (WFD) defines ground water status in part by considering qualitative and quantitative data allowing the evaluation of potential impacts of pressures carried by ground water to related aquatic ecosystems.

In the studied area (Don catchment, South Yorkshire, UK), several aquifers of secondary economical importance are highly related to surface water ecosystems, in terms of flow contribution and pollutants discharge. These aquifers are characterized by a high heterogeneity of sedimentary and structural features, which impose a high complexity of flow and transport behaviour. The WFD imposed a better understanding of their interaction with streams.

The hyporheic zone, a saturated zone between streams and aquifers where a mixing of waters occurs, is a key interface because of its pollutant attenuation ability and its ecology, strongly linked to stream ecology. Its heterogeneity, in terms of hydrology and hydrochemistry, has to be discussed with the aim of understanding how potential bio-controlling mechanisms and parameters such as groundwater pollutant discharge, stream water infiltration or riverbed sedimentology interact in this interface.

The assessment of the relevant scales of spatial and temporal heterogeneity of aquifers/streams interactions will allow us to discuss modelling issues aiming to propose predicting tools to catchment managers.

Principle Resources and Objectives
Four reaches of 500m length have been chosen in the upper river Don according to their variability of aquifers types (shallow alluvial aquifers/deep aquifers) and natural and anthropogenic river features. Deep and shallow boreholes in aquifers, and mini-piezometers in riverbeds will be sampled for water physico-chemistry to identify aquifers interacting with streams and quantify mixing within the hyporheic zone. Hydraulic tests will be use to discuss variability of riverbed hydraulic conductivity and riverbed temperature profiles will allow us to estimate groundwater discharge.

In collaboration with Anna Ritchie, hyporheic piezometers will be equipped with unscreened sampling tubes to give a first assessment of the impact of potential bio-controlling parameters on hyporheic ecology.

In a later stage, other techniques such as geophysical imaging, riverbed coring or tracer test, and natural tracer analysis will be developed to give a better understanding of the subject.