ESR10: Aquifer-surface water interfaces as hotspots for interconnected carbon and nitrogen cycling and greenhouse gas production


Research Fellow: Paul Romeijn, University of Birmingham (UK)


ESR10 will quantify the enhanced greenhouse gas production resulting from increased biogeochemical processing rates at interface hotspots such as aquifer-surface water interfaces.

Tasks and methodology

ESR 10 will combine laboratory mesocosm experiments [at CSIC, ESR8] with field experiments ranging from plot to stream reach scales in order to analyse the progressive consumption of oxygen along interface exchange flow paths. High-resolution oxygen micro-electrodes will be deployed alongside diffusion gas samplers in sediments for quantifying flow path and residence time specific chemical turnover of nitrate and organic carbon and associated generation of respiration and denitrification by-products (in collaboration with ESR8) and compared to quantitative measures of functional microbial groups (together with ESR 6). To identify complex patterns of groundwater-surface water exchange and residence time distributions, FO-DTS will be applied with support from ER1. The impact of interfaces as hotspots of biogeochemical turnover will be analysed by Raz/Rru smart tracers for monitoring microbial reactivity (with ESR5+8).


  • Stefan Krause, David Hannah, Mike Rivett (University of Birmingham)
  • Co-Supervisor: Eugenia Marti (CSIC)

Video Introduction

Watch this short video of ESR10 Paul Romeijn introducing himself and talking about his Interfaces project:



Aquifer-surface water interface

In streams or lakes, the aquifer-surface water interface is where there is contact between the lowest part of the water body (as an imaginary plane) and groundwater upwelling from an aquifer (groundwater reservoir). At this interface there is mixing between the two types of waters. The two types of water typically show different characteristics, such as oxygen content and temperature.

Conventional tracers

Tracers are used to gain insight into water discharge of streams. It works simply by injecting it in the stream in the upstream area. In the downstream area you can then measure the concentration in the water. An important property of a tracer should be that it does not change its chemical properties. In this way you know how much of the tracer has arrived and you can estimate water discharge. Examples of tracer monitoring are by measuring the electrical conductivity of the water (when using chloride from salt as a tracer), or, when using fluorescent tracers, by measuring fluorescence with a spectrofluorometer (see glossary).

‘Smart’ tracers

Recently, a new type of tracer has been proposed for use in streams. When compared to conventional tracers, a so-called smart tracer does change its chemical properties, but only in such a way that those changes can also be measured. For example, the fluorescent compound Resazurin (Raz) undergoes a transformation to differently fluorescent Resurofin (Rru) when it comes in contact with oxygen-respiring microbes while flowing through a stream. When measuring the amount of both Raz and Rru downstream using a spectrofluorometer, the amount of tracer that has been degraded by the microbial community’s metabolism is a measure for the activity. And the combined amount still gives the same information as a conventional tracer would, hence making it a little “smarter”.


Chemical compounds sometimes have fluorescent properties. This means that when light of a certain wavelength (or: colour), is absorbed by that chemical compound, the light excites molecules in the material, absorbs a part of the energy of the light, and then emits light of a lower energy and longer wavelength. When using fluorescent tracers in water, a spectrofluorometer can be used to measure how much of the tracer is present in the water, compared to standard solutions with known concentrations of the tracer compound.

Hyporheic zone

The hyporheic zone is where water flows through the soil, underneath and alongside the riverbed and gravel bars. It literally means below (hypo) flow (rheos) in Greek. Because water flows much slower compared to the free flowing water, it can be seen as a temporary water and nutrient storage compartment within the river system. This zone is also the zone where mixing between groundwater and surface water takes place, which influences oxygen and nutrient levels of the water. It is considered a zone that plays an important role in river health, water quality and breakdown of waste water effluents.  

Sorption to mineral surfaces

Substances that are transported by water across mineral material, such as sand and clay, can be held to the surface of that mineral material. This is caused by natural forces of attraction and usually limited to the formation of only a thin film. When using tracers to understand river flow and the metabolic activity of microorganisms in, for example, the riverbed and hyporheic zone, not all of the injected tracer material is recovered. This is an effect caused by sorption processes.

Biogeochemical cycles of Carbon and Nitrogen

Life in general relies on many building blocks. Carbon and Nitrogen (C and N) are one of the most important of those nutrients. In the nature, they are often transformed through a typical cycle. For example for   C, Carbondioxide (CO2) is taken by plants from the atmosphere, the dead biomass is broken down or transformed to humus, peat or fossil fuels and eventually respired or burnt and sent back into the atmosphere, and the cycle starts over.   

Metabolically Active Transient Storage (MATS)

While water flows through the river system and exchanges with the hyporheic zone, the water resides in that zone for a period of time, anywhere between hours and days. This is called transient storage. The concept of MATS comprises the processes that occur while the water is temporarily stored. If stored longer, more nutrients can be metabolised by the microbial communities.

Greenhouse gas production in river systems

In several zones of the river system there is active cycling of nutrients. The riverbed, thin biofilms of microorganisms on rocks and the hyporheic zone are considered important. The metabolism of nutrients produces several types of greenhouse gases. Measuring these gases, combined with oxygen levels and oxygen consumption in water, gives insight into how active the microbial communities are.

Measuring hotspots of biogeochemical turnover using ‘smart’ tracers

By using ‘smart’ tracers we can measure microbial activity in transient storage in hyporheic zone or biofilms in a stream, among others. Combined with measuring a combination of greenhouse gases, oxygen consumption and water temperature, we can potentially identify where biogeochemical turnover may be concentrated, i.e. hotspots. We are interested in these hotspots, because they can help understand the responsible processes governing stream health or water quality.