Measuring Greenhouse Gases in Aquatic Environments with floating chambers

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Figure 1: Measure of the GHG fluxes
with a buoyant chamber

Background

UNESCO has designated greenhouse gases (GHG) as being one of the causes of global warming. The four major gases involved are carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) and water vapour. Certain human activities produce a lot of these gases. Electricity production in Canada represents between 30 to 45% of the nation’s emissions. It is therefore important to adequately quantify the impacts of this industry on the GHG emissions. In the case of hydroelectricity, GHG emissions must be determined prior to and after the creation of a reservoir. This is the task Hydro-Québec has undertaken with the Eastmain-1 Project: several teams have been monitoring GHG emissions in this region since 2003, that is, before the creation of the reservoir in 2005. The study also includes GHG emissions in forests, peatlands and aquatic environments. This Information Sheet explains the techniques used to determine GHG emissions of the lakes, rivers and reservoirs.

Why GHG Fluxes at the Surface of Lakes and Reservoirs?

Figure 2: Linear regression between
the CO₂ concentration and time

In aquatic systems, the concentration of CO₂, CH₄ and N₂O depends on certain biological and chemical phenomena. For example, to feed themselves, bacteria degrade organic material (plant or animal) present in water or sediments. This leads to an increased concentration of these gases in the water. At the same time, the CO₂ is utilized by photosynthesizers present in water (algae, marine plants) for their growth, which reduces the concentration of these gases in the water (see Information Sheet concerning lakes). Several other chemical reactions also impact on the concentrations of these gases in the water. In the end, if the concentration of a gas in water is higher than its concentration in air, then this gas will be released from the water, since the concentrations tend to balance out naturally. Inversely, if the concentration of a gas in water is lower than its concentration in air, then this gas will tend to be absorbed in the water. This is what is respectively called the upward or downward flux of a gas or emission versus a gas being absorbed by the aquatic environment. It is possible to measure these fluxes thanks to two distinct techniques. The first one, the floating chamber technique, is a method said to be “direct” because it measures fluxes at the water/air interface and the second method said to be “indirect” in which the flux is calculated based on the gas concentrations in the water.

Direct Measurement of Fluxes

Figure 3: Chromatography in gaseous phase
used to measure the concentration
of greenhouse gases in water

A floating chamber for gas capture is deposited on the surface of the lake, river or reservoir for approximately 10 minutes. The chamber is coupled in closed circuit to two gas analyzer systems (Ciras-sc from PPSystems and Gasmet) measuring concentrations of trapped CO₂, CH₄ and N₂O gases (Figure 1). Afterwards, the slope of the linear regression is used (Figure 2) to calculate a GHG flux representing the quantity of GHG emitted or absorbed per unit area and per unit time. In order to obtain an average representative value, three fluxes are measured at each station.

Indirect Measurement of Fluxes

With this method, gas concentrations in the water are measured to calculate upward or downward GHG fluxes in the body of water. Two methods are utilized to measure gas concentrations. The first method consists in drawing water samples which are brought back to the laboratory. At the laboratory, the gas concentrations in the samples are analyzed in a gas chromatograph (Figure 3). The second method consists in measuring the concentration of CO₂ directly on the field. Water is pumped and sent to an air exchanger (cylinder containing capillaries that draw the gas contained in the water) coupled to a CO₂ analyzer (EGM-4; Figure 4). Afterwards, using a mathematical model (Boundary Layer or Thin Boundary Layer Model), a theoretical gas flux is calculated based on the gas concentrations measured and wind speed. Wind speed is determined either with a conventional weather station (Figure 5) or portable weather station (Figure 6).

Figure 4 : Measure of the partial pressure of CO₂

	*Figure 5 :Conventional weather station used to determine wind speed*

	Figure 6 : Portable weather station used to determine wind speed. The weather station (held by technician Pierre-David) is used on the floats of a hydroplane. It is also frequently used onboard the boats

Maud Demarty
maud.demarty@envill.com
and
Julie Bastien
julie.bastien@envill.com

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