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Measuring Greenhouse Gases in Aquatic Environments Using Automated Systems

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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. Some human activities produce a lot of these gases. In Canada, electricity production 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.

Automated Systems

An automated system is a system utilized to measure several variables at regular intervals that stores this information in the computer’s memory without any human intervention. These systems can be installed on floating docks (Figure 1), along the bank of a watercourse (Figure 2) or in a hydroelectric powerhouse (Figure 3). The systems installed measure CO2, CH4 and of O2 concentrations in the water and air, as well as water and air temperature. This data is collected every 3 hours, 7 days out of 7.
 


 

Figure 1: Automated system installed on a floating dock.  The solar panels charge two batteries


Figure 2: Automated system installed along the bank of a watercourse.
The solar panels charge two batteries
 


 

Figure 3: Automated system installed
in the Eastmain-1 hydroelectric
powerhouse


Figure 4: Sketch of a hydroelectric powerhouse.
When an automated system is utilized in a powerhouse,
the water is collected at the level of the scroll case


A small computer called a datalogger (the system’s brain) controls the system by indicating when the machine must warmup, start measurement or stop functioning to save energy. In addition, it stores in its memory data generated by the various measurement devices of the automated system. Therefore, at the beginning of the cycle, the datalogger activates the CO2 probe to provide sufficient time for it to warmup before the start of the analyses. Following warmup, it activates two pumps, one that allows the water to circulate in the system and the other for the air. The water is drawn directly from the watercourse (lake or river) or in the scroll case when the automated system is installed in a hydroelectric powerhouse (Figure 4). Gases present in the water are balanced with the air and it is the latter that is analyzed by the various probes.

The automated system is made up of two distinct boxes (Figure 5): the first box called the “humid” box contains the water pump (known as peristaltic), some valves (to alternatively measure GHG concentrations in water or air) and the air exchanger cartridge, a cylinder containing capillaries used to extract the gases found in the water. The second box, called the “dry” box, contains the CO2, CH4 and O2 probes, as well as the system that records data which are sensitive to humidity.
 

(A) (B)


Figure 5: «Dry» box containing the air circuit probes and datalogger (A) and “humid” box (B)
containing the water circuit, peristaltic pump and water/air exchanger


Calculating the Fluxes

The automated systems are measuring GHG concentrations in water (see Information Sheet on Measuring GHG in Aquatic Environments with Floating Chambers). With the GHG concentrations measured in the water, we calculate theoretical fluxes with a mathematical model (Boundary Layer Model or Thin Boundary Layer Model) which includes the following variables:

  • gas concentrations measured in the water;
  • gas concentrations in the air (constant);
  • wind speed measured by a nearby weather station.

Further details on this measurement technique can be found in Bastien et al. (2008).


Figure 6: Concentrations of CO2 measured in
the water (in black) and in the air (in grey)
in the Eastmain-1 powerhouse


Figure 6 shows the type of information obtained by an automated system installed in the Eastmain-1 powerhouse. The increase in CO2 concentration that occurred between December and March was caused by the formation of ice during the winter months. The decrease in CO2 concentration observed during the month of May results from ice melt in spring. There are several advantages of using this method, notably, a follow-up in time of the GHG concentrations and a reduced cost compared to the utilization of the floating chamber technique (see the printable on Measuring GHG in Aquatic Environments) despite the maintenance and repair required every 4 to 6 weeks.

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

References:
Bastien, J., Fréchette, J-L. & R.H. Hesslein. 2008. Continuous Greenhouse Gas Monitoring System – Operating Manual. Report prepared by Environnement Illimité inc. for Manitoba Hydro and Hydro-Québec, 54 p. and appendices

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