Greenhouse gas emissions (CO2, CH4, and N2O) from several perialpine and alpine hydropower reservoirs by diffusion and loss in turbines
Summary (3 min read)
Introduction
- In the early 1990s artificial lakes and reservoirs were discovered as potential greenhouse gas emitters (Rudd et al. 1993; Kelly et al. 1994).
- The question was put forward whether hydroelectric reservoirs, especially in the tropics, could still be considered cleaner energy sources compared to fossil alternatives (Fearnside 1997, 2002; Delmas et al.
- In total, Swiss reservoirs cover an area of nearly 120 km2 (approximately 0.01 % of the area Electronic supplementary material.
Present Address:
- The main emission pathways for greenhouse gases from reservoir surfaces are the diffusive flux across the air–water interface and bubble flux resulting from supersaturation in the sediment.
- Changes in isotopic signature caused by methane emission are small (Knox et al. 1992), while turbulent diffusion has no effect.
- Furthermore, the authors examined the importance of river inflows for the methane content of reservoirs at different altitudes and the contribution of methane loss to total methane emissions.
Study sites
- Between September 2003 and August 2006, 11 Swiss reservoirs from different regions and elevations were sampled for greenhouse gases (Table 1; Fig. 1 for reservoir properties and locations, Table 3 for sampling dates).
- A drop of reservoir water of several hundred meters through pipes and tunnels before it reaches the turbines is the result.
- Two of the reservoirs investigated (Lakes Oberaar, alpine and Sihl, lowland) are pump-storage reservoirs, which receive water from a reservoir or lake located at lower altitude (Lake Grimsel for Lake Oberaar and Lake Zurich for Lake Sihl).
- Sampling time was restricted to late spring until autumn, as access to the high altitude reservoirs was limited due to weather conditions and water content was low after ice-melt.
Methods
- Sampling A SBE 19 CTD probe (Sea Bird Electronics) equipped with an oxygen and pH sensor was used to collect hydrographic data (conductivity, temperature, depth, light transmission, pH and dissolved oxygen).
- Winkler samples were used to correct the offset in the oxygen sensor.
- Samples for dissolved gas analysis were flushed with 2–3 times the bottle volume before the samples were preserved with NaOH (pH [ 12) or Cu(I)Cl, then closed with a butyl septa while carefully avoiding air bubbles in the bottles.
- Inflows, outflows Methane concentrations were measured in the in- and outflowing water of six reservoirs.
- If possible the CTD probe was used, but if depth of the river was not sufficient, temperature and conductivity were measured with a WTW LF 330 conductivity meter, pH with a Metrohm 704 pH-meter and oxygen with a WTW Multi 340i multi probe.
CO2
- Dissolved CO2 (DIC) was calculated using the measured alkalinity, temperature, pH, and the dissociation constants of H2CO3 and HCO3 - (Plummer and Busenberg 1982).
- Samples for alkalinity were taken at the surface and at the bottom of the water column.
CH4 and N2O
- Concentrations of dissolved methane and nitrous oxide were measured by the headspace technique similar to McAuliffe (1971).
- The oven temperature was kept constant at 70 C and the detector temperature was 340 C.
- The carbon isotopic signature of methane was determined similar to the method described by Sansone et al. (1997).
- The model estimates the air–water flux F [mg m-2 day-1] using the water saturation concentration Ceq [M], the measured water concentration Cw [M] of the greenhousegas, the transfer velocity k [cm h-1] and a unit conversion factor f.
Results
- CO2 concentrations and emissions Surface concentrations of CO2 were supersaturated in all five reservoirs for which data are available (Table 2) with concentrations ranging from 40–280 lmol L-1.
- In nearly all lakes, alkalinity measured above at the bottom of the lake was nearly 0.5 units higher than at the lake surface, except for Lake Luzzone and Lake Wohlen , where values were similar (data not shown).
- Figure 2a, b show a typical profile for an alpine reservoir (Lake Grimsel) and for a lowland reservoir (Lake Lungern).
- Methane concentrations, d13C isotopic composition and emissions.
Concentrations and isotopic composition
- In the 11 reservoirs sampled, three characteristic types of methane profiles were identified.
- In the following, one example for each profile type will be illustrated.
- In Lake Santa Maria , methane concentrations on all three sampling dates (June, July, and August) increased towards the bottom (Fig. 3b).
- These profiles showed a local maximum of methane concentrations in intermediate water layers.
- Concentrations increase again towards the sediment and reach the highest concentrations above the sediment at 100 nmol L-1 in August.
Emissions
- Concentrations in Lake Bianco were at saturation (*3 nmol L-1), therefore the methane emissions were negligible (Table 2; Fig. 4).
- Right Temperature (black line), light transmission (yellow line), conductivity (green line) and dissolved oxygen concentration (red) profiles of Lake Bianco.
- B Left Methane concentrations (open symbols) and isotopic composition (full symbols) in Lake Santa Maria on 7 June , 6 July and 23 August 2005 .
- Concentrations tend do decrease later in the year, but this is not a common trend for all reservoirs.
Discussion
- One reason the CH4 emissions the authors measured are low compared to diffusive fluxes from other reservoirs in general could be that they have been measured at deep sites of the reservoirs where emissions are lower compared to shallow, littoral areas (Duchemin et al.
- This increase causes a shift away from DIC and H2CO3 towards CO3 2- causing lower concentration differences between water and the atmosphere and thus smaller fluxes.
- When looking at the methane profiles of reservoirs (Fig. 3; supplementary material 1–3), there is an obvious difference between alpine reservoirs which have dissolved methane concentrations below 60 nmol L-1 and subalpine/lowland reservoirs which have maximum concentrations above 100 nmol L-1 and up to 6,500 nmol L-1.
- A third reason is that ebullition, a potential pathway for methane emission, is not included in their calculations.
- Lower concentrations of DIC (only Lake Luzzone, subalpine) and CH4 in reservoirs of higher elevations (Table 2; Fig. 3 and supplementary material 1) reflect the less favourable conditions for internal productivity and respiration (lower temperatures, shorter ice-free periods, less nutrients) compared to lower elevations.
Methane sources
- Generally, the carbon cycle in oxic lakes and reservoirs assumes methane production in the sediments followed by methane oxidation during the diffusion into the water column (e.g. Kuivila et al. 1988).
- An exception is Lake Oberaar , which is a pumpstorage reservoir and receives substantial amounts of water from Lake Grimsel , and thus is more likely controlled by the methane inflow from Lake Grimsel than by the inflow of glacial melt water.
- This implies that methane loss from water passing the turbine could be equally important as methane loss via the reservoir surface in alpine and subalpine reservoirs, while being of less importance for lowland reservoirs.
Conclusions
- The most important greenhouse gas emitted from the perialpine and alpine reservoirs the authors sampled in Switzerland is CO2.
- Temperature and organic matter input are presumably the most important factors for the decrease the authors found, while reservoir morphology of the predominantly steep and deep subalpine/alpine reservoirs could be an important factor as well.
- The amount of external methane entering via inflows is sufficient to explain the emission rates found in some reservoirs in spring and early summer, while contributions from other sources (e.g. sediments) increase towards autumn for two lowland reservoirs.
- The authors would like to thank MeteoSchweiz for supplying wind speed data.
- Additionally the authors would like to thank Markus Fette, Michael Schurter, Michael Meyer, Ilia Ostrovsky, David Finger and Lorenz Jaun for their assistance during sampling.
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References
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"Greenhouse gas emissions (CO2, CH4,..." refers background in this paper
...Increasing pH is a common occurrence in lakes during stratification in summer caused, among others, when photosynthetic activity is larger than respiratory activity (Maberly 1996)....
[...]
264 citations
"Greenhouse gas emissions (CO2, CH4,..." refers background in this paper
...Emissions from these two pathways contribute methane amounts similar to reservoir surface loss (Guérin et al. 2006; Kemenes et al. 2007) and are thus highly relevant for greenhouse gas (especially methane) emissions from reservoirs....
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...…emissions from the reservoir surface, other emission pathways that can significantly contribute to total gas emissions have recently drawn attention, i.e. gas release immediately below the turbine and emissions further downstream (Abril et al. 2006; Roehm and Tremblay 2006; Kemenes et al. 2007)....
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...The average loss for the three other lakes (Lake Sihl, Lake Luzzone and Lake Grimsel) was 46 ± 18 % (range 16–73 %), which matches the findings of Kemenes et al. (2007)....
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245 citations
"Greenhouse gas emissions (CO2, CH4,..." refers background or result in this paper
...As methane loss by ebullition (bubbles rising from the sediment) is definitely a factor in lowland reservoirs (DelSontro et al. 2010) the importance of loss at the turbines will even decrease....
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...Diffusive fluxes in Lake Wohlen were one order of magnitude higher than in the other lowland reservoirs at an average of 1.8 ± 0.9 mg CH4 m -2 day-1 for all sampling campaigns confirming results by DelSontro et al. (2010)....
[...]
...So far there is limited information about emissions from reservoirs in the temperate climate zone (e.g. Soumis et al. 2004; DelSontro et al. 2010), which account for approximately 40 % of all reservoirs (Barros et al. 2011), and to our knowledge none from alpine reservoirs....
[...]
...…production and thus to a higher rate of ebullition for lower lying reservoirs, the total rate of methane emission (diffusive ? ebullition) could be significantly higher than for reservoirs at higher elevations (as for example the very high ebullition rates of Lake Wohlen in DelSontro et al. 2010)....
[...]
...This would lead to higher total methane emissions via bubble flux from the sediment (DelSontro et al. 2010 for Lake Wohlen) and in the end make lowland reservoirs significantly more important emitters of methane to the atmosphere....
[...]
245 citations
"Greenhouse gas emissions (CO2, CH4,..." refers background in this paper
...The question was put forward whether hydroelectric reservoirs, especially in the tropics, could still be considered cleaner energy sources compared to fossil alternatives (Fearnside 1997, 2002; Delmas et al. 2001; Pacca and Horvath 2002)....
[...]
244 citations
"Greenhouse gas emissions (CO2, CH4,..." refers background in this paper
...The lack of differences between reservoirs at different altitudes (and thus different temperatures) is somewhat astonishing as methane production was shown to be temperature dependent (e.g. Zeikus and Winfrey 1976; Nguyen et al. 2010) as did CO2 emissions from lakes (Kosten et al....
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...The lack of differences between reservoirs at different altitudes (and thus different temperatures) is somewhat astonishing as methane production was shown to be temperature dependent (e.g. Zeikus and Winfrey 1976; Nguyen et al. 2010) as did CO2 emissions from lakes (Kosten et al. 2010)....
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...The strong temperature dependence of methane production (e.g. Zeikus and Winfrey 1976; Kelly and Chynoweth 1981; Nguyen et al. 2010) suggests a decrease of methane emissions with decreasing temperatures at higher elevations....
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Frequently Asked Questions (2)
Q2. What have the authors stated for future works in "Greenhouse gas emissions (co2, ch4, and n2o) from several perialpine and alpine hydropower reservoirs by diffusion and loss in turbines" ?
Further studies are needed to support this and determine up to which altitude bubble flux plays a role in reservoirs of the Alps. As a result the reservoir stores methane from rivers, which otherwise would probably emit on the way down the mountain, and exposes it to potential methane oxidation inside the reservoir. And finally two anonymous reviewers for their helpful comments and suggestions.