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Monitoring and verification

6.6.1 Background

Monitoring (Figure 6.22) would be done for at least two different purposes: (1) to gain specific information relating to a particular CO2 storage operation and (2) to gain general scientific understanding. A monitoring program should attempt to quantify the mass and distribution of CO2 from each point source and could record related biological and geochemical parameters. These same issues may relate to monitoring of potential leakages from subsea geologic storage, or for verification that such leakage does not occur. Monitoring protocols for submarine sewage disposal for example are already well established, and experience may be drawn from that.

6.6.2 Monitoring amounts and distributions of materials released

6.6.2.1 Monitoring the near field

It appears that there is no serious impediment to verifying plant compliance with likely performance standards for flow through a pipe. Once CO2 is discharged from the pipe then the specific monitoring protocols will depend upon whether the plume is buoyant or sinking. Fixed location injections present fewer verification difficulties than moving ship options.

For ocean injection from large point sources on land, verifying compliance involves above ground inspection of facilities for verification of flow and the CO2 purity being consistent with environmental regulations (e.g., trace metal concentrations, etc.). For a power plant, flue gases could be monitored for flow rate and CO2 partial pressure, thus allowing a full power plant carbon audit.

There are a variety of strategies for monitoring release of CO2 into the ocean from fixed locations. Brewer et al. (2005) observed a plume of CO2-rich sea water emanating from a small-scale experimental release at 4 km depth with an array of pH and

Figure 6.22 Schematic of possible approaches for monitoring the injection of CO2 into the deep ocean via a pipeline. The grey region represents a plume of high CO2/low pH water extending from the end of the pipeline. Two sets of chemical, biological and current sensors and two underwater cameras are shown at the end of the pipeline. An array of moored sensors to monitor the direction and magnitude of the resulting plume can be seen around the pipe and are also located along the pipeline to monitor for possible leaks. A shore-based facility provides power to the sensors and for obtaining real-time data and an autonomous underwater vehicle maps the near-field distribution of the plume. A towed undulating pumping system monitors at distances of more than a few kilometres from the injection site. The towed system could provide much greater measurement accuracy and precision, but would also be able to provide measurements over large areas in a relatively short period of time. Moored systems are used to monitor the plume between mapping cruises. These moorings have surface buoys and make daily transmissions back to the monitoring facility via satellite. The very far-field distributions are monitored with hydrographic section cruises conducted every 2-5 years using standard discrete sampling approaches. These approaches provide the accuracy and precision required to detect the small CO2 signals that add to background variations.

conductivity sensors. Measurements of ocean pH and current profiles at sufficiently high temporal resolution could be used to evaluate the rate of CO2 release, local CO2 accumulation and net transport away from the site (Sundfjord et al., 2001). Undersea video cameras can monitor the point of release to observe CO2 flow. The very large sound velocity contrast between liquid CO2 (about 300 m s1) and sea water (about 1,500 m s1) offers the potential for very efficient monitoring of the liquid CO2 phase using acoustic techniques (e.g., sonar).

The placement of CO2 directly in a lake on the sea floor can be verified, and the quantity and loss rate determined by a combination of acoustic, pH, and velocity measurements, and by direct inspection with underwater vehicles. Undersea vehicles, tethered or autonomous, could play a prominent role in monitoring and verification. Autonomous vehicles have been developed that can be programmed to efficiently follow a variety of complex trajectories over large areas (Simonetti, 1998), but accurate pH sensing in a rapidly changing pressure and temperature field has yet to be demonstrated. Deep-sea pH monitoring from tethered vehicles has been shown to be very precise (Brewer et al., 2004), and these vehicles can routinely collect precisely located samples for later analysis.

6.6.2.2 Monitoring the far field

It will be possible to monitor the far field distributions of injected CO2 using a combination of shipboard measurements and modelling approaches. The ability to identify pH plumes in the ocean has been well demonstrated (Figure 6.23). Current analytical techniques for measuring total CO2 in the ocean are accurate to about ±0.05% (Johnson et al., 1998). Thus, measurable changes could be seen with the addition of approximately 90 tonnes of CO2 per km3. In other words,

Figure 6.23 Measurements showing the ability to measure chemical effects of a natural CO2 plume. Profiles for pH were taken in June 1999 near the Axial Volcano at 46°N 130°W, in the ocean near Portland, Oregon, United States.

1 GtCO2 could be detected even if it were dispersed over 107 6.7 km3 (i.e., 5000 km x 2000 km x 1 km), if the dissolved inorganic carbon concentrations in the region were mapped out with high-density surveys before the injection began.

Variability in the upper ocean mixed layer would make it difficult to directly monitor small changes in CO2 in waters shallower than the annual maximum mixed-layer depth. Seasonal mixing from the surface can extend as deep as 800 m in some places, but is less than 200 m in most regions of the ocean. Below the seasonal mixed layer, however, periodic ship-based surveys (every 2 to 5 years) could quantify the expansion of the injection plume.

We do not have a direct means of measuring the evasion of carbon stored in the ocean to the atmosphere. In most cases of practical interest the flux of stored CO2 from the ocean to atmosphere will be small relative to natural variability and the accuracy of our measurements. Operationally, it would be impossible to differentiate between carbon that has and has not interacted with the atmosphere. The use of prognostic models in evaluating the long-term fate of the injected CO2 is critical for properly attributing the net storage from a particular site.

Given the natural background variability in ocean carbon concentrations, it would be extremely difficult, if not impossible, to measure CO2 injected very far from the injection source. The attribution of a signal to a particular point source would become increasingly difficult if injection plumes from different locations began to overlap and mix. In some parts of the ocean it would be difficult to assign the rise in CO2 to intentional ocean storage as opposed to CO2 from atmospheric absorption.

Environmental impacts, risks, and risk management

6.6.3 Approaches and technologies for monitoring environmental effects

Techniques now being used for field experiments could be used to monitor some near field consequences of direct CO2 injection (Section 6.7). For example, researchers (Barry et al., 2004, 2005; Carman et al, 2004; Thistle et al., 2005) have been developing experimental means for observing the consequences of elevated CO2 on organisms in the deep ocean. However, such experiments and studies typically look for evidence of acute toxicity in a narrow range of species (Sato, 2004; Caulfield et al, 1997; Adams et al, 1997; Tamburri et al, 2000). Sub-lethal effects have been studied by Kurihara et al. (2004). Process studies, surveys of biogeochemical tracers, and ocean bottom studies could be used to evaluate changes in ecosystem structure and dynamics both before and after an injection.

It is less clear how best to monitor the health of broad reaches of the ocean interior (Sections 6.7.3 and 6.7.4). Ongoing long-term surveys of biogeochemical tracers and deep-sea biota could help to detect long-term changes in deep-sea ecology.

6.7.1 Introduction to biological impacts and risk

Overall, there is limited knowledge of deep-sea population and community structure and of deep-sea ecological interactions (Box 6.4). Thus the sensitivities of deep ocean ecosystems to intentional carbon storage and the effects on possibly unidentified goods and services that they may provide remain largely unknown.

Most ocean storage proposals seek to minimize the volume of water with high CO2 concentrations either by diluting the CO2 in a large volume of water or by isolating the CO2 in a small volume (e.g., in CO2 lakes). Nevertheless, if deployed widely, CO2 injection strategies ultimately will produce large volumes of water with somewhat elevated CO2 concentrations (Figure 6.15). Because large amounts of relatively pure CO2 have never been introduced to the deep ocean in a controlled experiment, conclusions about environmental risk must be based primarily on laboratory and small-scale in-situ experiments and extrapolation from these experiments using conceptual and mathematical models. Natural analogues (Box 6.5) can be relevant, but differ significantly from proposed ocean engineering projects.

Compared to the surface, most of the deep sea is stable and varies little in its physiochemical factors over time (Box 6.4). The process of evolutionary selection has probably eliminated individuals apt to endure environmental perturbation. As a result, deep-sea organisms may be more sensitive to environmental disturbance than their shallow water cousins (Shirayama, 1997).

Ocean storage would occur deep in the ocean where there is virtually no light and photosynthesizing organisms are lacking, thus the following discussion primarily addresses CO2 effects on heterotrophic organisms, mostly animals. The diverse fauna that lives in the waters and sediments of the deep ocean can be affected by ocean CO2 storage, leading to change in ecosystem composition and functioning. Thus, the effects of CO2 need to be identified at the level of both the individual (physiological) and the ecosystem.

As described in Section 6.2, introduction of CO2 into the ocean either directly into sea water or as a lake on the sea floor would result in changes in dissolved CO2 near to and down current from a discharge point. Dissolving CO2 in sea water (Box 6.1; Table 6.3) increases the partial pressure of CO2 (pCO2, expressed as a ppm fraction of atmospheric pressure, equivalent to ^atm), causes decreased pH (more acidic) and decreased CO32- concentrations (less saturated). This can lead to dissolution of CaCO3 in sediments or in shells of organisms. Bicarbonate (HCO3-) is then produced from carbonate (CO32-).

The spatial extent of the waters with increased CO2 content and decreased pH will depend on the amount of CO2 released and the technology and approach used to introduce that CO2 into the ocean. Table 6.3 shows the amount of sea water needed to dilute each tonne of CO2 to a specified ApH reduction. Further dilution would reduce the fraction of ocean at one ApH

Box 6.4 Relevant background in biological oceanography.

Photosynthesis produces organic matter in the ocean almost exclusively in the upper .00 m where there is both light and nutrients (e.g., PO4, NO3, NH4+, Fe). Photosynthesis forms the base of a marine food chain that recycles much of the carbon and nutrients in the upper ocean. Some of this organic matter ultimately sinks to the deep ocean as particles and some of it is mixed into the deep ocean as dissolved organic matter. The flux of organic matter from the surface ocean provides most of the energy and nutrients to support the heterotrophic ecosystems of the deep ocean (Gage and Tyler, 1991). With the exception of the oxygen minimum zone and near volcanic CO. vents, most organisms living in the deep ocean live in low and more or less constant CO. levels.

At low latitudes, oxygen consumption and CO. release can produce a zone at around 1000 m depth characterized by low O. and high CO. concentrations, known as the 'oxygen minimum zone'. Bacteria are the primary consumers of organic matter in the deep ocean. They obtain energy predominately by consuming dissolved oxygen in reactions that oxidize organic carbon into CO.. In the oxygen minimum layer, sea water pH may be less than 7.7, roughly 0.5 pH units lower than average pH of natural surface waters (Figure 6.6).

At some locations near the sea floor, especially near submarine volcanic CO. sources, CO. concentrations can fluctuate greatly. Near deep-sea hydrothermal vents CO. partial pressures (pCO., expressed as a ppm fraction of atmospheric pressure, equivalent to ^atm) of up to 80,000 ppm have been observed. These are more than 100 times the typical value for deep-sea water. Typically, these vents are associated with fauna that have adapted to these conditions over evolutionary time. For example, tube worms can make use of high CO. levels for chemosynthetic CO. fixation in association with symbiotic bacteria (Childress et al., 1993). High CO. levels (up to a pCO. of 16,000 ppm; Knoll et al., 1996) have been observed in ocean bottom waters and marine sediments where there are high rates organic matter oxidation and low rates of mixing with the overlying seawater. Under these conditions, high CO. concentrations are often accompanied by low O. concentrations. Near the surface at night, respiratory fluxes in some relatively confined rock pools of the intertidal zone can produce high CO. levels. These patterns suggest that in some environments, organisms have evolved to tolerate relatively wide pH oscillations and/or low pH values.

Deep-sea ecosystems generally depend on sinking particles of organic carbon made by photosynthesis near the ocean surface settling down through the water. Most species living in the deep sea display very low metabolic rates (Childress, 1995), especially in oxygen minimum layers (Seibel et al., 1997). Organisms living in the deep seawaters have adapted to the energy-limited environment by conserving energy stores and minimizing energy turnover. As a result of energy limitations and cold temperatures found in the deep sea, biological activities tend to be extremely low. For example, respiration rates of rat-tail fish are roughly 0.1% that of their shallow-water relatives. Community respiration declines exponentially with depth along the California margin, however, rapid turnover of large quantities of organic matter has been observed on the ocean floor (Mahaut et al., 1995; Smith and Demopoulos, 2003). Thus, biological activity of some animals living on the deep sea floor can be as great as is found in relatives living on the sea floor in shallow waters.

Deep-sea ecosystems may take a long time to recover from disturbances that reduce population size. Organisms have adapted to the energy-limited environment of the deep sea by limiting investment in reproduction, thus most deep-sea species produce few offspring. Deep-sea species tend to invest heavily in each of their eggs, making them large and rich in yolk to provide the offspring with the resources they will need for survival. Due to their low metabolic rates, deep-sea species tend to grow slowly and have much longer lifespans than their upper-ocean cousins. For example, on the deep-sea floor, a bivalve less than 1 cm across can be more than 100 years old (Gage, 1991). This means that populations of deep-sea species will be more greatly affected by the loss of individual larvae than would upper ocean species. Upon disturbance, recolonization and community recovery in the deep ocean follows similar patterns to those in shallow waters, but on much longer time scales (several years compared to weeks or months in shallow waters, Smith and Demopoulos, .003).

The numbers of organisms living on the sea floor per unit area decreases exponentially with depth, probably associated with the diminishing flux of food with depth. On the sea floor of the deepest ocean and of the upper ocean, the fauna can be dominated by a few species. Between .000 and 3000 m depth ecosystems tend to have high species diversity with a low number of individuals, meaning that each species has a low population size (Snelgrove and Smith, .00.). The fauna living in the water column appear to be less diverse than that on the sea floor, probably due to the relative uniformity of vast volumes of water in the deep ocean.

Box 6.5 Natural analogues and Earth history.

There are several examples of natural systems with strong CO2 sources in the ocean, and fluid pools toxic to marine life that may be examined to better understand possible physical and biological effects of active CO2 injection.

Most natural environments that are heavily enriched in CO2 (or toxic substances) host life forms that have adapted to these special conditions on evolutionary time scales. During Earth history much of the oceans may have hosted life forms specialized on elevated pCO2, which are now extinct. This limits the use of natural analogues or Earth history to predict and generalize effects of CO2 injection on most extant marine life.

• Venting of carbon dioxide-rich fluids: Hydrothermal vents, often associated with mid-ocean-ridge systems, often release CO2 rich fluids into the ocean and can be used to study CO2 behaviour and effects. For example, Sakai et al. (1990) observed buoyant hydrate forming fluids containing 86-91% CO2 (with H2S, and methane etc. making up the residual) released from the sea floor at 1335-1550 m depth from a hydrothermal vent field. These fluids would be similar to a heavily contaminated industrial CO2 source. These fluids arise from the reaction of sea water with acid and intermediate volcanic rocks at high temperature; they are released into sea water of 3.8°C. A buoyant hydrate-coated mass forms at the sea floor, which then floats upwards dissolving into the ocean water. Sea floor venting of aqueous fluids, rich in CO2 and low in pH (3.5-4.4), is also to be found in some hydrothermal systems (Massoth et al., 1989; Karl, 1995).

Near volcanic vents, deep-sea ecosystems can be sustained by a geochemical input of chemical energy and CO2. While there has been extensive investigation of these sites, and the plumes emanating from them, this has not yet been in the context of analogues for industrial CO2 storage effects. Such an investigation would show how a fauna has evolved to adapt to a high-CO2 environment; it would not show how biota adapted to normal ocean water would respond to increased CO2 concentrations.*

• Deep saline brine pools: The ocean floor is known to have a large number of highly saline brine pools that are anoxic and toxic to marine life. The salty brines freely dissolve, but mixing into the overlying ocean waters is impeded by the stable stratification imparted by the high density of the dissolving brines. The Red Sea contains many such brine pools (Degens and Ross, 1969; Anschutz et al., 1999), some up to 60 km2 in area, filled with high-temperature hyper-saline, anoxic, brine. Animals cannot survive in these conditions, and the heat and salt that are transported across the brine-seawater interface form a plume into the surrounding bottom water. Hydrothermal sources resupply brine at the bottom of the brine pool (Anschutz and Blanc, 1996). The Gulf of Mexico contains numerous brine pools. The largest known is the Orca Basin, where a 90 km2 brine pool in 2250 m water depth is fed by drainage from exposed salt deposits. The salt is toxic to life, but biogeochemical cycles operate at the interface with the overlying ocean (van Cappellen et al., 1998). The Mediterranean also contains numerous large hypersaline basins (MEDRIFF Consortium, 1995).

Taken together these naturally occurring brine pools provide examples of vast volumes of soluble, dense, fluids, hostile to marine life, on the sea floor. The number, volume, and extent of these pools exceed those for scenarios for CO2 lake formation yet considered. There has been little study of the impact of the plumes emanating from these sources. These could be examined to yield information that may be relevant to environmental impacts of a lake of CO2 on the ocean floor.

• Changes over geological time: In certain times in Earth's geological past the oceans may have contained more dissolved inorganic carbon and/or have had a lower pH.

There is evidence of large-scale changes in calcifying organism distributions in the oceans in the geological record that may be related in changes in carbonate mineral saturation states in the surface ocean. For example, Barker and Elderfield (2002) demonstrated that glacial-interglacial changes in the shell weights of several species of planktonic foraminifera are negatively correlated with atmospheric CO2 concentrations, suggesting a causal relationship.

Cambrian CO2 levels (i.e., about 500 million years ago) were as high as 5000 ppm and mean values decreased progressively thereafter (see. Dudley, 1998; Berner, 2002). Two to three times higher than extant ocean calcium levels ensured that calcification of, for example, coral reefs was enabled in paleo-oceans despite high CO2 levels (Arp et al., 2001). High performance animal life appeared in the sea only after atmospheric CO2 began to diminish. The success of these creatures may have depended on the reduction of atmospheric CO2 levels (reviewed by Portner et al., 2004, 2005). CO2 is also thought to have been a potential key factor in the late Permian/Triassic mass extinction, which affected corals, articulate brachiopods, bryozoans, and echinoderms to a larger extent than molluscs, arthropods and chordates (Knoll et al., 1996; Berner, 2002; Bambach et al., 2002). Portner et al. (2004) hypothesized that this may be due to the corrosive effect of CO2 on heavily calcified skeletons. CO2 excursions would have occurred in the context of large climate oscillations. Effects of temperature oscillations, hypoxia events and CO2 excursions probably contributed to extinctions (Portner et al., 2005, see section 6.7.3).

Table 6.3 Relationships between ApH, changes in pCO2, and dissolved inorganic carbon concentration calculated for mean deep-sea conditions. Also shown are volumes of water needed to dilute 1 tCO2 to the specified ApH, and the amount of CO2 that, if uniformly distributed throughout the ocean, would produce this ApH.

Table 6.3 Relationships between ApH, changes in pCO2, and dissolved inorganic carbon concentration calculated for mean deep-sea conditions. Also shown are volumes of water needed to dilute 1 tCO2 to the specified ApH, and the amount of CO2 that, if uniformly distributed throughout the ocean, would produce this ApH.

pH change ApH

Increase in CO2 partial pressure ApCO2 (ppm)

Increase in dissolved inorganic carbon ADIC (^mol kg-1)

Seawater volume to dilute 1 tCO2 to

ApH (m3)2

GtCO2 to produce ApH in entire ocean volume

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