•Approaches for CO2 leakage detection, attribution and quantification monitoring exist.•Many approaches cover multiple monitoring tasks simultaneously.•Sonars and chemical sensors on ships or AUVs ...can cover large areas.•Newer, more specific technologies can detect, verify and quantify smaller, localised leaks.
Environmental monitoring of offshore Carbon Capture and Storage (CCS) complexes requires robust methodologies and cost-effective tools to detect, attribute and quantify CO2 leakage in the unlikely event it occurs from a sub-seafloor reservoir. Various approaches can be utilised for environmental CCS monitoring, but their capabilities are often undemonstrated and more detailed monitoring strategies need to be developed. We tested and compared different approaches in an offshore setting using a CO2 release experiment conducted at 120 m water depth in the Central North Sea. Tests were carried out over a range of CO2 injection rates (6 - 143 kg d−1) comparable to emission rates observed from abandoned wells. Here, we discuss the benefits and challenges of the tested approaches and compare their relative cost, temporal and spatial resolution, technology readiness level and sensitivity to leakage. The individual approaches demonstrate a high level of sensitivity and certainty and cover a wide range of operational requirements. Additionally, we refer to a set of generic requirements for site-specific baseline surveys that will aid in the interpretation of the results. Critically, we show that the capability of most techniques to detect and quantify leakage exceeds the currently existing legal requirements.
The fragility of thin Clark‐type glass microelectrodes used in aquatic eddy covariance measurements of benthic oxygen fluxes is a challenge when using this powerful technique. This study presents a ...new fast‐responding dual oxygen‐temperature sensor for eddy covariance measurements that is far more robust. Response time tests in the lab, where the sensor was inserted from air into water, revealed 90% response times of 0.51 s and 0.34 s for oxygen and temperature measurements, respectively. In wave tank tests, the new sensor showed no stirring sensitivity in contrast to Clark‐type microelectrodes. Other tests in a flume and in a particle‐free water tank revealed how close the sensor can be positioned to the measuring volume of the Acoustic Doppler Velocimeter without disturbing velocity recordings. In field tests at river sites, all > 24 h, the new sensor recorded high‐quality eddy covariance data for the entire deployment. Similar positive results were obtained in deployments at a marine site with unidirectional current flow overlaid with minor wave action. Concurrently deployed eddy covariance systems based on the new sensor and a traditional Clark‐type microelectrode revealed that they recorded statistically equivalent fluxes and similar velocity‐oxygen cospectra until the microelectrode broke after 2 h. The significant increase in robustness of the new sensor was achieved by relying on a larger sensor tip. This put some constrains on how the sensor should be deployed and fluxes extracted, but given the substantial gain in performance, it is a viable alternative for eddy covariance measurements in many aquatic environments.
We report primary production and respiration of Posidonia oceanica meadows determined with the non-invasive aquatic eddy covariance technique. Oxygen fluxes were measured in late spring at an ...open-water meadow (300 m from shore), at a nearshore meadow (60 m from shore), and at an adjacent sand bed. Despite the oligotrophic environment, the meadows were highly productive and highly autotrophic. Net ecosystem production (54 to 119 mmol m-2 d-1) was about one-half of gross primary production. In adjacent sands, net primary production was a tenth- to a twentieth smaller (4.6 mmol m-2 d-1). Thus, P. oceanica meadows are an oasis of productivity in unproductive surroundings. During the night, dissolved oxygen was depleted in the open-water meadow. This caused a hysteresis where oxygen production in the late afternoon was greater than in the morning at the same irradiance. Therefore, for accurate measurements of diel primary production and respiration in this system, oxygen must be measured within the canopy. Generally, these measurements demonstrate that P. oceanica meadows fix substantially more carbon than they respire. This supports the high rate of organic carbon accumulation and export for which the ecosystem is known.
We quantified oxygen flux in a coastal stream in Virginia using a novel combination of the conventional open water technique and the aquatic eddy covariance technique. The latter has a smaller ...footprint (sediment surface area that contributes to the flux; ∼ 10 m²), allowing measurements to be made at multiple sites within the footprint of the open water technique (∼ 1000 m²). Sites included an unvegetated stream pool with cohesive sediment, a macrophyte bed with sandy sediment, and an unvegetated sand bed with rippled bedforms. Nighttime eddy covariance oxygen uptake was always smaller than uptake produced by the open water technique. At the pool and unvegetated sand bed sites, nighttime eddy covariance uptake was 20-fold smaller than open water uptake. At the macrophyte bed site, gross primary production quantified with the two techniques was similar but eddy covariance uptake was 2.4-fold smaller. The difference in oxygen uptake between eddy covariance and open water techniques could not be accounted for by uncertainties in the gas transfer velocity but could be accounted for by anoxic groundwater inflow through stream banks outside of the eddy covariance footprint. Nighttime oxygen uptake was also measured with eddy covariance in a tidal freshwater part of the stream, where pore space in the sandy sediment near the sediment–water interface was flushed with stream water at peak water velocities. As a result of this advective hyporheic exchange, nighttime oxygen flux increased fourfold with a doubling of water velocity.
•A subsea release of CO2 was detected and quantified with a novel approach.•Autonomous lab-on-chip sensors on a lander measured pH and total alkalinity.•The pH signals showed decreases in pH of >0.6 ...and strong vertical gradients.•pH and hydrodynamic measurements enabled an estimate of CO2 emission rate.•This technique could be adapted to a wide range of in situ benthic gradient studies.
We present a novel approach to detecting and quantifying a subsea release of CO2 from within North Sea sediments, which mimicked a leak from a subsea CO2 reservoir. Autonomous lab-on-chip sensors performed in situ measurements of pH at two heights above the seafloor. During the 11 day experiment the rate of CO2 release was gradually increased. Whenever the currents carried the CO2-enriched water towards the sensors, the sensors measured a decrease in pH, with a strong vertical gradient within a metre of the seafloor. At the highest release rate, a decrease of over 0.6 pH units was observed 17 cm above the seafloor compared to background measurements. The sensor data was combined with hydrodynamic measurements to quantify the amount of CO2 escaping the sediments using an advective mass transport model. On average, we directly detected 43 ± 8% of the released CO2 in the water column. Accounting for the incomplete carbonate equilibration process increases this estimate to up to 61 ± 10%. This technique can provide long-term in situ monitoring of offshore CO2 reservoirs and hence provides a tool to support climate change mitigation activities. It could also be applied to characterising plumes and quantifying other natural or anthropogenic fluxes of dissolved solutes.
Oxygen depletion in bottom waters of lakes and coastal regions is expanding worldwide. To examine the causes of hypoxia, we quantified the drivers of benthic oxygen uptake in Green Bay, Lake ...Michigan, USA, using 2 techniques, aquatic eddy covariance and sediment core incubation. We investigated benthic oxygen uptake along a gradient in C deposition, including shallow water near the riverine source of eutrophication and deeper waters of lower Green Bay where high net sediment deposition occurs. Time-averaged eddy covariance oxygen uptake was high near the source of eutrophication (11.5 mmol m−2 d−1) and at the shallower of the high deposition sites (9.8 mmol m−2 d−1). The eddy covariance technique revealed a decrease in benthic oxygen uptake with depth at the high deposition sites. These patterns were consistent with benthic uptake being driven by the deposition of autochthonous production. Additionally, eddy covariance revealed a nearly proportional relationship between benthic oxygen uptake and current velocity at all sites. Specifically, because of the lake seiche, water velocity typically varied 3× at a site and caused a 3× variation in benthic oxygen uptake. A summer storm also doubled bottom-water velocities and caused a further doubling of uptake to 28 mmol m−2 d−1. This high sensitivity of benthic oxygen uptake to seiche-driven water velocities indicates that redox conditions in surficial cohesive sediments are highly dynamic.
●We developed a pH eddy covariance system to detect a sub-seafloor CO2 release.●It detected CO2 emission to the water column at injection rates of 5.7–143 kg d − 1.●It was also sensitive enough to ...quantify benthic biological CO2 production.●Close to bubble streams, the kinetics of aqueous CO2 equilibration are important.●This system can be used to detect, attribute, and quantify seafloor sources of CO2.
We detected a controlled release of CO2 (g) with pH eddy covariance. We quantified CO2 emission using measurements of water velocity and pH in the plume of aqueous CO2 generated by the bubble streams, and using model predictions of vertical CO2 dissolution and its dispersion downstream. CO2 (g) was injected 3 m below the floor of the North Sea at rates of 5.7–143 kg d − 1. Instruments were 2.6 m from the center of the bubble streams. In the absence of injected CO2, pH eddy covariance quantified the proton flux due to naturally-occurring benthic organic matter mineralization (equivalent to a dissolved inorganic carbon flux of 7.6 ± 3.3 mmol m − 2d − 1, s.e., n = 33). At the lowest injection rate, the proton flux due to CO2 dissolution was 20-fold greater than this. To accurately quantify emission, the kinetics of the carbonate system had to be accounted for. At the peak injection rate, 73 ± 13% (s.d.) of the injected CO2 was emitted, but when kinetics were neglected, the calculated CO2 emission was one-fifth of this. Our results demonstrate that geochemical techniques can detect and quantify very small seafloor sources of CO2 and attribute them to natural or abiotic origins.
A new seepage meter design that increases sensitivity to the small hydraulic gradients that drive submarine groundwater discharge was developed as an inexpensive alternative to traditional seepage ...meters. The new design replaces seepage meter bags with open‐ended tubing through which the displacement of injected dye is a highly reproducible measure of discharge. Laboratory measurements, detailed mathematical modeling of groundwater flow around seepage meters, and parallel field tests of bag and dye displacement seepage meters demonstrated that the new design can improve the precision and temporal resolution of submarine groundwater discharge measurement. In parallel field tests, groundwater discharge to 15 cm diameter, bag‐equipped seepage meters was half of the discharge to adjacent 15 cm diameter dye displacement seepage meters. Model results confirm that similar discrepancies, caused by groundwater diversion around bag‐equipped seepage meters, may occur during the deployment of 57 cm diameter seepage meters in common field conditions. A second field deployment of 15 cm diameter dye displacement seepage meters showed saline inflow into sandy sediments at high tide and a high rate of discharge at low tide, similar to observations made with far more expensive, open flow path automated seepage meters in other studies.
Recent studies have shown that in a number of humic-rich surface waters in North America, NH sub(4) super(+) is released when dissolved organic matter (DOM) is exposed to sunlight. However, ...photochemical NH sub(4) super(+) production has not been observed in all surface waters, and factors that contribute to it are not well understood. We hypothesized that the presence or absence of NH sub(4) super(+) photoproduction may be affected by the light exposure history of DOM. The present study was undertaken to determine whether DOM from surficial groundwaters, with minimal light exposure history, would produce labile nitrogen (N) photoproducts more consistently. In this study, estuarine surface waters and surficial groundwaters, collected just adjacent to estuaries in Georgia and South Carolina, USA, were exposed to sunlight to quantify the photochemical production of NH sub(4) super(+), dissolved primary amines (DPA), and NO sub(2) super(-). The photoproduction of NH sub(4) super(+) was observed in 4 of 5 irradiated estuarine surface water samples but in only 2 of 13 groundwater samples. In contrast, NH sub(4) super(+) concentrations decreased in 5 of 13 groundwater samples when exposed to sunlight. The results indicate that a small amount of NH sub(4) super(+) may be lost from waters in which groundwater-derived DOM is first exposed to sunlight. No consistent trends were observed in the photoproduction or loss of DPA and NO sub(2) super(-).
•The STEMM-CCS project completed a unique field experiment in the central North Sea.•The experiment mimicked a leakage of CO2 from an offshore storage site.•A custom setup released CO2 into shallow ...sediment at relevant leakage rates.•Diverse established methods and novel technologies characterised the CO2.•The outcomes show such a release can be detected, attributed, and quantified.
Carbon capture and storage (CCS) is a key technology to reduce carbon dioxide (CO2) emissions from industrial processes in a feasible, substantial, and timely manner. For geological CO2 storage to be safe, reliable, and accepted by society, robust strategies for CO2 leakage detection, quantification and management are crucial. The STEMM-CCS (Strategies for Environmental Monitoring of Marine Carbon Capture and Storage) project aimed to provide techniques and understanding to enable and inform cost-effective monitoring of CCS sites in the marine environment. A controlled CO2 release experiment was carried out in the central North Sea, designed to mimic an unintended emission of CO2 from a subsurface CO2 storage site to the seafloor. A total of 675 kg of CO2 were released into the shallow sediments (∼3 m below seafloor), at flow rates between 6 and 143 kg/d. A combination of novel techniques, adapted versions of existing techniques, and well-proven standard techniques were used to detect, characterise and quantify gaseous and dissolved CO2 in the sediments and the overlying seawater. This paper provides an overview of this ambitious field experiment. We describe the preparatory work prior to the release experiment, the experimental layout and procedures, the methods tested, and summarise the main results and the lessons learnt.
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