Robert van Geldern

Field-based stable isotope analysis of carbon dioxide by mid-infrared laser spectroscopy for carbon capture and storage monitoring

A newly developed isotope ratio laser spectrometer for CO2 analyses has been tested during a tracer experiment at the Ketzin pilot site (northern Germany) for CO2 storage. For the experiment, 500 tons of CO2 from a natural CO2 reservoir was injected in supercritical state into the reservoir. The carbon stable isotope value (δ(13)C) of injected CO2 was significantly different from background values. In order to observe the breakthrough of the isotope tracer continuously, the new instruments were connected to a stainless steel riser tube that was installed in an observation well. The laser instrument is based on tunable laser direct absorption in the mid-infrared. The instrument recorded a continuous 10 day carbon stable isotope data set with 30 min resolution directly on-site in a field-based laboratory container during a tracer experiment. To test the instruments performance and accuracy the monitoring campaign was accompanied by daily CO2 sampling for laboratory analyses with isotope ratio mass spectrometry (IRMS). The carbon stable isotope ratios measured by conventional IRMS technique and by the new mid-infrared laser spectrometer agree remarkably well within analytical precision. This proves the capability of the new mid-infrared direct absorption technique to measure high precision and accurate real-time stable isotope data directly in the field. The laser spectroscopy data revealed for the first time a prior to this experiment unknown, intensive dynamic with fast changing δ(13)C values. The arrival pattern of the tracer suggest that the observed fluctuations were probably caused by migration along separate and distinct preferential flow paths between injection well and observation well. The short-term variances as observed in this study might have been missed during previous works that applied laboratory-based IRMS analysis. The new technique could contribute to a better tracing of the migration of the underground CO2 plume and help to ensure the long-term integrity of the reservoir.

Stable isotope geochemistry of pore waters and marine sediments from the New Jersey shelf: Methane formation and fluid origin

Interstitial water and sediment samples of Integrated Ocean Drilling Program (IODP) Expedition 313 (New Jersey Shallow Shelf) were analyzed for chemical composition and stable isotope ratios. The analyses indicate a previously unknown complex geometry of the underlying fresh-water lens with alternating fresh-water–salt-water intervals divided by sharp boundaries in the upper part of the cores. Three fluid sources were identified: (1) meteoric fresh water, (2) marine seawater, and (3) brine. The pore-fluid stable isotope values define a mixing line with end members that have δ18O and δ2H values of –7.0‰ and –41‰ for fresh water, and –0.8‰ and –6‰ for salt water, respectively. This is similar to the modern mean value of New Jersey precipitation and today’s New Jersey shelf water. For fresh water, this either indicates modern meteoric recharge via aquifers that crop out on mainland New Jersey or emplacement at a time with climatic and hydrologic conditions similar to modern. An origin from Pleistocene glacial meltwaters with depleted isotope values is not confirmed by stable isotope data of this study. Salt water also represents modern isotope values suggesting an infiltration along permeable, coarse-grained sandy units. The lower core parts are characterized by mixing with brine fluids that originate from evaporites in the deep underground. Stable carbon isotope analyses of gas and fluids prove the existence of methane formation from degradation of marine organic matter and CO2 reduction in the lower core parts below ∼350 m below seafloor. Methane concentrations above 10000 ppm and δ13Cmethane values of ∼–80‰ were measured. Methane formation is also indicated by authigenic carbonates with low δ13Ccarbonate values. Although not reaching the surface at present conditions, the venting out of variable fluxes of methane from passive continental margins due to sea-level fluctuations is significant for the long-term carbon cycle. Authigenic carbonates indicate the precipitation from pore fluids with marine oxygen stable isotope ratios at low temperatures. The geochemical data and interpretations presented in this study supply the missing link between existing onshore and offshore data and may provide the basis for an integrated approach to construct a geochemical transect across the New Jersey shallow shelf.

Stable carbon isotope analysis of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in natural waters- results from a worldwide proficiency test

RATIONALE Stable carbon isotope ratios of dissolved inorganic (DIC) and organic carbon (DOC) are of particular interest in aquatic geochemistry. The precision for this type of analysis is typically reported in the range of 0.1‰ to 0.5‰. However, there is no published attempt that compares δ(13)C measurements of DIC and DOC among different laboratories for natural water samples. METHODS Five natural water samples (lake water, seawater, two geothermal waters, and petroleum well water) were analyzed for δ(13)CDIC and δ(13)CDOC values by five laboratories with isotope ratio mass spectrometry (IRMS) in an international proficiency test. RESULTS The reported δ(13)CDIC values for lake water and seawater showed fairly good agreement within a range of about 1‰, whereas geothermal and petroleum waters were characterized by much larger differences (up to 6.6‰ between laboratories). δ(13)CDOC values were only comparable for seawater and showed differences of 10 to 21‰ for other samples. CONCLUSIONS This study indicates that scatter in δ(13)CDIC isotope data can be in the range of several per mil for samples from extreme environments (geothermal waters) and may not yield reliable information with respect to dissolved carbon (petroleum wells). The analyses of lake water and seawater also revealed a larger than expected difference and researchers from various disciplines should be aware of this. Evaluation of analytical procedures of the participating laboratories indicated that the differences cannot be explained by analytical errors or different data normalization procedures and must be related to specific sample characteristics or secondary effects during sample storage and handling. Our results reveal the need for further research on sources of error and on method standardization.

Pleistocene paleo-groundwater as a pristine fresh water resource in southern Germany – evidence from stable and radiogenic isotopes

Shallow groundwater aquifers are often influenced by anthropogenic contaminants or increased nutrient levels. In contrast, deeper aquifers hold potentially pristine paleo-waters that are not influenced by modern recharge. They thus represent important water resources, but their recharge history is often unknown. In this study groundwater from two aquifers in southern Germany were analyzed for their hydrogen and oxygen stable isotope compositions. One sampling campaign targeted the upper aquifer that is actively recharged by modern precipitation, whereas the second campaign sampled the confined, deep Benkersandstein aquifer. The groundwater samples from both aquifers were compared to the local meteoric water line to investigate sources and conditions of groundwater recharge. In addition, the deep groundwater was dated by tritium and radiocarbon analyses. Stable and radiogenic isotope data indicate that the deep-aquifer groundwater was not part of the hydrological water cycle in the recent human history. The results show that the groundwater is older than ~20,000 years and most likely originates from isotopically depleted melt waters of the Pleistocene ice age. Today, the use of this aquifer is strictly regulated to preserve the pristine water. Clear identification of such non-renewable paleo-waters by means of isotope geochemistry will help local water authorities to enact and justify measures for conservation of these valuable resources for future generations in the context of a sustainable water management.

A Brief Overview of Isotope Measurements Carried Out at Various CCS Pilot Sites Worldwide

About 1800 geochemical measurements including more than 1000 isotope analyses have been published as a result of geochemical monitoring programs established at several CO2 storage and enhanced oil and gas recovery projects worldwide. These projects are briefly discussed here in order to compare sampling techniques to obtain fluid and gas samples for chemical and isotopic analyses. In all the projects, changes of stable isotope ratios of CO2 and dissolved inorganic carbon in samples obtained pre- and post-injection were used to quantify solubility and ionic trapping of CO2 via stable isotope mass balances. Further applications include, monitoring of underground CO2 migration and early detection of potential CO2 leaks into overlying formations. Other benefits of these stable isotope tracers include a better understanding of water-rock-gas interactions with CO2 under supercritical conditions and often highly saline reservoir fluids that are present in the storage formations. While the results of these projects need further laboratory and experimental confirmation, further increase in field applications of stable isotope tracer techniques are anticipated with the introduction of new portable laser stable isotope mass spectrometers.

Controls of evaporative irrigation return flows in comparison to seawater intrusion in coastal karstic aquifers in northern Sri Lanka: Evidence from solutes and stable isotopes.

Groundwater in Miocene karstic aquifers in the Jaffna Peninsula of Sri Lanka is an important resource since no other fresh water sources are available in the region. The subsurface is characterized by highly productive limestone aquifers that are used for drinking and agriculture purposes. A comprehensive hydrogeochemical study was carried out to reveal the processes affecting the groundwater quality in this region. Major and trace element composition and environmental isotope ratios of oxygen and hydrogen (δ(18)OH2O and δ(2)HH2O) were determined in 35 groundwater samples for this investigation. The ion abundance of groundwater in the region was characterized by an anion sequence order with HCO3->Cl->SO4->NO3-. For cations, average Na(+)+K(+) contents in groundwater exceeded those of Ca(2+)+Mg(2+) in most cases. Ionic relationships of major solutes indicated open system calcite dissolution while seawater intrusions are also evident but only close to the coast. The solute contents are enriched by agricultural irrigation returns and associated evaporation. This was confirmed by the stable isotope composition of groundwater that deviated from the local meteoric water line (LMWL) and formed its own regression line denoted as the local evaporation line (LEL). The latter can be described by δ(2)HH2O=5.8×δ(18)OH2O -- 2.9. Increased contents of nitrate-N (up to 5mg/L), sulfate (up to 430mg/L) and fluoride (up to 1.5mg/L) provided evidences for anthropogenic inputs of solutes, most likely from agriculture activities. Among trace elements Ba, Sr, As and Se levels in the Jaffna groundwater were higher compared to that of the dry zone metamorphic aquifers in Sri Lanka. Solute geochemistry and stable isotope evidences from the region indicates that groundwater in the area is mainly derived from local modern precipitation but modified heavily by progressive evaporative concentration rather than seawater intrusion.

A high-resolution carbon balance in a small temperate catchment: Insights from the Schwabach River, Germany

Abstract The present study is one of only few to address the stable carbon isotope (δ13C) and concentration dynamics of DIC, DOC, and POC over an entire hydrologic year, using a dataset of high sampling resolution (4–11 samples retrieved per month). This research was performed in the catchment of the Schwabach River, a typical mid-latitude small headwater stream in Germany emplaced mainly in karstic bedrock. The DIC data indicated the dominance of mineral weathering as a DIC source, with a noticeable dilution effect during periods of high river flow. A weakly negative relationship between discharge and δ13CDIC hints at a greater importance of plant-derived organic matter during flooding events, likely transported to river waters via overland runoff and intermediate flow. DOC inputs appeared to be enhanced during periods of high discharge, indicating a greater importance of overland runoff as a DOC source. POC concentrations seem unaffected by changes in discharge, although a slight negative correlation between δ13CPOC and discharge may be derived from increased inputs of C4 plant material. Estimated CO2 concentrations were in excess of ambient atmospheric values throughout the year, confirming that the surface waters of the Schwabach River are a net CO2 source. The total riverine carbon flux was dominated by DIC (70%), followed by CO2 outgassing (21%), DOC (7%), and POC (2%). While the selection of a bi-monthly sampling scheme yielded a broadly similar carbon flux estimate to that utilizing the entire dataset, the use of a monthly sampling interval differed by as much as 19% from values using the high-resolution data set. This discrepancy is due to the inability of a monthly sampling scheme to capture sudden and large variations in river discharge and associated changes in dissolved/particulate carbon concentrations, such as those observed during flooding. We suggest that bi-monthly sampling is the minimum timeframe required to achieve an acceptable degree of accuracy in the calculation of carbon fluxes, at least during periods of high runoff. The application of high sampling frequencies and comprehensive DIC, DOC, and POC studies in future research would greatly reduce uncertainties in local riverine carbon budgets, and help clarify the role of smaller streams in the global carbon cycle.

Stable water isotope patterns in a climate change hotspot: the isotope hydrology framework of Corsica (western Mediterranean)

The Mediterranean is regarded as a region of intense climate change. To better understand future climate change, this area has been the target of several palaeoclimate studies which also studied stable isotope proxies that are directly linked to the stable isotope composition of water, such as tree rings, tooth enamel or speleothems. For such work, it is also essential to establish an isotope hydrology framework of the region of interest. Surface waters from streams and lakes as well as groundwater from springs on the island of Corsica were sampled between 2003 and 2009 for their oxygen and hydrogen isotope compositions. Isotope values from lake waters were enriched in heavier isotopes and define a local evaporation line (LEL). On the other hand, stream and spring waters reflect the isotope composition of local precipitation in the catchment. The intersection of the LEL and the linear fit of the spring and stream waters reflect the mean isotope composition of the annual precipitation (δP) with values of−8.6(±0.2) ‰ for δ18O and−58(±2) ‰ for δ2H. This value is also a good indicator of the average isotope composition of the local groundwater in the island. Surface water samples reflect the altitude isotope effect with a value of−0.17(±0.02) ‰ per 100 m elevation for oxygen isotopes. At Vizzavona Pass in central Corsica, water samples from two catchments within a lateral distance of only a few hundred metres showed unexpected but systematic differences in their stable isotope composition. At this specific location, the direction of exposure seems to be an important factor. The differences were likely caused by isotopic enrichment during recharge in warm weather conditions in south-exposed valley flanks compared to the opposite, north-exposed valley flanks.

Advances in Stable Isotope Monitoring of CO2 Under Elevated Pressures, Temperatures and Salinities: Selected Results from the Project CO2ISO-LABEL

The BMBF project CO2ISO-LABEL (Carbon and Oxygen ISOtopes under extreme conditions LABoratory EvaLuations for CO2-storage monitoring) investigated stable isotope methods in laboratory studies for transferral to carbon capture and storage (CCS) field sites including enhanced gas and oil recovery (EGR and EOR). The isotope composition of injected CO2 and water are useful tracers for migration and water-rock-gas interactions during such operations. However, quantification of carbon and oxygen equilibrium isotope effects at elevated pressures and temperatures are so far scarce. They thus need more investigations under p/T conditions that are characteristic for reservoirs and overlying aquifers. With this, the main objective of the project was to improve stable carbon and oxygen isotope methods for monitoring CO2 storage sites and their impact of injected CO2 on reservoir geochemistry under controlled laboratory settings. An important finding was that isotope fractionations of carbon between CO2 and dissolved inorganic carbon (DIC) were not significantly different from each other in experiments with pure CO2 and pressures between 59 and 190 bar. Furthermore, influences of rock types (limestone, dolomite and sandstone) and fluid salinities were found to be negligible for carbon isotope fractionation between CO2 and DIC. Another finding was that water oxygen isotope ratios changed systematically in response to different CO2/H2O molar ratios in closed system equilibration experiments. This helps to reconstruct the amounts of CO2 that equilibrated with formation waters. Results of the project will enable better assessment of geochemical conditions in underground carbon storage sites or other subsurface systems where large amounts of CO2 interact with water and rocks.

Stable isotope mass balances versus concentration differences of dissolved inorganic carbon – implications for tracing carbon turnover in reservoirs

ABSTRACT The aim of this study was to identify sources of carbon turnover using stable isotope mass balances. For this purpose, two pre-reservoirs in the Harz Mountains (Germany) were investigated for their dissolved and particulate carbon contents (dissolved inorganic carbon (DIC), dissolved organic carbon, particulate organic carbon) together with their stable carbon isotope ratios. DIC concentration depth profiles from March 2012 had an average of 0.33 mmol L–1. Increases in DIC concentrations later on in the year often corresponded with decreases in its carbon isotope composition (δ13CDIC) with the most negative value of –18.4 ‰ in September. This led to a carbon isotope mass balance with carbon isotope inputs of −28.5 ‰ from DOC and −23.4, −31.8 and −30.7 ‰ from algae, terrestrial and sedimentary matter, respectively. Best matches between calculated and measured DIC gains were achieved when using the isotope composition of algae. This shows that this type of organic material is most likely responsible for carbon additions to the DIC pool when its concentrations and δ13CDIC values correlate negatively. The presented isotope mass balance is transferable to other surface water and groundwater systems for quantification of organic matter turnover.