Week 2 in the Geochemistry lab (JUNE 17-21, 2019)

Me flushing out the sample with helium and monitoring its flow.

Today Mike and I discussed GC Gas Chromatography which is a separation science that takes a complex mixture and break it down into it components.

We first started off the day by manually injecting certified gas standards to see if the machine is properly calibrated.

Water can not get into the GC so a head space is needed. A head space is created in the sample by removing 8mL of solution from the sample and replacing it with helium. Henry’s law is used to recalibrate. The law states that the amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid.

C= kP

C is the solubility of a gas at a fixed temperature in a particular solvent (in units of M or mL gas/L)

k is Henry’s law constant (often in units of M/atm)

Pgas is the partial pressure of the gas (often in units of atm)

The gas runs through 4 different channels. 450GC: hydrocarbons, carbon dioxide and H2S, and a fixed gas channel. 456GC: Argon and oxygen. The results should be approximately 100% with a 5% error. The samples each took 7 minutes to run so I had plenty of time to do some readings in between each one.

Friday June 14, 2019- I met with Dr. Pauline Humez who is a research associate at the University of Calgary. We discussed the methane (CH4) formation, mitigation and fate in shallow aquifers. The purpose of her research is to characterize the groundwater resources in Alberta and record their vulnerability to potential contamination from development of unconventional hydrocarbon reservoirs. Migration of fugitive gas into shallow aquifers is viewed as a potential risk associated with the exploitation of coalbed methane and shale gas reservoirs. One challenge/objective is to distinguish for instance between natural in-situ production of methane (biogenic, due to microbial breakdown of organic matter) or methane migration from deep sources due to human activities(thermogenic, due to thermal breakdown of organic matter).

Redox ladder concept. Microbial formation of CH4 can only be produced when the dissolved O2 is consumed, nitrate(NO3-)is removed by denitrification, sulphate(SO4^2-) is reduced

In-situ formation of methane (methanogenesis) in shallow aquifers requires highly reducing conditions consistent with the redox ladder concept (see figure). Hence, analyzing a variety of water chemistry parameters can provide important clues to whether in situ formation of methane within a shallow aquifer is possible. Isotopes analyses on methane ( 𝛿13CCH4; ( 𝛿2HCH4) and other dissolved groundwater constituents such as dissolved inorganic carbon ( 𝛿13CDIC), nitrate ( 𝛿15NNO3, 𝛿18ONO3) and sulfate ( 𝛿18OSO4, 𝛿18OSO4) can provide important additional constraints about redox conditions and methanogenesis processes (CO2 reduction or acetate fermentation) or methane sinks processes such as methane oxidation.

Sample preparation for analyses of sulphur (S) and oxygen (O) isotope ratios of sulfate (SO4): For the water samples I added 5mL of Hydrochloric (HCl) acid and 10mL of Barium Chloride (BaCl2)and 400mL of water sample into a beaker. If the solution becomes cloudy from Barium Sulfate(BaSO4) precipitating then it is evident that the water sample had a significant amount of sulfate and therefore according to the redox ladder concept is not compatible with the presence of elevated concentrations of methane. If the beaker remained clear then it was placed onto a hot plate to let the water evaporates and refill the beaker with water samples (up to a liter) if there is only a little amount of sulfate present (minimum is 4 mg/L of sulphate to form a BaSO4 precipitate). Low/negligible amount of sulfate would thus be favourable with the presence of elevated methane. These “prediction” on occurrence of low or elevated concentrations of methane have been confirmed with the gas compositions analyses performed with the Gas Chromatography (GC) technique.

The BaSO4 precipitates obtained from the different samples were filtered and then dried and stored until the mass spectrometric analyses.

The access S and O isotopes ratios of sulfate will permit to identify the source of sulfate (ex: pyrite oxidation) or if sulfate has been reduced (in case of low sulfate concentrations). Such sulfate reduction could be couple with methane oxidations processes.

Preparation for carbon isotope ratios of DIC (Dissolved Inorganic Carbon) analyses: Vials containing 250 μl phosphoric acid (H2PO3) are first flushed with helium.We monitored the flow and made sure it was near between 70 and 100 mL/min.Then, the water samples are injected into the vials. Such acid conditions will permit to convert the DIC into CO2 gas that will then be analyzed with the mass spectrometric technique. The DIC isotope ratios would permit to identify the source of DIC and processes that have generated or affected DIC. Example very positive 𝛿13CDIC values (> +10 ‰) could be evidence of in situ methane formation.

Unfortunately I couldn’t help her with the rest of the experiment that would look at the isotope content because I was going to be busy that afternoon with Geoscout training.

That afternoon I went downtown to GeoLOGIC to take part in a training session on how to use their GeoSCOUT program.https://www.geologic.com/ GeoSCOUT is an exploration system with many features such as mapping and cross-section tools. It also has software that lets us handle data and analyze. Unfortunately my program kept freezing so I had to watch the demo instead of following along because I had to start over.

Geochemical Resource Characterization of Alberta Groundwater. (n.d.). Retrieved from https://albertainnovates.ca/wp-content/uploads/2018/06/Nightingale-Geochem-Characterization-of-AB-Groundwater.pdf

Humez P, Mayer B, Nightingale M, Becker V, Kingston A, Taylor S, Bayegnak G, Millot R, Kloppmann W. Redox controls on methane formation, migration and fate in shallow aquifers. Hydrology and Earth System Sciences Discussions. 2016:1–33. doi:10.5194/hess-2016-85

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