A Review on the Application of Isotopic Techniques to Trace Groundwater Pollution Sources within Developing Countries

Owing to a lack of efficient solid waste management (SWM) systems, groundwater in most developing countries is found to be contaminated and tends to pose significant environmental health risks. This review paper proffers guidelines on the application of isotopic techniques to trace groundwater pollution sources from data spanning from 2010 to 2020 within developing countries. Earlier groundwater studies in those countries were mainly focused on using hydrochemical and geophysical techniques. The limitation of these techniques is that they can only monitor the concentration of pollutants in the water bodies and possible leachate infiltration but cannot determine the specific sources of the pollution. Stable isotopes of δ18O, δ2H and δ13C can confirm leachate migration to water bodies due to methanogenesis. The high tritium in landfill leachates is useful to identify leachate percolation in groundwater. The δ15N technique has been used to distinguish between synthetic and organic nitrogen sources but its application is limited to differentiating between atmospheric vs. inorganic nitrogen sources. The use of a dual isotope of δ15N–NO3− and δ18O–NO3− is beneficial in terms of identifying various sources of nitrogen such as atmospheric and inorganic fertilizers but is yet to be used to differentiate between nitrogen pollution sources from dumpsites, sewage and animal manure. The coupling of the 11B isotope with δ15N–NO3− and δ18O–NO3− and other hydrochemical parameters has proven to be effective in distinguishing between nitrate fertilizer, animal manure, seawater contamination and sewage. Therefore, in areas affected by agricultural activities, landfill leachates, domestic or sewage effluent and seawater intrusion, it is incumbent to couple hydrochemical (Cl−, NO3−, B, DO) and isotope techniques (δ18O, 2H, δ13C, δ18O–NO3−, δ15N–NO3−, δ11B and 3H) to effectively determine pollution sources of groundwater in developing countries. The foregoing review will provide guidelines for studies that may aim to critically distinguish between seawater intrusion, dumpsites, sewage and septic leachates.

The dual C and O isotope – gas exchange model: A concept review for understanding plant responses to the environment and its application in tree rings

The combined study of C and O isotopes in plant organic matter has emerged as a powerful tool for understanding plant functional response to environmental change. The approach relies on established relationships between leaf gas exchange and isotopic fractionation to derive a series of model scenarios that can be used to draw inferences about changes in photosynthetic assimi-lation and stomatal conductance driven by changes in environmental parameters (CO2, water availability, air humidity, temperature, nutrients). We review the mechanistic basis for model and research to date, and discuss where isotopic observations don’t match our current under-standing of plant physiological response to environment. We demonstrate that 1) the model has been applied successfully in many, but not all studies, and 2), while originally conceived for leaf isotopes, the model has been applied extensively to tree ring isotopes in the context of tree phys-iology and dendrochronology. Where isotopic observations deviate from physiologically plau-sible conclusions, this mismatch between gas-exchange and isotope response provides valuable insights on underlying physiological processes. Overall, we found that isotope responses can be grouped into situations of increasing resource limitation versus higher resource availability. Thus, the dual isotope model helps to interpret plant responses to a multitude of environmental factors.

Filter Feeding and Carbon and Nitrogen Assimilation of a Freshwater Bivalve (Unio douglasiae) on a Toxic Cyanobacterium (Microcystis aeruginosa)

We investigated the possible intake of toxic cyanobacteria (Microcystis aeruginosa) as a nutrient resource for a filter-feeder bivalve (Unio douglasiae) based on the measurement of feeding and assimilation rates of carbon and nitrogen in a limited space with no current for 72 h using 13C and 15N dual isotope tracers. With high clearance rates, the unionid rapidly removed Microcystis cells within 24 h, but only a small amount of carbon and nitrogen were incorporated into the tissues. Even with the low assimilation rates, the mussels showed more favorable uptake of carbon than of nitrogen from toxic Microcystis water, and of tissues, the gills and gut accumulated more carbon and nitrogen than the muscle and mantle. Collectively, our findings indicate that although Unio douglasiae effectively uptake toxic Microcystis cells, they can assimilate only low amounts of nutrients into tissues within three days, despite a non-flowing system.

Hydrogen and carbon isotope fractionation factors of aerobic methane oxidation in deep-sea water

Isotope fractionation factors associated with various biogeochemical processes are important in ensuring the reliable use of isotope tracers in biogeosciences at large. Methane is a key component of the subsurface biosphere and a notable greenhouse gas, making the accurate evaluation of methane cycles, including microbial methanotrophy, imperative. Although the isotope fractionation factors associated with methanotrophy have been examined under various conditions, the dual-isotope fractionation factors of aerobic methanotrophy in oxic seawater remain unclear. Here, we investigated hydrogen and carbon isotope ratios of methane as well as the relevant biogeochemical parameters and microbial community compositions in hydrothermal plumes in the Okinawa Trough. Methanotrophs were found to be abundant in plumes above the Hatoma Knoll vent site, and we succeeded in simultaneously determining hydrogen and carbon isotope fractionation factors associated with the aerobic oxidation of methane (εH=49.4±5.0 ‰, εC=5.2±0.4 ‰) – the former being the first of its kind ever reported. This εH value is comparable with values reported from terrestrial ecosystems but clearly lower than those from aerobic and anaerobic methanotroph enrichment cultures, as well as incubations of methanotrophic isolates. The covariation factor between δ13CCH4 and δDCH4, Λ (9.4 or 8.8 determined using two different methods), was consistent with those from methanotrophic isolate incubations. These values are valuable for understanding dynamics of methane cycling in the marine realm, and future applications of the approach to other habitats with methanotrophic activity will help reveal whether the small εH value observed is a ubiquitous feature across all marine systems.

Diagnostic value of simultaneous 99mTc-sestamibi/123I-BMIPP imaging parameters for predicting the improvement of left ventricular wall motion after acute myocardial infarction using CZT SPECT system

Background Although the presence of 99mTc-sestamibi/123I-BMIPP-mismatch, the reverse redistribution (RR) of 99mTc-sestamibi and RR of 123I-BMIPP in patients with acute myocardial infarction (AMI) are known to significant markers for predicting the improvement of LV wall motion in the infarcted territory in chronic phase, few studies were performed to analyze them by simultaneous dual-isotope imaging using cadmium-zinc-telluride (CZT) SPECT system. Purpose The purpose of this study was to evaluate whether the presence of 99mTc-sestamibi/123I-BMIPP-mismatch or RR of 99mTc-sestamibi, RR of 123I-BMIPP make better prediction of the improvement of LV wall motion in the infarcted territory. Methods We evaluated 42 consecutive patients with AMI who had undergone both dual-isotope SPECT in acute phase and stress myocardial SPECT using 99mTc-tracers in chronic phase by Discovery NM530c. The presence of 99mTc-sestamibi/123I-BMIPP-mismatch, RR of 99mTc-sestamibi and RR of 123I-BMIPP were determined using traditional definition. The improvement of LV wall motion in the infarcted territory from acute phase to chronic phase was assessed using QGS. Results Of 42 patients, the improvement of LV wall motion in the infarcted territory from acute phase to chronic phase was found in 29 patients. The presence of 99mTc-sestamibi/123I-BMIPP-mismatch and RR of 99mTc-sestamibi and RR of 123I-BMIPP were significantly linked to predict the improvement of LV wall motion (p=0.0001, p=0.0001 and p=0.002, respectively). To predict the improvement of LV wall motion in the infarcted territory in chronic phase, the presence of 99mTc-sestamibi/123I-BMIPP-mismatch showed sensitivity of 93%, specificity of 85% and accuracy of 91%, while RR of 99mTc-sestamibi and RR of 123I-BMIPP had sensitivities of 72%, 48%, specificities of 85%, 100% and accuracies of 76%, 64%, respectively. The multivariate discriminant analysis revealed that the combination of 99mTc-sestamibi/123I-BMIPP-mismatch, RR of 99mTc-sestamibi and RR of 123I-BMIPP best predicted the improvement of LV wall motion in the infarcted territory in chronic phase with sensitivity of 93%, specificity of 85% and accuracy of 91% (chi-square=40.6), compared with RR of 99mTc-sestamibi and RR of 123I-BMIPP only (sensitivity 79%, specificity 85% and accuracy of 81%, chi-square=16.9). Conclusions The addition of 99mTc-sestamibi/123I-BMIPP-mismatch on RR of 99mTc-sestamibi and RR of 123I-BMIPP in patients with AMI, help better predict the improvement of LV wall motion in the infarcted territory in chronic phase. FUNDunding Acknowledgement Type of funding sources: None.

Elucidation of the Structure of Lignin-Carbohydrate Complexes in Ginkgo CW-DHP by 13C-2H Dual Isotope Tracer

To elucidate the chemical linkages between lignin and carbohydrates in ginkgo cell walls, 13C-2H-enriched cell wall-dehydrogenation polymers (CW-DHP) were selectively prepared with cambial tissue from Ginkgo biloba L. by feeding D-glucose-[6-2H2], coniferin-[α-13C], and phenylalanine ammonia-lyase (PAL) inhibitor. The abundant detection of 13C and 2H confirmed that D-glucose-[6-2H2] and coniferin-[α-13C] were involved in the normal metabolism of ginkgo cambial cells that had been effectively labelled with dual isotopes. In the ginkgo CW-DHP, ketal and ether linkages were formed between the C-α of lignin side chains and carbohydrates, as revealed by solid state CP/MAS 13C-NMR differential spectroscopy. Furthermore, the DMSO/TBAH ionic liquids system was used to fractionate the ball-milled CW-DHP into three lignin-carbohydrate complex (LCC) fractions: glucan–lignin complex (GL), glucomannan–lignin complex (GML), and xylan–lignin complex (XL). The XRD determination indicated that the cellulose type I of the GL was converted into cellulose type II during the separation process. The molecular weight was in the order of Ac-GL > Ac-GML > XL. The 13C-NMR and 1H-NMR differential spectroscopy of 13C-2H-enriched GL fraction indicated that lignin was linked with cellulose C-6 by benzyl ether linkages. It was also found that there were benzyl ether linkages between the lignin side chain C-α and glucomannan C-6 in the 13C-2H-enriched GML fraction. The formation of ketal linkages between the C-α of lignin and xylan was confirmed in the 13C-2H-enriched XL fraction.

Laboratory Experiments to Evaluate the Effectiveness of Persulfate to Oxidize BTEX in Saline Environment and at Elevated Temperature Using Stable Isotopes

In this study, batch experiments were carried out to investigate the effectiveness of persulfate (PS) as an oxidant agent to remediate benzene, toluene, ethylbenzene, and xylenes (BTEX) in saline environments and at high water temperatures (30°C). This hydrological setting is quite common in contaminated groundwater aquifers in Middle Eastern countries. In general, increasing the system temperature from 10 to 30°C greatly enhanced the effectiveness of PS, and resulted in a faster oxidation rate for the target contaminants. When PS was added to the reactor at 30 °C, the targeted contaminants were almost completely oxidized over a 98-day reaction period. During the chemical oxidation of the BTEX, carbon and hydrogen isotope fractionations were monitored and utilized as potential proof of contaminant degradation. The calculated carbon-enrichment values were −1.9‰ for benzene, −1.5‰ for ethylbenzene and toluene, −0.4‰ for ρ,m-xylene, and -1.4‰ for o-xylene, while the hydrogen enrichment values were −9.5‰, -6.8‰, −2.1‰, −6.9‰, and −9.1‰, respectively. In comparison with other processes, the hydrogen and carbon isotope fractionations during the chemical oxidation by PS were smaller than the isotope fractionations resulting from sulfate reduction and denitrification. This observation demonstrates the differences in the transformation pathways and isotope fractionations when compounds undergo chemical oxidation or biodegradation. The distinct trend observed on the dual isotope plot (Δδ13C vs. Δδ2H) suggests that compound-specific isotope analysis can be utilized to monitor the chemical oxidation of BTEX by PS, and to distinguish treatment zones where PS and biodegradation technologies are applied simultaneously.