Luis Araguas-Araguas

Stable isotope composition of precipitation over southeast Asia

Spatial and temporal variability of the stable isotope composition of precipitation in the southeast Asia and western Pacific region is discussed, with emphasis on the China territory, based on the database of the International Atomic Energy Agency/World Meteorological Organization Global Network Isotopes in Precipitation and the available information on the regional climatology and atmospheric circulation patterns. The meteorological and pluviometric regime of southeast Asia is controlled by five different air masses: (1) polar air mass originating in the Arctic, (2) continental air mass originating over central Asia, (3) tropical-maritime air mass originating in the northern Pacific, (4) equatorial-maritime air mass originating in the western equatorial Pacific, and (5) equatorial-maritime air mass originating in the Indian Ocean. The relative importance of different air masses in the course of a given year is modulated by the monsoon activity and the seasonal displacement of the Intertropical Convergence Zone (ITCZ). Gradual rain-out of moist, oceanic air masses moving inland, associated with monsoon circulation, constitutes a powerful mechanism capable of producing large isotopic depletions in rainfall, often completely overshadowing the dependence of δ 18 O and δ 2 H on temperature. For instance, precipitation at Lhasa station (Tibetan Plateau) during rainy period (June-September) is depleted in 18 O by more than 6 ‰ with respect to winter rainfall, despite of 10°C higher surface air temperature in summer. This characteristic isotopic imprint of monsoon activity is seen over large areas of the region. The oceanic air masses forming the two monsoon systems, Pacific and Indian monsoon, differ in their isotope signatures, as demonstrated by the average δ 18 O of rainfall, which in the south of China (Haikou, Hong Kong) is about 2.5‰ more negative than in the Bay of Bengal (Yangoon). Strong seasonal variations of the deuterium excess values in precipitation observed in some areas of the studied region result from a complete reversal of atmospheric circulation over these areas and changing source of atmospheric moisture. High d-excess values observed at Tokyo and Pohang during winter (15-25‰) result from interaction of dry air masses from the northern Asian continent passing the Sea of Japan and the China Sea and picking up moisture under reduced relative humidity. The isotopic composition of precipitation also provides information about the maximum extent of the ITCZ on the continent during summer.

Deuterium and oxygen-18 isotope composition of precipitation and atmospheric moisture

The stable isotopes of oxygen and hydrogen incorporated in the water molecule ( 18 O and 2 H) have become an important tool not only in Isotope Hydrology, routinely applied to study the origin and dynamics of surface and groundwaters, but also in studies related to atmospheric circulation and palaeoclimatic investigations. A proper understanding of the behaviour of these tracers in the water cycle is required for a meaningful use of these tools in any of these disciplines. Our knowledge of the vertical distribution and the factors controlling the stable isotope ratios of oxygen and hydrogen in atmospheric moisture derives from a limited number of observations and vertical profiles in the atmosphere. An international programme jointly operated by the International Atomic Energy Agency (IAEA) and the World Meteorological Organization (WMO), and operational since 1961. has resulted in the development of a dedicated database to monitor isotope ratios in precipitation in more than 500 meteorological stations world-wide. The main features of the spatial and temporal variations of stable isotope ratios of oxygen and hydrogen in precipitation and atmospheric moisture at the global scale are presented based on the analysis of limited data on water vapour, data obtained by the Global Network for Isotopes in Precipitation IGNIP) and the few observations at high latitudes.

Isotope effects accompanying vacuum extraction of soil water for stable isotope analyses

Abstract The vacuum distillation method of extracting soil water for stable isotope analysis was tested for three different types of soil characterized by high water content: (1) pure sand, (2) cambisol with high organic matter content, developed on calcareous sandstone under temperate climatic conditions (Austria), and (3) tropical latosol poor in organic matter, developed on sandy clay sediment (Brazil). The method yields accurate and reproducible results for sand, provided that more than 98% of the original soil water is extracted. The time required for complete extraction is a function of sample size and the applied extraction temperature. Column experiments with the clayey soils revealed existence of a weakly bound, easily exchangeable pool of water which is isotopically different from the mobile water. The experiments showed that the extracted soil water is depleted in both deuterium and oxygen-18 by 5–10% and 0.3–0.5%, respectively, when compared with the percolate (mobile water). This depletion depends strongly on the soil type. The reproducibility for replicate extractions of soil water from clayey soils is around ±3% and 0.3% for δD and δ 18 O, respectively.

Chapter 7 – Isotopes in Groundwater Hydrology

Publisher Summary The concentration of stable isotopes in groundwater depends mainly on the origin of the water. The concentration of radioactive isotopes and dissolved compounds in groundwater depends on the initial concentration and residence time of groundwater in the aquifer. These two factors determine the amount of radioactive isotope(s) decayed, removed, or added during water-rock processes. As geochemical tools, stable and radioactive environmental isotopes provide information on the geochemical processes operating on groundwater and on the hydrogeological characteristics of aquifers. Information provided by environmental isotopes is also useful in modeling groundwater systems. This chapter discusses the scientific background of applications of environmental isotope techniques to groundwater hydrology. The stable isotope composition of groundwater reflects that of the precipitation in the recharge area that seeps through the soil and the unsaturated zone to reach the water table. Stable isotopes are often used to identify groundwater recharge by rivers and lakes. These water bodies frequently have isotopic compositions different from that of precipitation over the study area.

Lower Groundwater 14C Age by Atmospheric CO2 Uptake During Sampling and Analysis

Uptake of atmospheric CO₂ during sample collection and analysis, and consequent lowering of estimated ages, has rarely been considered in radiocarbon dating of groundwater. Using field and laboratory experiments, we show that atmospheric CO₂ can be easily and rapidly absorbed in hyperalkaline solutions used for the extraction of dissolved inorganic carbon, resulting in elevated ¹⁴C measurements. Kinetic isotope fractionation during atmospheric CO₂ uptake may also result in decrease of δ¹³C, leading to insufficient corrections for addition of dead carbon by geochemical processes. Consequently, measured ¹⁴C values of groundwater should not be used for age estimation without corresponding δ¹³C values, and historical ¹⁴C data in the range of 1 to 10% modern Carbon should be re-evaluated to ensure that samples with atmospheric contamination are recognized appropriately. We recommend that samples for ¹⁴C analysis should be collected and processed in the field and the laboratory without exposure to the atmosphere. These precautions are considered necessary even if ¹⁴C measurements are made with an accelerator mass spectrometer.

Global Hydrological Isotope Data and Data Networks

Isotopes of light elements constitute a set of powerful and widely used environmental tracers that often provide unique information about hydrological, climatological, and ecological processes. Environmental isotopes are extensively used in groundwater and surface water hydrology, palaeoclimatic reconstructions, atmospheric circulation processes, ocean dynamics, archaeology, palaeontology, anthropology, ecology, food webs, forensics and food authentication. Basic data on spatial and temporal distribution of isotopes at varying scales in the different components of the water cycle are required for a meaningful application of these tracers. A major source of isotope data on a global scale has been provided since the 1960s by the International Atomic Energy Agency (IAEA), which collects and disseminates isotope data and related hydrological information obtained as part of global or regional monitoring programmes and isotope hydrology studies. Available isotope data are gathered and compiled through global networks such as the global network of isotopes in precipitation (GNIP); global network of isotopes in rivers (GNIR); and moisture isotopes in biosphere and atmosphere (MIBA) network. In addition, global isotope data from surface waters and groundwaters are also being compiled. Other important hydrological isotope databases not covered by these networks are the Global Seawater Oxygen-18 Database; and GNIP-Antarctica, an extensive data set containing isotope composition of samples collected in Antarctic snow pits and ice cores. This chapter reviews the current status of and the basic information provided by global isotope networks and databases, and includes some examples of how such data are used to understand regional- to global-scale processes.

A simple polymer electrolyte membrane system for enrichment of low-level tritium (3H) in environmental water samples

ABSTRACT Tritium (3H) is an essential tracer of the Earths water cycle; yet widespread adoption of tritium in hydrologic studies remains a challenge because of analytical barriers to quantification and detection of 3H by electrolytic pre-concentration. Here, we propose a simple tritium electrolytic enrichment system based on the use of solid polymer electrolyte membranes (PEMs) that can be used to enrich 3H in 250–3000 mL environmental water samples to a 10-mL final volume. The IAEA PEM-3H system reported here can produce high enrichment factors (>70-fold) and, importantly, removes some of the deterrents to conventional 3H enrichments methods, including the use of toxic electrolysis and neutralization chemicals, spike standards, a complex electrolysis apparatus that requires extensive cooling and temperature controls, and improves precision by eliminating the need for tracking recovery gravimetrics. Preliminary results with varying operating conditions show 3H enrichments to 70-fold and higher are feasible, spanning a wide range of tritium activities from 5 to 150 TU with a precision of ∼4.5 %. Further work is needed to quantify inter-sample memory and to establish lower 3H detection limits. The IAEA PEM-3H system is open source, with 3-D CAD and design files made freely available for adoption and improvement by others.