[Show abstract][Hide abstract] ABSTRACT: During the last five decades, isotope concentrations (O-18, D, tritium) have been extensively measured in precipitation, surface- and ground-waters to derive information on residence times of water in aquifers and rivers, recharge processes, and groundwater dynamics. The unique properties of the isotopes of the water molecule as tracers are especially useful for understanding the retention of water in river basins, which is a key parameter for assessing water resources availability, addressing quality issues, investigating interconnections between surface- and ground-waters, and for predicting possible hydrological shifts related to human activities and climate change. Detailed information of the spatial and temporal changes of isotope contents in precipitation at a global scale was one of the initial aims of the Global Network of Isotopes in Precipitation (GNIP), which has provided a detailed chronicle of tritium and stable isotope contents in precipitation since the 1960s. Accurate information of tritium contents resulting of the thermonuclear atmospheric tests in the 1950s and 1960s is available in GNIP for stations distributed world-wide. Use of this dataset for hydrological dating or as an indicator of recent recharge has been extensive in shallow groundwaters. However, its use has been more limited in surface waters, due to the absence of specific monitoring programmes of tritium and stable isotopes in rivers, lakes and other surface water bodies. The IAEA has recently been compiling new and archival isotope data measured in groundwaters, rivers, lakes and other water bodies as part of its web based Water Isotope System for Data Analysis, Visualization and Electronic Retrieval (WISER). Recent additions to the Global Network of Isotopes in Rivers (GNIR) contained within WISER now make detailed studies in rivers possible. For this study, we are re-examining residence time estimates for the Danube in central Europe. Tritium data are available in GNIR from 15 Danube monitoring sites in Germany, Austria, Slovakia, Hungary, Slovenia and Serbia. Most of these sites have continuous stable isotope and tritium records of over 10 years. The longest and most complete record of isotopes in precipitation and the Danube is from Vienna, which contains continuous tritium and stable isotope records since the 1960s. Previous estimates of residence time using tritium in the upper Danube are about 3-5 years (Rank et al., 1998, Yurtsever, 1999). However, these estimates were based on a tritium record up to 1995 and some of the parts of the observed time series were not represented well by the models. We are now re-evaluating the upper Danube residence time using a complete record covering the entire tritium transient created by atmospheric nuclear weapons testing (1964-2005). Several combinations of lumped parameter models are being tested using MULTIS and LUMPY. The models assume two main water components in parallel; a "fast" component that represents water with a short residence time (less than one year), resulting from recent precipitation and fast runoff, and a "slow" or "old" component representing discharge of older groundwaters to the river. Preliminary results obtained during this exercise, as well as those determined using other environmental tracers, are providing new insights into the age distribution of water in the upper Danube. Initial calculations with the complete tritium record for Vienna suggest that the mean residence time is substantially older than previous estimates. This study also demonstrates the value of the GNIP/GNIR/WISER dataset for examining dynamics of surface water systems.
[Show abstract][Hide abstract] ABSTRACT: Increases in world population and water needs have led to greater demands on groundwater as a key resource to provide sufficient water to meet the growing needs for agricultural, industrial, and domestic purposes. Increasing groundwater withdrawals on shallow systems may have adverse effects such as decreases of baseflow in rivers, threatening the existence of wetlands or reducing water quality. In other cases, exploitation of fossil groundwater resources leads to marked lowering of the water table. To ensure a rational management of these resources and minimal long term negative effects, a clear understanding of the hydrological conditions in the area is required. Global climate change brings another factor to consider when planning future water requirements. For these reasons it is important to develop a more comprehensive and predictive understanding of aquifer systems, specifically the recharge rates, flow paths, and residence times of groundwater. Studies should aim to ensure accurate water resource assessments and the development of effective strategies for sustainable withdrawal and protection. The assessment of aquifer systems under different climatic and geological conditions can be effectively conducted with isotope age dating of groundwater, which may be the only means of building relevant information in many instances. A lack of easy access to analytical facilities and discordant ages estimated from multiple isotope tracers have been two of the important impediments in the wider use of age dating tools. The IAEA has recently established a helium/noble gas isotope facility to increase the availability of groundwater residence time data from shallow aquifers and river baseflow. A number of pilot studies have been conducted to demonstrate the value of the tritum-helium-3 dating technique for recently recharged groundwaters (< 50 years). In the case of large sedimentary aquifer systems, groundwater ages beyond the limit of radiocarbon are commonly found. The combined use of radioactive tracers such as carbon-14, chlorine-36 and krypton-81, and numerical hydrodynamic modelling showed inconsistencies in residence time estimates made before the extensive use of accelerator mass spectrometry (AMS) dating methods. Most of the inconsistencies in the use of radiocarbon were the result of contamination with atmospheric CO2 during sampling by the conventional precipitation method. Duplicate analysis of radiocarbon by both sampling methods in selected large aquifers showed consistent differences, up to 10 pMC. The lack of radiocarbon in some deep groundwaters has been confirmed by other long-lived radioisotopes. We have re-evaluated groundwater ages of several of these aquifers coupled with 3-D groundwater models. Our study has demonstrated the usefulness of conservative long-lived radioisotopes for assessing groundwater dynamics of large aquifers containing fossil groundwaters, such as the Nubian Sandstone Aquifer System in Africa and the Guarani aquifer in South America. This presentation will discuss these and related IAEA initiatives in the field of isotope age dating of both young and old groundwaters under various climatic conditions.
[Show abstract][Hide abstract] ABSTRACT: 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.