[en] The present chapter offers a qualitative or semi-quantitative step towards a synthesis of the information that the different tracers provide. It also offers some criteria that can be used to help assess the reliability of selected tracer data in evaluating tracer model ages from measurements of concentrations of multiple environmental tracers in the system. In most of the literature on isotope hydrology, the term ‘apparent age’ is used instead of ‘tracer model age’, and within this chapter the two terms are considered synonymous. This approach testing reliability of tracer data is only a first step and performs a black and white selection of the tracer data. Some consistency tests are performed, and for data passing these tests there is at least no obvious reason known that the tracer model age is not a valid description. For data that fail these tests, it is obvious that a straightforward calculation of tracer model ages will not give the intended result.

[en] This chapter provides background on the behaviour of uranium isotopes in groundwater and shows that uranium isotope disequilibrium presents a method that has the potential to play an important role within the concerted application of techniques to date old groundwater. Depending on the geochemistry of the aquifer system, the potential dating range extends from about 10 ka up to 1 M a, which overlaps the ¹⁴C and the ³⁶Cl/⁸¹Kr dating ranges. Uranium isotopes can provide independent information relevant to groundwater age that complements other dating approaches and may help resolve ambiguities in data interpretation. Moreover, uranium isotopes in groundwater have the potential for quantifying geochemical parameters of the water–rock interaction and represent natural analogues for radioactive waste. Any multitracer application to date old groundwater should integrate uranium isotope measurements, which can easily be afforded by the use of modern mass spectrometric techniques.

[en] Any estimate of ‘age’ of a groundwater sample based on environmental tracers requires some form of geochemical model to interpret the tracer chemistry (chapter 3) and is, therefore, referred to in this chapter as a tracer model age. the tracer model age of a groundwater sample can be useful for obtaining information on the residence time and replenishment rate of an aquifer system, but that type of data is most useful when it can be incorporated with all other information that is known about the groundwater system under study. groundwater fl ow models are constructed of aquifer systems because they are usually the best way of incorporating all of the known information about the system in the context of a mathematical framework that constrains the model to follow the known laws of physics and chemistry as they apply to groundwater flow and transport.

[en] In many parts of the world, groundwater constitutes a major source of water for agricultural, energy, industrial and urban use, and it is expected to play an even greater role in the next decades on a global scale. The rising importance of groundwater is a result of increasing water demands deriving from population growth and concerns about the impact of predicted climate change on the hydrological cycle. Unfortunately, in many cases, water officials and managers lack the knowledge of the local groundwater resources required to ensure adequate and long term access to available water resources. In order to adopt adequate policies and to share resources with limited accessibility, sound and comprehensive information on the amount and condition of existing water resources is required. New scientific, technical, social and legal questions and a growing number of conflicts and issues regarding water usage require a better understanding of the movement, origin and age of groundwater. Isotope hydrology methods have great potential to provide the hydrogeological information required to rapidly and effectively assess and map groundwater resources. For several decades, one of the major tools for obtaining information about groundwater origin, and its properties and movement has been the use of isotopes, which has often provided insights not available using other techniques. Information on groundwater age is required to address aspects such as recharge rates and mechanisms, resource renewability, flow rate estimation in aquifers and vulnerability to pollution, especially when dealing with shared water resources. Age information, mainly provided by radionuclides and modelling, is considered highly relevant for validating conceptual flow models of groundwater systems, calibrating numerical flow models and predicting the fate of pollutants in aquifers. Isotope tracers are now used to study groundwater age and movement, covering time spans from a few months up to a million years. The understanding of groundwater occurrence and movement in large continental basins has been a matter of debate among experts. Despite the large number of studies which have been carried out in the past, many open questions remain, and ideas and concepts are often revised based on new conceptual models, isotope and tracer analyses or water flow models. The book's 14 chapters explain what is currently understood about the use and application of radionuclides and related geochemical tracers and tools to assess groundwater age and movement over time spans beyond a few thousand years

[en] This book investigates applications of selected chemical and isotopic substances that can be used to recognize and interpret age information pertaining to ‘old’ groundwater (defined as water that was recharged on a timescale from approximately 1000 to more than 1 000 000 a). However, as discussed below, only estimates of the ‘age’ of water extracted from wells can be inferred. These groundwater age estimates are interpreted from measured concentrations of chemical and isotopic substances in the groundwater. Even then, there are many complicating factors, as discussed in this book. In spite of these limitations, much can be learned about the physics of groundwater flow and about the temporal aspects of groundwater systems from age interpretations of measured concentrations of environmental tracers in groundwater systems. This chapter puts the concept of ‘age’ into context, including its meaning and interpretation, and attempts to provide a unifying usage for the rest of the book.

[en] Chemical and isotopic patterns in groundwater can record characteristics of water sources, flow directions and groundwater age information. This hydrochemical information can be useful in refining conceptualization of groundwater flow, in calibration of numerical models of groundwater flow and in estimation of palaeo- and modern recharge rates. This case study shows how chemical and isotopic data were used to characterize sources and flow of groundwater in the Middle Rio Grande Basin, New Mexico, USA. The ¹⁴C model ages of the groundwater samples are on the tens of thousands of years timescale. These data changed some of the prevailing ideas about flow in the Middle Rio Grande Basin, and were used to improve a numerical model of the aquifer system.

[en] The radioactive isotope of carbon, radiocarbon (14C), was first produced artificially in 1940 by Martin Kamen and Sam Ruben, who bombarded graphite in a cyclotron at the Radiation Laboratory at Berkeley, CA, in an attempt to produce a radioactive isotope of carbon that could be used as a tracer in biological systems (Kamen (1963) [101]; Ruben and Kamen (1941) [102]). Carbon-14 of cosmogenic origin was discovered in atmospheric CO2 in 1946 by Willard F. Libby, who determined a half-life of 5568 a. Libby and his co-workers (anderson et al. (1947) [103]; Libby et al. (1949) [104]) developed radiocarbon dating of organic carbon of biological origin, which revolutionized research in a number of fields, including archaeology and quaternary geology/climatology, by establishing ages and chronologies of events that have occurred over the past approximately 45 ka.

[en] This chapter discusses some of the fundamental concepts, data needs and approaches that aid in developing a general understanding of a groundwater system. Principles of the hydrological cycle are reviewed; the processes of recharge and discharge in aquifer systems; types of geological, hydrological and hydraulic data needed to describe the hydrogeological framework of an aquifer system; factors affecting the distribution of recharge to aquifers; and uses of groundwater chemistry, geochemical modelling, environmental tracers and age interpretations in groundwater studies. Together, these concepts and observations aid in developing a conceptualization of groundwater flow systems and provide input to the development of numerical models of a flow system. Conceptualization of the geology, hydrology, geochemistry, and hydrogeological and hydrochemical framework can be quite useful in planning, study design, guiding sampling campaigns, acquisition of new data and, ultimately, developing numerical models capable of assessing a wide variety of societal issues — for example, sustainability of groundwater resources in response to real or planned withdrawals from the system, CO₂ sequestration or other waste isolation issues (such as nuclear waste disposal).

[en] Chlorine-36 has been widely used for dating old groundwater. The ³⁶Cl used for this purpose is produced in the atmosphere through the interaction of cosmic rays with argon atoms. The ³⁶Cl then mixes with ordinary atmospheric chloride (mostly derived from the ocean). The mixture is deposited on the land surface dissolved in rain or snow or as dry aerosols. Groundwater recharge carries the ³⁶Cl into the subsurface where the radiometric ‘clock’ is set. Chloride is conservative in the subsurface and, thus, the ³⁶Cl is rarely retarded with respect to the water velocity by adsorption or geochemical reactions. Groundwater age can be estimated using the radiometric decay equation and the decrease in ³⁶Cl from the amount in the recharge water. The main complication is variations in the Cl concentration of groundwater. This can potentially be due to variable evapotranspiration during recharge or to the addition of Cl in the aquifer. If the cause of Cl concentration variations is understood, the age calculation can be corrected to account for the process. Chlorine-36 dating is generally applicable to water in the age range 100 ka–1 Ma.

[en] The Milk river aquifer is a complex system in terms of origin and evolution of the chemical and isotopic composition of the groundwater. Fortunately, a wealth of physical and chemical data of the groundwater and the bonding shales was collected by numerous authors over a period of about 15 a that allowed applying various complementary approaches in dating the groundwater. Although these Milk river aquifer studies were carried out more than two decades ago, they can still be considered an outstanding example for dating of old groundwater.