Tracers in Hydrology

Christian Leibundgut, Piotr Maloszewski, Christoph Külls

• Publisher: Wiley
• Year: 2011
• ISBN:
  9781119965015
• Edition: 1
• Topic: Ciencias físicas
• Subtopic: Hidrología

Tracers in Hydrology and Water Research is a comprehensive overview of the application of natural and artificial tracers in hydrology and environmental research. Taking a unique approach by providing the reader with a systematic and state of the art description of natural and artificial tracers, the book also covers key analytical techniques and applications, and modern tracer methods in the context of systematic hydrology. Tracers have become a primary tool for process investigation, qualitative and quantitative system analysis and integrated resource management. This book will outline the fundamentals of the subject, and examine the latest research findings, clearly showing the entire process of tracer application through the inclusion of numerous integrated case studies.

As many techniques derive from different scientific disciplines (chemistry, biology, physics), the effort of compilation and integration into modern hydrology and environmental science research and application requires substantial continuity and experience, which certifies this group of authors.

This book will be an invaluable reference not only for students and researchers within the field of Hydrology and Hydrogeology but also for engineers and other tracer techniques applying users.

1

Introduction

‘Tracers in Hydrology’ defines the scientific field that aims at understanding the hydro-logic system by making use of environmental and artificial tracers and modelling. Tracing of water provides unique methods for a direct insight into the dynamics of surface and subsurface water bodies. The relevance of tracer techniques in hydrological investigations and in applied hydrology ensues from the astounding complexity of water flow in natural systems. How much runoff in rivers really stems from rainstorms? How does water flow through a hill-slope ora glacier? How large is the storage of water resources in aquifers? Where, how and when was the water found in an aquifer formed? Tracer techniques are a useful tool in understanding the transport processes and quantifying their parameters. Tracers help to identify and quantify the phase changes (evaporation, condensation, sublimation), shed light on the origin of pollution and assist in the respective remediation processes. The natural tracers constitute a tool of prime importance in the reconstruction of the climate during the Holocene period when studying ice cores, old groundwater and the unsaturated zone in arid and semiarid regions. Tracer methods are also a major tool for calibration and validation both of strategies in modelling catchment hydrology and hydrological models of groundwater systems. Furthermore, tracer approaches are commonly used to address issues like surface water–groundwater interactions, paleohydrology, water movement in very low permeable rocks, calibrating and validating numerical flow and transport models and evaluating vulnerability of water resources. Finally for Integrated Water Resource Management, tracer techniques have great potential as tracers that provide integrated information and can be very efficient in characterising complex systems in remote areas.

The empirical observation of flow and transport processes with tracers and the theoretical formulation of flow and transport processes depend on each other and have resulted in a beneficial coevolution of both approaches if adequately combined. Tracers provide empirical data of real and often unexpected flow patterns – models provide tools for flow and transport predictions.

The term ‘Tracerhydrology’ is used as a short expression for the use of tracers in hydrology understood as an advanced method that allows for an integrative investigation of the hydrologic system. It is not regarded merely as an isolated technique for solving particular problems of applied hydrology, although it certainly can be useful in those fields.

Figure 1.1 Tracerhydrology as a method of application tracers in water sciences understood as a holistic approach of hydrology and water research.

This book originated from the idea of interweaving knowledge from the fields of artificial and environmental tracers and of modelling, in order to present the options, opportunities and limits of tracers in hydrology to students, scientists, engineers and other users. In the following chapters the explanation of tracer and modelling basics builds a foundation for students and users to be able to understand the techniques, which are then applied to case studies of both specific applications and integrated studies. Herein, tracer techniques are described with regard to their relevance for advancing hydrological science and to their role in solving problems in applied hydrology. Students, scientists and consultants will find a wealth of information on tracers and modelling in order to introduce them to the field of tracerhydrology. A methodological chapter provides specific techniques such as the calculation of injection mass and the chloride method and also case studies dealing with the different approaches and problems of applied tracerhydrology (groundwater recharge).

Scientists can see the range of opportunities that tracer techniques offer through the variety of comprehensive case studies that are presented. Engineers and other users will find a large collection of work examples and may apply the methods described, for example tasks in integrated water resources management or the allocation of water supply protection zones, as well as many others. In this book the application of tracers in hydrology is understood basically as the integrated use of tracers in hydrology and therefore as a part of an integrated hydrological approach (Figure 1.1).

In Chapter 2 a detailed concept of tracerhydrology will be presented. The role of modelling in integrated tracerhydrology will be defined in a separate chapter. The combined application of tracerhydrology and modelling is presented by means of selected examples of applications in various hydrological compartments (glaciers, rivers, lakes, groundwater). The authors wish to present a textbook that starts from a simple and general overview and moves on to the more complex topics of tracerhydrology in order to facilitate an easy understanding by the readers, be they students, water research scientists, engineers or applied hydrologists.

The application of tracers in hydrology has a long tradition among the geo- and water sciences. After what were at first somewhat ‘trial and error’ – based experiments about 150 years ago a fascinating development began. Artificial salt and fluorescence tracers have been used for decades. In the 1950s a wide variety of new artificial tracers were included in tests designed to trace water, mainly in karst aquifers. At the same time a compelling new direction in tracerhydrology based on the use of natural, mainly isotopic tracers began to develop. Most of the fundamental principles had been developed during this phase. Stable isotopes have provided a major input into the study of hydrological processes such as runoff generation and runoff component separation as well as recharge and groundwater flow and are still at the centre of defining the conceptual models of hydrological processes. The role of isotopes in the validation of circulation models and response of ecosystems to climate change is not yet fully explored.

In addition to an increasing number of papers on tracerhydrology published in international hydrological journals, there are many publications on the use of tracers for water research issued by international organizations, such as (i) the IAHS (International Association on Hydrological Sciences), (ii) the symposia proceedings of the IAEA (International Atomic Energy Agency), and (iii) the proceedings and project volumes of the ATH (International Association of Tracers). These publications are an excellent resource for all matters concerning the methodological aspects and application of tracers.

Comprehensive presentations of large combined tracerhydrological studies are given in the reports of the ATH (International Association of Tracers). The focus of these investigations was on groundwater systems but the approach was holistic within the respective river basins. Increasingly, investigations on runoff generation and catchment modelling have adopted an integrated tracerhydrological approach.

Innovations in analytical techniques will provide new tools for tracerhydrology. There are trends towards reducing sample volumes, increasing the number of samples analysed, reducing detection limits and identifying new natural and industrial substances that can be used for tracer studies. Certainly, natural remediation and reactive transport processes will be explored increasingly with tracers. For the advance of hydrological science, empirical data provided by tracer methods have and will continue to play an important role. Further integration of experimental and theoretical approaches leading to an integration of tracers into soil water atmosphere transfer schemes and catchment and groundwater models, will provide additional means of validating the hydrological concepts.

2

The Integrated Concept of Tracers in Hydrology

2.1 System approach

The system analysis of watersheds and aquifers draws key insights from artificial and environmental tracer data. Artificial tracers help to understand flow processes, to estimate main hydrological system parameters and to visualize the movement and mixing of otherwise indiscernible distinct water volumes. Hence, they provide a tool for understanding and characterizing complex flow through the soil, on surfaces, in channels, through and along hill-slopes, in aquifers or in artificial systems. Environmental tracers have become key tools for estimating water resources in the catchment areas, for the reconstruction of hydrological processes from the past, in ungauged basins or for the integration of hydrological processes that otherwise would be far beyond observation. Both environmental and artificial tracers have their own theoretical basis. This textbook will provide an introduction to both environmental and artificial tracer techniques along with their respective theoretical background and will demonstrate how both techniques can be modelled, combined and integrated into hydrological applications that work.

When trying to analyse hydrological systems’ hydrometric data, hydro-chemical information and system characteristics need to be reconciled within a common system model (Dyckand Peschke, 1995). The aim of tracerhydrology is to develop, test and validate those representations of the hydrological system that best agree with the available data by making use of environmental and artificial tracers and modelling.

The general approach in system hydrology is based on the determination of:

• a known or measured input (volume, concentration, energy) as a function of time and space,
• a function characterizing the system (e.g. catchment, spring. . .) by a set of equations describing the flow and/or transport processes at the atmosphere-surface boundary, in surface water or in subsurface-water,
• a known or measured output of the same parameters as a function of space and time (Figure 2.1).

Figure 2.1 Hydrological system approach adapted to tracerhydrology by the convergence approach. Q: volume of water; C: concentration; E: energy.

Linking this to tracer techniques, the input will be the concentration of tracer in infiltrating water and effective precipitation (for environmental tracers) or the injection mass of artificial tracers. After flowing through the system the output will be characterized by the runoff volume, the environmental tracer concentrations (for isotopes and geochemical compounds) and/or artificial tracer concentrations. This concept has been described by Leibundgut (1987) and named the convergence approach. In other words tracerhydrology is based on decoding of information contained in the output parameters of a system. The simplest example is the system of a spring.

Both input and output parameters will be measured in order to understand the processes in a natural hydrological system be it a catchment, an aquifer or surface water. Besides the hydrological water balance parameters in particular, data of environmental and artificial tracers are measured. Models are simplified abstractions of nature that are used to obtain information from measured data about the system. The transfer function between input and output is identified from tracer data and can be used for predictions or system characterization. The modelling of both environmental tracers and artificial tracer experiments is a necessary tool for evaluating the application of tracers.

The application of the convergence approach in tracerhydrology can be used to derive concepts of hydrological systems (Figure 2.2). These conceptual models can be simple or more structured. They represent the principal functioning of the investigated hydrological system (Leibundgut, 1987; Attinger, 1988). Predictions derived from an existing, conceptual system model allow for an improved design of the experimental planning and the observation network.

The combined and simultaneous use of several independent methods and techniques in investigating a hydrological issue is considered as an axiom of tracerhydrology. This principle is applied using the different tracer techniques (natural, artificial tracers) in combination with independent hydrological methods. First, this means that several techniques should be combined in multi-tracer experiments, if possible. The combination of different tracers ensures that the specific limitations of single tracers or methods do not bias our understanding of the hydrological system. While it has become almost common practice to combine different artificial tracers, the combination of environmental and artificial tracers is the most promising approach. Furthermore, tracer methods should also be combined and integrated with other hydrological and scientific methods (hydrometry, geophysics, hydrochemistry, remote sensing, etc.).

Figure 2.2 Conceptual model (Structure model) of a complex system (catchment) evaluated by tracer techniques serving as a base for further research and mathematical modelling (see colour plate section P3b-d.

The fascination of an integrated approach is the reconciliation of results obtained by different independent methods. If different methods provide consistent or concordant data, the scientific conclusion is more soundly based and validated. The fuzziness of individual methods can be overcome if different methods point in the same direction. Finally, contradictions between different methods can be very instructive and push for new experiments or research aimed at resolving the problem.

2.2 Definition of tracers

Environmental tracers are defined as inherent components of the water cycle. Sometimes, accidental injections can be used for hydrological studies. Global input functions have been created as the side effects of industrial or military activities (CFCs, ⁸⁵Krypton or bomb-tritium). Artificial tracers are defined by their active injection into the hydro-logic system in the context of an experiment.

In nature tracers are widely used as markers, such as wildlife marking their territory or ants using pheromones for marking itineraries. Such markers are effective at extremely low concentrations (10^–15). All tracers carry discernable and preferably unique information. These two properties – carrying information that can be identified most effectively at low concentrations – categorize substances as trace elements. Hydrological tracers are dissolved, suspended or floating substances according to their purpose and field of application. Some natural and artificial substances which are suitable for scientific studies or can be applied for the investigation of hydrological systems and subsystems are given in Table 2.1 (Leibundgut, 1982). In principle, hydrological trace elements have to be detectable in solutions with mass ratios of water:tracer of >10⁹.

Table 2.1 Systematic tracer classification, distinguished by their application. Pollution tracers originate from anthropogenic activities, however their input in the hydrological system can be similar to that of natural tracers

Environmental tracers	Artificial tracers
Utilization	Application
Environmental isotopes	Chemicals
Hydrochemical substances	Biological substances
Pollution tracers	Drift substances
Characteristics:	Characteristics:
Spatial input via precipitation, geogenic sources	punctual input (injection), defined by time, place, hydrological situation
Pollution tracers (e.g. Cl^– SF6 CFCs)

Environmental tracers are inherent components of the water cycle, thus we speak of their utilization, while artificial tracers are brought actively into the hydrologic system, so that we refer to their application. Not belonging completely to either of the two groups, pollution tracers are substances introduced into the water cycle by anthropogenic activity, coming either from punctual contaminations such as waste deposits or brought in by accidents, or originating from the production of pollution gases released into the atmosphere. Consequently, they are not natural but feature the same input channels as environmental tracers.

The input of environmental tr...