Trends in groundwater quality in relation to groundwater age
Outline
Two lines of research are combined in this thesis: (1) investigating trends in groundwater quality in relation to groundwater ages and (2) determining groundwater age of sampled groundwater and in numerical models. Therefore the thesis consists of two parts, each consisting of three chapters:
PART I: TRENDS IN GROUNDWATER QUALITY
Demonstrating trends in groundwater quality in relation to time of recharge (Chapter 2): Using groundwater dating, the measured concentrations of pollutants can be directly related to the time of recharge. This provides an objective basis for the aggregation of groundwater quality data of an entire groundwater body. The aggregated data can then be analyzed for trends and trend reversal in groundwater quality.
· Can we detect trends and trend reversal in groundwater quality using groundwater age?
Analyzing trends in reactive solute concentrations in groundwater in relation to time of recharge (Chapter 3): Using the aggregation of groundwater quality data presented in Chapter 2, the concentrations of reactive solutes can be analyzed in relation to groundwater age. The observed trends were compared to the results of a 1D reactive numerical hydrogeochemical model, simulating the reactions along a representative flow-line.
· Can we understand patterns of concentrations of reactive solutes using groundwater age?
Comparison of methods for the detection and extrapolation of trends in groundwater quality (Chapter 4): The collaboration of the four research groups in work package TREND 2 of the FP6 project Aquaterra formed the basis for a unique comparison of hands-on experience with different methods for the detection of trends in groundwater quality. The different methods were compared in terms of prerequisites, costs, the system under study, the understanding of the system and the potential for extrapolation.
· Which method is best suitable for the detection of trends in groundwater quality given a variety of available data, settings, etc.?
PART II: GROUNDWATER AGE
Dating degassed groundwater with 3H/3He (Chapter 5): Denitrification of nitrate from agricultural pollution leads to the formation of gas bubbles below the water table. These bubbles cause “degassing” and affect the transport of the tracer gases that are used to determine the groundwater age. Chapter 5 presents a method to determine the groundwater age based on degassed groundwater.
· Can we date groundwater that is affected by geochemical reactions?
Degassing of groundwater age tracers: measurements and two-phase transport simulations (Chapter 6): A two-phase numerical groundwater flow and transport model is used to reproduce the measured concentrations of tracer gases. The model results are used to assess the accuracy of the method presented in Chapter 5.
· What is the accuracy of degassed groundwater ages?
Travel time distributions derived from particle tracking in groundwater models containing weak sinks (Chapter 7): The contribution of groundwater to surface water quality depends on the travel time distribution of discharging groundwater. Such a travel time distribution can be quickly calculated based on a particle tracking analysis in numerical groundwater flow models. However, so-called weak sinks in the flow model pose a problem to the particle tracking theory. Chapter 7 presents an objective method to calculate the travel time distribution, avoiding the problem of weak sinks.
· What is the value of simulating groundwater travel time distributions?
Conclusions and new research questions (Chapter 8): The conclusions of the individual chapters are summarized and combined in Chapter 8. These conclusions have lead to new views on groundwater quality and age, but also inspired many new research questions.