Multiple tracer approaches to resolve the age structure of groundwater in European aquifers

Corcho Alvarado, Jose Antonio (2005). Multiple tracer approaches to resolve the age structure of groundwater in European aquifers (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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In the frame of this thesis the tracer methods were applied in several groundwater bodies that range from shallow and vulnerable aquifers to deep and confined aquifers, and also in a geothermal system. The groundwater bodies investigated are: a) the shallow and semiconfined Odense sands aquifer, located in Denmark; b) the semiconfined Fontainebleau sands aquifer, located in France; c) the shallow and semiconfined Turonian aquifer and the deep and confined Cenomanian aquifer of the Bohemian Cretaceous Basin, located in the Czech Republic; and d) the Bath thermal water, located in Great Britain (Fig. 2). The research carried out in these systems, with the exception of the Bath thermal water, was partially supported by the European Project “Baseline” (EVK1-CT-1999-00006). The project aimed to establish criteria for defining the natural groundwater quality background and to develop a standardised Europe-wide approach which may be used in emerging water directives. Such a standard, based on geochemical principles, is needed as a reference to be able to assess quantitatively whether or not anthropogenic pollution is taking place. The baseline water quality is mainly controlled by the duration of water-rock interaction, a parameter that is directly related to the age of groundwater. This work was meant to provide information about this last parameter.
The research of the groundwater systems, of which very scarce or vague information was available before this work, relies on measurements in groundwater of a large set of environmental tracers (e.g. 3 H, 3
He, 85 Kr, SF 6 , CFCs, 39 Ar, 14 C and 4 He). The combination of several tracers enables other processes or parameters of the aquifer system than the mean residence time (e.g. mixing processes, dispersion, sorption, degradation, degassing, contamination, etc.) to be investigated (Ekwurzel et al., 1994; Plummer et al., 2003, Corcho et al., 2005); and also refining the interpretation of groundwater age. The investigated groundwater bodies show different hydrogeological conditions; therefore slightly different methods were required in each case. More details about the specific methods applied are given further below. The interpretation of the tracer concentrations is commonly carried out by models that try to mathematically describe the age distribution of sampled groundwater. The limited number of sampling sites in the aquifer systems investigated in the frame of this thesis suggested the use of lumped parameter approaches for the assessment of groundwater dynamics (Zuber, 1986; Zuber and Maloszewski, 2001). The choice of an age weighting function in the lumped parameter models that appropriately represents the hydrogeological situation in each study case is validated using the measured tracer data. The use of more complicated models such as numerical flow and transport models is not justified in most of the systems due to the lack of sufficient information.
A brief introduction to the subject areas is given in the following. The thesis is organized in four main chapters which are written in an article form. Each article is intended for publication or has already been submitted. A short summary of the four main chapters of this thesis and some general remarks on the findings are also given. Each chapter concentrates in the characterization of one aquifer system, and they are named as follows:

a) Chapter 2: 36 Cl in modern groundwater dated by a multi tracer approach (3 H/3 He, SF 6 , CFC-12 and 85 Kr): A case study in Quaternary sand aquifers in the Odense Pilot River Basin, Denmark. This chapter has been published in: Applied Geochemistry (Corcho Alvarado et al., 2005)
b) Chapter 3: Constraining groundwater age distribution using 39 Ar: a multiple environmental tracer (3 H/3 He, 85 Kr, 39 Ar and 14 C) study in the semi-confined Fontainebleau Sands aquifer (France). This chapter has been submitted to: Journal of Hydrology (Corcho Alvarado et al., submitted).
c) Chapter 4: Groundwater dating in the Turonian and Cenomanian aquifers of the Bohemian Cretaceous Basin: A first step in getting insights on underground processes and recharge conditions. This chapter is in preparation for publication (Corcho Alvarado et al., in preparation).
d) Chapter 5: The age and origin of the Bath thermal waters. This chapter is in preparation for publication (Edmunds et al., in preparation).

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