3H/3He ages show that heterogeneities in flow patterns cause quite some spatial variation in groundwater ages. The question is whether groundwater ages are also temporarily variable. In this thesis, groundwater ages are assumed to be constant in time at a certain location: a monitoring well always samples water with the same age. Time series of 3H/3He ages at monitoring location are needed to confirm the assumption. Time series of CFC concentrations from a granular unconsolidated aquifer in Denmark showed temporal variations (Laier 2004), which were attributed to varying groundwater ages at the monitoring screen. The variations of CFC concentrations may also have been the result of local variation of the recharge concentrations or the non-linearity of the leaching of CFC through the unsaturated zone. Variations in apparent 3H/3He groundwater age may have been caused by actual age fluctuations as the result of transient groundwater flow conditions, or by temporal variations in the entrapment of 3He at the groundwater table.
Dating mixed groundwater ages and age spread
The new challenges in the field of groundwater dating are the application of age tracers in more complex systems. The first challenge is dating mixed groundwater (Rueedi et al. 2005) from production wells (Osenbrück et al. 2006; Zinn and Konikow 2007a, 2007b), springs (Plummer
et al. 2001; Hendrix and Meinardi 2004) or highly dispersive aquifers (Weissmann et al. 2002;
LaBolle et al. 2006). For this goal, time-series of age tracer concentrations should be analyzed with deconvolution techniques developed for the interpretation of 3H data (Maloszewski and Zuber 1993, 1998). The use of multiple tracers will yield additional information on the distribution of groundwater ages – or age spread – in sampled groundwater (Castro et al. 1998). Furthermore, a combination of groundwater age tracers provide more insight into the contribution of groundwater to surface water (Ojiambo et al. 2001; Rose 2007; Koeniger et al. 2008) or the water balance and transit time of entire catchments (McGuire and McDonnell 2006).
Groundwater age and modeling
Comparing modeled groundwater ages with tracer ages is a challenging task. Groundwater ages are determined most reliably on point samples, and a direct comparison to a single point in the modeled 3D groundwater age field seems logical. But small local heterogeneities (e.g. clay lenses) may have dramatic effects on the groundwater age at the specific location (Weissmann et al. 2002). The question is whether the differences between model and tracer age, which are possibly only due to local discrepancies, can effectively be used to improve groundwater flow models, if these differences are not systematically biased. Even the use of groundwater ages to estimate recharge rates suffers from this dilemma. Both groundwater ages and recharge rates may be spatially very variable. To improve the understanding of the discrepancies between modeled and measured ages, groundwater age spread should be modeled simultaneously (Varni and Carrera 1998). A wide simulated groundwater age distribution will then keep us from rejecting the groundwater model if the measured age is a little off.
Simulating groundwater age spread provides a new approach to study and possibly quantify the numerical dispersion in groundwater transport models. The sensitivity of the simulated groundwater age spread to the model parameters describing hydrodynamic dispersion will reveal the numerical dispersion of the model. The results of such an analysis provides a substitute or
effective numerical dispersion length, that is characteristic to the groundwater transport model, or sub-domains of the model space.
Despite the uncertainties characteristic of groundwater ages derived from particle tracking in coarse models, these quick and easy groundwater ages are very valuable. Because groundwater ages represent fluxes, rather then pressures, a few modeled ages are likely to be more restrictive calibration targets than a large number of additional head measurements. So far, most work has focused on the simulation of groundwater age in stationary models. However, especially if travel times of a few years are studied, seasonal and transient effects are expected to play an important role. These short travel times require a very fine and accurate representation of the surface water system in the model. Current developments to implement drains and watercourses as line elements in models are promising; to facilitate the (automatic) model refinement needed for accurate groundwater age simulations. At such a fine scale, the simulated arrival time of particles at (weak) sinks is very accurate and the travel time within the sink cells – which is neglected by particle tracking algorithms – is actually be negligible.
Dating groundwater in reactive two-phase systems
The application of age tracers in exactly the environments where they are most useful – heavily polluted and reactive sites – becomes a challenge, because of the possible formation of a gas phase at such sites. In such relevant environments, 3H/3He is the most reliable method to date degassed groundwater, if anoxic conditions prevent the use of CFCs. 85Kr (Smethie et al. 1992) is an alternative, but it requires large volumes of groundwater to be sampled, in the order of tens of m3. Collecting such sample volumes from near the water table will likely affect the local groundwater flow pattern, which is undesirable.
If degassed groundwater is to be dated with 3H/3He, an estimate of the relative timing of degassing is necessary. Under the assumption of downward flow, the total dissolved gas (TDG) pressure yields the depth, and thereby relative timing, of degassing. In other circumstances, detailed geochemical information on the reactivity of the aquifer or high-resolution vertical profiles of the concentrations of reactive solutes is needed to provide the relative timing of degassing. Such detailed information should also confirm the use of the TDG pressure to estimate the depth of degassing. In this study, the TDG pressure was assumed to equal the dissolved N2 pressure because other sources of gas were unlikely. Measurements of the concentrations of all dissolved gases are needed to confirm this. In any case, a TDG probe is a highly recommended piece of field equipment in areas where gas production is possible, because it provides the minimum pressure to be maintained in the noble gas sampling line to prevent degassing during sampling.
Noble gases to study gas-water interactions
While the production of gases may pose a problem for the straightforward analysis of groundwater age tracers, these tracers are valuable to studies in gas-water phase interactions. The analysis of noble gas concentrations and isotope ratios yield valuable information on the pore- scale processes acting on gases in semi-saturated porous media. One important aspect of fluid flow in porous media is the area of contact or interfacial area between the gas and the water phase (Joekar-Niasar et al. 2008; Niessner and Hassanizadeh 2008). Laboratory studies analyzing the transport behavior of partitioning tracers (Gupta et al. 1994) provide new insight into the contact area between the water and gas phase in semi-saturated porous media (Holocher 2002; Holocher
et al. 2002; Holocher et al. 2003; Balcke et al. 2007). Upward movement of gas in semi-saturated
porous media tends to be discontinuous. This burping behavior of gas movement may have a large impact on the transport of tracers and limit diffusion through the gas phase. Analyzing noble gas concentration in ebullition experiments (e.g. Geistlinger et al. 2005; Geistlinger et al. 2006) are likely to reveal the connectivity of a trapped gas phase. This information is essential to improve the two-phase flow and transport models available today (White and Oostrom 2000; Holocher et al. 2003; Amos and Mayer 2006b).
Depleted noble gas concentrations in natural waters reveal degassing has taken place and noble gas concentrations have been used to quantify degassing of N2 (Blicher-Mathiesen et al. 1998). Noble gas analysis have great potential to be applied with that particular aim: to quantify the escape of gases from natural systems (Brennwald et al. 2002; Holzner et al. 2004; Brennwald
et al. 2005; Holzner et al. 2008; Ma et al. 2009). Two systems would be very interesting in that
context: peatlands and gas-hydrates (Winckler et al. 2002). Both systems represent an important but poorly quantified source of methane to the atmosphere. As a result, their contribution to the greenhouse effect is uncertain. The concentrations of noble gases, in combination with the 13C/12C ratio and pore water ages, are a promising approach to quantify the rate of escape of methane from peatlands. If noble gas concentrations appear to be a valid proxy for methane emissions, groundwater originating from peatlands forms a new environmental archive of a key climate variable.