Groundwater residence time

Figure 9 Hydrologic properties of a sandy silicate aquifer in northern Wisconsin (a) hydrologic conductivities (log K, tn s ), (b) groundwater potentials (tn), and (c) groundwater residence times (yr) (reproduced by permission of American Geophysical Union from Water Resources Research, 1992, 28, 579-589).

The solubility of ordinary atmospheric noble gases (neon, argon, krypton, and xenon) in water is temperature dependent (Benson, 1973). The stable isotope composition of precipitation (6 H, 6 0) also depends on ternperamre. If variations in these constituents can be related to a known history of temperature variation, then groundwater residence times can be estimated (Stute and Schlosser, 1993). 

Simulation of He transport in the basin coupled with a groundwater flow model allowed the estimation of hydraulic conductivities for the different aquifers and, consequently, the estimation of groundwater residence times. Average turnover times for different aquifers are highly variable, ranging from 8,700 yr for the shallowest aquifer (Ypresian) to 30 Myr for the deepest... 

Earle and Drever (1983) found He concentrations of 0.05-236 pL/L in groundwaters from various depths (up to 500 m) in the Athabasca Basin, Saskatchewan. They claimed there to be a moderate correlation between the occurrence of He anomalies and known U mineralisation, provided the He concentrations were divided by sample salinity in a rough correction for groundwater residence time. 

It does not appear possible to assume a priori a He accumulation rate in aged groundwaters. The studies presented in this section show that the He accumulation rates are highly variable, from virtually no external He contribution required, to He accumulation rates apparently exceeding the He flux from the whole continental crust. Hence the expectation that the He flux from the crust into aquifers could be used to estimate groundwater residence time is not valid. 

Unique information about groundwater residence times and cross formational flow can be derived from noble gases. The occurrence of He excesses that cannot be accounted for by production in the crust or in basins and are attributed to a mantle origin demonstrate without ambiguity the occurrence of cross formational flows. " He/ Ne /" °Ar ratios close to the production values found in shallow aquifers can hardly be established in situ because it would require temperatures much in excess of those prevailing in these aquifers (Ballentine et ah, 1994 Ballentine and Bumard 2002, this volume). They instead provide evidence for a contribution of a fluid component coming from much deeper regions of the basins or of the crust. 

The method was validated by hydraulic calculations carried out in a pleistocene aged watershed covering 220 km (Stoebber river) and applied for the calculation of groundwater residence times in the mesoscale watershed of the Ucker river (covering 2400 km ) and planned for modelling substance balance in the Oderbruch region (800 km2). 

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