Groundwater flow dynamics under past ice sheets and preservation of old glacial groundwater

Numerical modelling shows that groundwater flow velocities and hydraulic heads in northwestern Europe, especially in the relatively shallow aquifers, were significantly higher under ice sheets than they are at present. For an area bordering the Baltic Sea in Germany, Piotrowski (1997b) estimated flow velocities in the upper aquifer under the last ice sheet as about 30 times higher than at present. Furthermore, a reversal of flow direction occurred. At present, the groundwater drains to the Baltic Sea; during the glaciation it was forced in the opposite direction by the ice sheet advancing out of the Baltic Sea basin. A corresponding flow reversal was also determined by Boulton et al. (1996) and van Weert et al. (1997) in a large-scale, vertically integrated palaeoflow model (Fig. 9.4), by Glynn et al. (1999) in a transect between the ice-sheet centre in Scandinavia and its periphery in Poland, by Grasby et al. (2000) in a transect from South Dakota to Manitoba during the last glaciation, and by Mar-czinek & Piotrowski (this volume, Chapter 10) in a coastal area of northwest Germany. Maximum flow velocities in upper subglacial aquifers in northwestern Europe were estimated at 20myr-1 (Boulton et al., 1995), 200myr-1 (Boulton et al., 1996) and over 100myr-1 (van Weert et al., 1997).

Given the high flow velocities and hydraulic heads, groundwa-ter can penetrate deep into the subglacial sediments and rocks. Because of the typically layered structure of sedimentary beds, the flow pattern is highly anisotropic with preferential flow approximately horizontal in the transmissible, coarse-grained sediments and vertical in aquitards. Most studies indicate that upper aquifers with residence times in the order of several thousand years were completely flushed out during glaciations and the groundwater derived from precipitation during the interglacials was replaced by glacial meltwater. In Holland the entire upper (Quaternary) aquifer system, about 300 m thick, and a large part of the lower (Mesozoic) aquifer, up to 1500 m thick, are believed to have been flushed under the Saalian ice sheet, whereas the Tertiary aquitard in between was partly penetrated by meltwater (Boulton et al., 1993). Deep meltwater circulation is also suggested for northwestern Germany, where during the Weichselian glaciation the flow field in the whole Quaternary sequence consisting of two major aquifers and two aquitards up to ca. 200 m thick was reorganized by ice overriding (Piotrowski, 1997a,b). Similar conclusions were drawn for the Illinois basin, USA where the several-kilometres-thick drainage system of superposed aquifers and aquitards was significantly modified by the last ice sheet, as modelling (Breemer et al., 2002) and hydrochemical analyses (McIntosh et al., 2002) indicate. Under the Des Moines lobe in Iowa, subglacial meltwater penetrated over 300 m down into the

Figure 9.4 Horizontal groundwater velocity vectors and the velocity field (ranges in myr-1) in the major regional aquifer in northwest Europe at present and during the expansion of Fennoscandian and Scottish ice sheets (different palaeogeographical scenarios with maximum ice extent in C1). Note the reorganization of flow patterns under glacial conditions as compared with the present time. (From van Weert et al., 1997.)

Figure 9.4 Horizontal groundwater velocity vectors and the velocity field (ranges in myr-1) in the major regional aquifer in northwest Europe at present and during the expansion of Fennoscandian and Scottish ice sheets (different palaeogeographical scenarios with maximum ice extent in C1). Note the reorganization of flow patterns under glacial conditions as compared with the present time. (From van Weert et al., 1997.)

bed (Siegel, 1991), and the hydrogeology of the Atlantic continental shelf off New England experienced short-lived but dramatic reorganization during the Last Glacial Maximum, including large-scale disturbances in freshwater/seawater equilibrium (Person et al., 2003). Pressurized glacial meltwater pene trated the bedrock in Northwest Territories, Canada, to depths over 1.6km (Clark et al., 2000). Under some circumstances, however, a glacier may seal the substratum preventing ground-water recharge, as was the case in one Alpine valley covered by the last ice sheet (Beyerle et al., 1998).

Admittedly, numerical models of past groundwater flow are notoriously difficult to validate, and the range of possible solutions vary with the conceivable range of input parameters and boundary conditions, such as the ice sheet thickness and glacier persistence in a certain area, lithospheric response to loading, and spatial variability of hydraulic conductivities. Heuristic studies are promising but scarce (e.g. Fleming & Clark, 2000). A semiquantitative test of model results may be provided by the isotope composition of deep groundwater, preferably in aquitards where the preservation potential of old water is high. In most cases direct dating with radiocarbon is not possible due to its relatively fast decay, but unstable isotopes with half-lives comparable to glacial cycles may be used (see below).

Indeed, several studies have shown that glacial meltwater flushed through and remained trapped in fine-grained sediments since the last glaciation in lowland areas of North America and Europe. Remenda et al. (1994) reported old groundwater in shallow tills and glaciolacustrine deposits from sites spanning a distance of 2000 km along the margin of the Last Glacial Maximum ice sheet in North America, identified by S18O values around -25%o, much lower than the modern precipitation of around -13 to -14% (see also Stueber & Walter, 1994; Clark et al., 2000). Similarly, Marlin et al. (1997) have found old glacial porewater depleted in 18O and 2H in thick clay sediment inside the extent of the last glaciation in northern Germany. In another area in northern Germany at Gorleben, deep, saline water on top of a salt dome was formed in a cold Pleistocene climate, very likely under glacial conditions (Schelkes et al., 1999). Glacial meltwater has also been documented in several places, notably down to depths of at least 500 m, in fractured rocks of the Fennoscandian and Canadian shields (Wallin, 1995; Smellie & Frape, 1997; Tullborg, 1997a; Glynn et al., 1999; Laaksoharju & Rhén, 1999) and in deep Cambrian aquifers in Estonia (Vaikmae et al., 2001). These case studies show that, under favourable hydrogeological conditions, glacial meltwater injected into low-conductivity rocks under high pressure can remain there for thousands of years after deglaciation, and can be used as a proxy of global climatic changes of the past. In many other places groundwater recharged during cold periods of the Pleistocene also has been documented, but it may not necessarily be water derived from melting of glacier ice (e.g. Zuber et al., 2004).

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