Near-surface and deep groundwaters

Journal of

Hydrology ELSEVIER

Journal of Hydrology 165 (1995) 33-44

[2]

Near-surface and deep groundwaters Klaus-Peter Seiler a'*, W u l f Lindner b aGSF-lnstitutffir Hydrologie, D 85764 Oberschleissheim, Germany bErftverband D 50126 Bergheim, Germany Received 17 March 1994; revision accepted 14 July 1994

Abstract

Groundwater occurs in all climatic zones and differs in hydrodynamic, isotopic and chemical terms according to its activity within the water cycle and its interaction with rocks. Among the many chemical components in groundwaters, tritium concentrations allow differentiation between groundwater of high and low mobility and thus permit the distinction between shallow and deep groundwaters. In Germany shallow groundwaters range in depth between 10 and 100 m and account for more than 85% of groundwater recharge. Deep groundwaters, in contrast, participate in the water cycle only at intervals of hundreds or thousands of years and reach depths of several hundred metres. Both types of groundwater overlie connate waters that have been isolated from the water cycle since the accumulation of the sediments in which they occur.

The exploitation of deep groundwaters mostly causes long-term non-steady-state conditions, large-scale changes of flow patterns of the groundwaters and hydraulic short circuits to shallow aquifers. Thus, persistent contaminants may penetrate extraction wells in deep groundwaters even if the wells are believed to be well protected against pollution. In deep groundwaters it is difficult to apply the conventional use of groundwater protection zones to prevent this kind of intrusion of contaminants.

1. Introduction

In a lot of countries, drinking water originates from groundwater reservoirs. At the beginning of this century, springs in Europe were often used for water supply. Later, man-made deep wells (<50 m) replaced springs in order to secure water supplies during periods of dryness and to provide better sanitary protection. In a lot of areas groundwater abstraction has actually proceeded to a depth of

* Corresponding author. 0022-1694/95/$09.50 © 1995 - Elsevier Science B.V. All rights reserved SSDI 0022-1694(94)02584-3

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K.-P. Seller, W. Lindner / Journal of Hydrology 165 (1995) 33-44

Table 1 Distribution of continentaland atmosphericwaters Occurrence

Volume(km3)

Volume(%)

Groundwater Lakes Soil water Water vapour in the atmosphere River water

8 000 000 226 000 62 000 15 000

96.3 2.7 0.8 0.2

1 000

0.01

several hundred metres (deep groundwaters). These groundwater resources have a sufficient yield and seem better protected against contamination than groundwaters from shallow wells (shallow groundwaters). Little is known, however, about the regime of these deep groundwaters and their protection against pollutants. In order to gain a better understanding of this, the following features are of interest: the origin of these groundwaters; the available groundwater recharge at these depths; the shortand long-term hydrodynamic behaviour of these groundwaters, especially in relation to groundwater protection. These features are mainly linked to the dynamics of groundwater movement in the different subsurface zones. Isotopic, hydraulic and chemical research in the field and numerical simulations on the basis of field results have been used to study groundwater movement. Such studies have been carried out over several decades in a lot of areas and have been linked with detailed studies in southern Germany.

2. Occurrence of water and the dynamics of subsurface waters

The total volume of water on the Earth is made up of 97.4 vol.% ocean water and 2.6 vol.% (3.6 x 107 km 3) fresh water on the continents and in glaciers. About 78 vol.% of the fresh water occurs as ice and does not participate in the water cycle on short time-scales. The other 22 vol.% is in a gas or liquid phase. This distribution of fresh water has changed a number of times during the Pleistocene. A future change in the distribution of fresh water might occur with global climatic changes. Among liquid fresh water, groundwater predominates (Table 1). For a general understanding of groundwater dynamics its volume has to be compared with the global water cycle, which discharges approximately 48 000 km 3 of water onto the continents annually. Supposing that these discharges contribute only to groundwater recharge and postulating that all groundwaters would move in the same manner yields a mean turnover time of 165 years. Considering that only a small portion of the discharge contributes to groundwater recharge, the mean groundwater turnover time increases. Such high turnover times would, however, imply that groundwater contamination due to human activities in the industrial age are negligible and thus contradict experience. Therefore, one must suppose that groundwaters at different depths underground participate in different ways in the water cycle.

K.-P. Seiler, IV. Lindner / Journal of Hydrology 165 (1995) 33-44

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Weithofer (1936) divided the groundwater system into (i) an active zone reaching from the groundwater surface to approximately sea level, and (ii) an inactive zone below sea level. Weithofer argued that gravity forces act on groundwater motion only in the zone above sea level, whereas only convection would prevail in the deeper zones. This argument was supported by the occurrence of connate waters in sedimentary basins of great depth and also implies that no hydraulic continuum exists between these two zones. Today it is well known, e.g. through isotope hydrology, that such a hydraulic continuum exists at all depths and therefore the ideas of Weithofer (1936) are not valid.

3. Subdivision of groundwaters into shallow and deep groundwaters Lithostatic pressure and rock age increase with increasing depth, while sediment porosity and permeability generally decrease. These decreases are not systematic or continuous, because sedimentary changes (e.g. permeabilities along bedding interfaces of unconsolidated sediments) are generally discontinuous. In hard rocks a pronounced change in permeability is often observed without a change in bedding; this is related to the base of the near-surface dilatation zone in which fissures have greater widths than in a triaxial pressure state at greater depth. The effects of such permeability changes on groundwater recharge and discharge in different zones have been studied in Germany and elsewhere by observing the vertical distribution of isotopic and chemical parameters in the groundwaters. Additionally, numerical simulations have been used to confirm the interpretation of isotopic and chemical results. 3.1. Mobility of groundwaters deduced by isotope an d chemical parameters Groundwater studies during the last 20 years have shown in humid climates that the concentrations of the radioactive environmental isotopes tritium (half-life, 12.43 years) and carbon-14 (half-life, 5730 years) decrease spontaneously at between 10 and 100 m below the surface; here tritium decreases to negligible concentrations and carbon-14 decreases to concentrations below one half-life. At the same depths of 10-100 m, waters that have undergone ion exchange often appear for the first time, and such ion exchange is documented as being a generally slow process (Eichinger, 1981). As the isotopes tritium and carbon-14 enter groundwater systems exclusively through precipitation infiltration they make groundwater dynamics measurable over tens (tritium) and thousands (carbon-14) of years. The depth at which tritium concentrations become unmeasurable is called the 'tritium zero limit' (TNF -- Tritium-Null-Fl~iche); below it, water ages exceed 50 years and rise over short depth ranges to ages of several thousand years. In Germany, where the groundwater resources are not overexploited, this TNF is met at an average groundwater depth of between 40 and 100 m. Examples of depths and locations of TNF in Germany are: in the sands of the Molasse basin near the Munich

K.-P. Seiler, W. Lindner / Journal of Hydrology 165 (1995) 33-44

36

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area at 50 m (Egger, 1978), in the gravels of the upper Rhine valley at 40-60 m (Matthess et al., 1968), in the sands of the Hamburg area at about 40 m, in the sandstones of the Bunter at the Saar area at roughly 100 m (Seiler, 1969), and in the area between Madgeburg and Halle at about 10 m depth. In the areas studied this depth is found to be dependent on: the infiltration capacity o f the soils and/or the unsaturated zone, the quantity of groundwater recharge, and the porosity and the permeability of the aquifer. The T N F is at the groundwater table in areas without groundwater recharge, e.g. in arid and semi-arid areas, and reaches its greatest depth in areas of high precipitation

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K.-P. Seiler, HI. Lindner / Journal of Hydrology 165 (1995) 33-44

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! z

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Fig. 3. Schematicillustrationof a two-dimensionalx-z modelwith no limitationsin the y direction. containing rocks with low porosity, medium permeability and high infiltration capacity (e,g. sandstones). In the areas cited, the TNF sometimes coincides either with the first appearance of ion exchange waters (Fig. 1) or with the basal high sulphate and/or chloride waters underlying fresh waters. Further observations of the vertical hydraulic gradients indicate that groundwaters containing tritium discharge to the nearest river while groundwaters below the TNF are mostly directed to one of the deeper discharge systems in a region (Fig. 2). The fresh water-salt water interface outside coastal zones is characterised by very slow groundwater motion, otherwise salts would have already been dissolved. Its occurrence, however, depends on the availability of salt deposits or fossil marine waters in the sediment. Although saline groundwaters commonly indicate groundwater zones that are not actively involved in the water cycle, the fresh water-salt water interface need not necessarily be identical with the TNF (see below), in many cases it is found below the TNF.

3.2. Penetration depth of groundwater recharged into the underground A numerical two-dimensional model was set up covering 400 m in the z- and 15 000 m in the x-direction; in this model groundwater flow is directed from left to right (Fig. 3). Groundwater recharge penetrated homogeneously into the subsurface over the entire model surface and displayed the same results in all simulation runs. The

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K.-P. Seiler, I4". Lindner / Journal of Hydrology 165 (1995) 33-44

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Tritium, however, may be useless if the groundwater table in fine-grained rocks without fissures occurs at more than 25 m depth and preferential flow in the unsaturated zone is negligible. The TNF represents an interface between zones of different hydraulic character; it is more useful than the fresh water-salt water interface far inland from coastal zones, because this interface is related to the presence of evaporites and/or fossil marine waters and therefore does not necessarily coincide with the TNF. Finally, the use of the interface of ion exchange-normal groundwater cannot generally be recommended because the ion exchange velocity varies with concentration gradients (e.g. Ca, Na) and clays are not present everywhere to favour ion exchange. Therefore the fresh water-salt water and the ion exchange interfaces may vary in depth independently

K.-P. Seiler, W. Lindner / Journal of Hydrology 165 (1995) 33-44

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Fig. 6. Geologicalcross-sectionand the positionof TNF in the profilein southernGermanynear Munich. from the hydrodynamic activity of the groundwater zones; this, however, is not the case for the TNF. Comparison of the trend of the TNF in a 140 km longitudinal profile in southern Germany with the distribution of sands and clays (Fig. 6) demonstrates that the TNF: does not vary much in depth and often, but not necessarily, follows bedding interfaces with strong permeability changes, and crosses aquifers with more or less homogeneous permeability distributions. The position of the TNF is guided by strong permeability changes and is obviously not influenced by permeability windows. This effect has also been simulated (Fig. 7) and the simulation results agree fairly well with observations in nature. However, it is not yet known how an increase in the widths of these hydraulic windows may influence the course of the TNF in areas without groundwater extraction. Below deep groundwaters are connate waters that have been trapped in the sediments since their origin and therefore have not been recycled to the biosphere. This interface is found in Germany, generally at a depth of several hundreds of metres (usually >500 m below ground). Shallow and deep groundwaters are known all over the world (Fig. 8). Near surface groundwaters: are largely lacking in arid zones, form a thin layer in semi-arid and polar climates, and reach a greater thickness in humid climates than in tropical zones, because infiltration capacity of the surface sediments is better in humid than in tropical climates.

4. Implications of using deep groundwaters for water supply Exploitation of deep groundwaters by means of pumping seems to offer more

41

K.-P. Seiler, W. Lindner / Journal of Hydrology 165 (1995) 33-44

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K.-P. Seiler, W. Lindner / Journal of Hydrology 165 (1995) 33-44

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security against contamination than the exploitation of shallow groundwaters. This is true for short-term observations; however, as will be shown below, one may be faced with tremendous contamination problems in the long run.

4.1. Change of hydrodynamic regime due to abstraction of deep groundwaters As has been shown by field observations and numerical methods, deep groundwaters generally move slowly. Groundwater abstraction from this zone is mostly focused on groundwater recharge, determined at the atmosphere-lithosphere interface. These recharge quantities, however, are not representative for deep groundwaters. Without considering the reducing availability of groundwater recharge,

K.-P. Seiler, W. Lindner / Journal of Hydrology 165 (1995) 33-44

43

groundwater abstraction activates the hydrodynamics of deep groundwaters and may change the direction of mass flow. Activation of groundwater flow is triggered by a reduction of the hydraulic head. This, however, creates long-term non-steady-state conditions, which have an observed range from a few to about 10 years (DATAR/BRGM, 1980; Seiler, 1983). During these non-steady-state conditions interconnections between different aquifer systems are created, especially short-circuits to shallow groundwaters. Contaminants having entered deep groundwaters in this way have longer residence times and may cover larger areas in deep rather than shallow groundwaters (Fig. 9). Hydrodynamic changes in the surroundings of wells exploiting deep groundwaters are weak upstream of the well and pronounced downstream (Fig. 9). Linked to this, the contamination of deep groundwaters may be greater downgradient of the well. An additional result of this is that the initial hydrogeological conditions of an area may be judged wrongly on the basis of isotope and hydrochemical research when longterm groundwater abstraction is neglected in the data interpretation. 4.2. Protection areas for deep groundwaters

Under undisturbed hydrodynamic conditions there is no significant penetration of persistent contaminants into deep groundwaters. Contamination may be initiated by deep pumping wells, however, if groundwater abstraction is based on the quantity of groundwater recharge at the atmosphere-lithosphere interface. As shown in section 4.1, persistent contaminants reach deep groundwaters with a delay time. It appears, therefore, that deep groundwaters are well protected against contaminants for short durations, but are highly endangered over longer time intervals. This cannot easily be recognized by the usual monitoring programs. As long as contaminants enter such systems they may prove to be a long-term menace. Protection of deep groundwaters against pathogenic b/acteria is easy to verify and there is a 50-day travel time between the source and the sink of the system. These protection areas, however, cannot protect exploited deep groundwaters against persistent contaminants because: persistent contaminants reach deep wells from more extensive areas than they do in dug wells, and penetration of contaminants occurs in many cases by means of hydraulic windows, the positions of which usually are unknown.

5. Conclusions

Shallow and deep groundwaters can be distinguished according to their active role within the water cycle. From a hydrodynamic point of view, near-surface groundwaters flow to the nearest river, while deep groundwaters are oriented to one of the deepest discharge lines in a region and therefore belong to a flow system on a regional scale. Contaminants do not penetrate deep groundwaters in appreciable amounts if exploitation does not take place. The exploitation of deep groundwaters, however,

44

K.-P. Seller, W. Lindner / Journal of Hydrology 165 (1995) 33-44

m a y interconnect deep and near-surface groundwaters; such linkage m a y facilitate the penetration o f persistent c o n t a m i n a n t s into the deep groundwaters. This takes place with a long delay time, because exploitation o f deep groundwaters creates long-term non-steady-state conditions. Thus, a l t h o u g h these deep zones m a y at a first glance seem to be well protected, they m a y in reality be highly endangered. Therefore, the use o f deep groundwaters should be limited to special cases and should be continued only for short periods o f time.

References DATAR/BRGM, 1980. Atlas des eaux sonterraines de la France. R6gion parisienne. BRGM, Paris. Egger, R., 1978. Das Grundwasser in der oberen SiiBwassermolasse im Raum Miinchen und Augsburg. Diss. Univ. Munich, Munich. Eichinger, L., 1981. Bestimmung des Alters yon Grundwfissern mit Kohlenstoff-14: Messung und Interpretation der Grundw~isser des Fr/inkischen Albvorlands. Diss. Ludwig-Maximilians-Universitfit, Munich, 194 pp. Matthess, G., Thilo, L., R6ther, W. and Miinnich, K.O., 1968. Tritium im Grundwasser tieferer Grundwasserstockwerke. Gas- u. Wasserfach, 109: 353-355. Seiler, K.-P., 1969. Kluft- und Porenwasser im Mittleren Buntsandstein des Stidlichen Saarlands. Geol. Mitt., 9: 75-96. Seiler, K.-P., 1983. Tiefe Grundw/isser und Faktoren, die das Absenkungsgesehehen in tiefen Grundwfissern beeinflussen. Z. d. geol. Ges., 134: 845-855. Seiler, K.-P., 1992. ErschlieBung, Nutzung und Schutz erneuerbarer Grundw~isservorkommenin Entwicklungsl/indern. Z. d. geol. Ges., 143: 214-219. Weithofer, K.A., 1936. Das Tiefenstandswasser. Z. d. geol. Ges., 88: 29-41.