Characteristics of groundwater flow based on field and modelling studies

Groundwater recharge and discharge altitudes are driving forces of groundwater flow. Groundwater recharge occurs in all climate zones, albeit at different rates. In desert regions groundwater recharge is small (<5 mm/a) and occurs irregularly (Verhagen et al. 1979). In semi-arid regions it is >5 mm/a and <25 mm/a and undergoes significant annual fluctuations. In tropical regions it ranges mostly between > 25 mm/aand <100 mm/a and again varies considerably year-to-years. In temperate humid regions ground-water recharge may reach many hundred millimetres per annum. Groundwater recharge may also occur in permafrost areas (Michel & Fritz 1978), but at very low rates.

Supposing that an equivalent of 44 800 km3/a of continental runoff (UNESCO 1999) recharged ground-water resources (total volume about 8 200 000 km3) homogeneously throughout the profile, a minimum mean residence time of 183 years would ensue; this is a minimum number, because continental discharge does not completely contribute to ground-water recharge. The result of this bulk calculation, however, is in obvious disagreement with all field observations on pollution, hydrochemistry and environmental isotopes in groundwater resources. Hence, flow in the subsurface cannot be homogeneous throughout the vertical profile, as has been qualitatively stated by Toth (1995), Freeze & Witherspoon (1967) and many others, and has been quantified by numerical modelling by Seiler & Lindner (1995).

Groundwater flows in aquifer systems with particular hydraulic properties. To investigate this influence, numerical modelling was performed (Fig. 1) with: a groundwater movement between an underground water divide (left border in Fig. 1) and a river, collecting totally subsurface discharge (right border in Fig. 1); model dimensions of z = 400 m, x = 15 000 m and y = i m; a constant groundwater recharge rate of 150 mm/a over the entire catchment; and typical distributions of aquifer conductivities as show in Figure 2 (left column) and a hydraulic effective porosity of 25%.

From about 100 numerical scenario simulations (Fig. 2) it was found that generally more than 85% of the groundwater recharge rate (right column in Fig. 2) joins surface discharges through near-surface aquifers (active groundwater recharge zone or shallow groundwater) and less than 15% reaches deep aquifers (passive groundwater recharge zone or deep groundwater) (Seiler & Lindner 1995). This holds both for uniform and stratified aquifer systems, though is accentuated for stratified aquifer systems.

Fig. 1. Block diagram of a catchment underlain by an aquifer system (z = 400 m, x = 15000 m, y = 1 m). Numbers indicate the average percentage of turn-over of groundwater recharge (R), resulting from some 100 numerical runs with different distributions of hydraulic conductivities.

From modelling of the propagation of both pollutants and environmental isotopes in non-stressed aquifer systems it was found that tritium and surface pollutants propagate in quasi-identical profile sectors. Hence tritium, in concentrations above the measuring accuracy, is considered a good indicator for shallow groundwater, and at non-measurable concentrations for deep groundwater, for groundwater of very short-term and low susceptibility to pollution. Tritium traces subsurface water quasi-exclusively by means of infiltration. The interface between shallow and deep groundwater was therefore defined by the tritium nought line (TNL) (Seiler 1983), separating shallow, young (<100 years) and deep, old (>100 years) ground-water (Fig. 3).

Shallow and deep groundwaters are both of meteoric origin and belong to a hydraulic continuum; both have been identified by field investigations in tropical humid (Alvarado et al. 1996), temperate humid (Andres & Egger 1985) and semi-arid climate zones (H. Raanan, Ben Gurion University, pers. comm.). In dry and hyper-dry areas shallow groundwater often has a patchy distribution, because of extended catchment sizes as well as an uneven and sporadic occurrence and distribution of precipitation. Sometimes a thick percolation zone also plays a role in arid areas, through which groundwater recharge flows with very low apparent percolation velocities (<1m/year); in such dry areas it is therefore often difficult to detect the shallow, young groundwater, but it is always easy to find old, deep groundwater.

In semi-arid regions, shallow groundwater has a thickness of a few metres, in the tropics typically a few decametres and in temperate climates up to 100 m; this thickness depends essentially upon the rate of groundwater recharge as well as the porosities and hydraulic conductivities. Hence all general changes in groundwater recharge rates will lead to an increase or depletion of the thickness of the active groundwater recharge zone. In the case of a reduction of groundwater recharge because of global climate changes, many wells in the active groundwater recharge zone will shift into the passive groundwater recharge zone, leading to a change in the hydraulic behaviour from instantaneous to transient.

Under hydraulically non-stressed and stressed conditions, transient conditions in groundwater flow play only a random role in shallow ground-water as long as groundwater abstraction does not exceed the safe yield. In contrast, exploitation from deep groundwater produces regularly transient conditions (Fig. 4) over decades to millennia as a response to: groundwater extraction at rates exceeding the depth-related, available groundwater recharge rate; and changing meteorological or tectonic and eustatic boundary conditions. This transient response results from the slow adaptation of the groundwater flow field to recharge/discharge changes to reach a new steady-state condition.

The passive groundwater recharge zone is underlain by connate or formation water (von Engelhardt 1960), which is mostly of non-meteoric origin; it has been entrapped in sediments since sedimentation times, and has not returned to the atmosphere.

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