The natural topography-induced, the artificial fluid flow systems and the density-induced fluid flow in shallow parts of onshore Netherlands today and their related physico-chemical characteristics have been described and modelled in great detail (see e.g. the ‘National analysis of regional groundwater flow systems’ of TNO-GG 1991-1996, and the overview publication of groundwater in the Netherlands of Dufour 1998, 2000).
From the Miocene onward, an increasing part of the southeastern part of the Southern North Sea Basin emerged above sea level (Chapter 3). Since that time three types of topography-induced groundwater flow system developed in the Netherlands (Table 3;
Chapters 6 and 8): two of these – the local and regional systems – originating in the Dutch lowland area itself; the third is a supraregional system that originated at the southern and southeastern margins of the Netherlands. Local and regional topography-induced groundwater flow had relatively limited depths of penetration. Evidence for a prolonged existence of a supra-regional system with northward flow is provided by dating of the groundwater, hydrochemical indicators of flow and results of modelling studies concerning elevated temperatures (Chapters 6 and 8). The results of hydrochemical studies suggest that a pre-Holocene supra-regional groundwater flow system extended more deeply than the present-day system (Chapter 8).
Chapter 7 discussed the relation between Tertiary and Quaternary sedimentary loading and present-day distributions of fluid flow and observed overpressures.
These main characteristics of the present-day hydrodynamic setting of onshore and offshore Netherlands are summarised below. Figure 32 shows the hydrodynamic setting.
9.1 Topography-induced fluid flow systems
Figure 24 shows the close relation between the present-day surface topography of onshore Netherlands in relation to the elevation of the water table and depth of the fresh-brackish groundwater interface. As observed before, the isotherm map at 250 m depth shows the cooling effect of meteoric waters infiltrating in regional recharge areas of Veluwe and Utrechtse Heuvelrug.
The present-day depths of penetration of local and regional topography-induced fluid flow systems originating in the Netherlands lowland area are in the order of tens to hundreds of metres (Figure 32). The present-day main recharge areas of supra-regional fluid flow systems are the Belgian Kempenland and Ardennes, and the German Eifel, Sauerland and Teutoburgerwald, and their offshoots. An important part of this supra-regional flow is concentrated in the Roer Valley Graben, where meteoric water has been able to infiltrate since the Miocene. Although topography-induced fluid flow probably penetrates to a depth of >– 1000 m in the Graben, most flow is concentrated in the upper 250-500 m (e.g. Wiers 2001). Most of the groundwater below the fresh-brackish groundwater interface under the Netherlands participating in the supra-regional flow systems is pre-Holocene (Chapter 8).
The fresh-brackish and brackish-salt groundwater interfaces in the Roer Valley Graben are hundreds of metres apart. This is probably because the supra-regional flow system used to extend much deeper than today as indicated by the relatively low content of chlorides in groundwater in clays at depths of 1400 - 1500 m in the Roer Valley Graben (Chapter 8).
9.2 Artificial fluid flow systems
Figure 32 shows the distribution of the major artificial fluid flow systems in the onshore Netherlands.
People have been lowering the groundwater levels in the peat and clay areas in the western and northern parts of the Netherlands since the Middle ages. Apart from influencing the natural groundwater flow systems, the lowering of the groundwater levels also resulted in extensive irreversible mechanical compaction of Holocene peat and clay layers and associated surface subsidence (e.g. Huisman et al. 1998).
There has been additional artificial compaction of shallow and deeper parts of the subsurface as a result of human-induced loading of the subsurface (e.g. houses) and gas production (Doornhof 1992), respectively.
Extensive abstraction of fresh groundwater for public water supply and groundwater use and abstraction for industry and agriculture have affected local, regional and supraregional groundwater flow systems in onshore Netherlands (e.g. Dufour 2000, Stuurman and Vermeulen 2000, Stuurman et al. 2000, Wiers 2001).
9.3 Fluid flow systems induced by sedimentary loading and associated burial-related processes
Evaluation of observed groundwater pressure distributions (Chapter 7) identified a qualitative relation between the pressure distributions, fluid flow and Tertiary and Quaternary sedimentary loading.
Late sedimentary loading showed relations with groundwater flow, compaction, and normal pressures in sediments of the Upper North Sea Group today (Japsen 1999, Kooi and De Vries 1998, Kooi et al. 1998, Stuyfzand 1993). Neogene and Quaternary sedimentary loading was found to be related to undercompaction and overpressures in the Lower and Middle North Sea Groups and Chalk Group in the northernmost offshore part of the Netherlands (Japsen 1999, Winthaegen and Verweij 2003).
Ongoing active expulsion of compaction-derived water from poorly permeable Tertiary and Quaternary hydrostratigraphic units probably occurs in onshore and offshore Netherlands (corresponding to the shallow subsystem of burial-induced flow, Chapter 1) except in the northernmost offshore area, where compaction disequilibrium conditions probably prevail in the Lower Tertiary.
Major overpressuring of the Chalk occurs in the northern offshore. These overpressure gradients are indicative of southward flow through the Chalk. Such flow conditions are characteristic for the transitional subsystem of the burial-induced flow system (Chapter 1). The normally pressured Lower Cretaceous reservoirs in the West Netherlands Basin and the Broad Fourteens Basin contain chloride-dominated brines with TDS of approximately 74 000 - 140 000 mg/l. In the West Netherlands Basin,
water with the highest content of dissolved solids (TDS = 140 000 mg/l) occurs in the most deeply buried formation. In addition, the calcium content increases with increasing TDS, probably resulting from increasing water-rock interaction. These chemical characteristics point to only little groundwater flow through these Lower Cretaceous reservoirs in the southern part of the study area. This is in accordance with only minor late sedimentary loading of the area (Chapter 3).
It was found that Neogene and Quaternary sedimentary loading and even sedimentary loading since the start of the Tertiary can only explain part of the observed over-pressures in pre-Cretaceous sedimentary units in the northern offshore. More prolonged sedimentation or other processes and forces must also have been involved in shaping the present-day overpressure distribution. A wide variety of additional mechanisms may be jointly responsible for the overpressure distribution: a. mechanisms related to ongoing deeper burial of the reservoir not directly related to mechanical loading (pore volume reduction by chemical compaction and salt cementation; pressure redistribution by groundwater flow; aquathermal pressuring of the pore water); b. gas generation in e.g. Jurassic source rocks; c. changes in lateral compressive stresses;
d. pressure redistribution by density-controlled flow; e. salt deformation during the Cenozoic. Overpressures in these pre-Cretaceous units in part of the Central North Sea Graben and in the Terschelling Basin are close to the regional maximum values indicating that present-day conditions are probably favourable for hydraulic fracturing of brittle rocks allowing groundwater flow. The Jurassic and Early Cretaceous
claystones form a thick seal for the underlying severely overpressured syn-rift reservoir-type hydrostratigraphic units. The early-rift reservoir type units of the Lower Germanic Trias Group are sealed by evaporites and shales of the Upper Germanic Trias Group. Fluid flow from these severely overpressured units is probably episodic and concentrated through hydrofractures or reactivated pre-existing fractures and faults resulting from the constant interaction between the fluid pressure and the stress regime (such restricted flow conditions correspond to the deep subsystem of characteristics of the burial-induced flow system; Chapter 1). In the North Sea, severe overpressures occur along the northern limit of the Slochteren Formation below a thick Zechstein seal. Zechstein salts may hold overpressures close to lithostatic. The magnitude of overpressures decreases in the direction of decreasing sealing capacity of the Zechstein Group, e.g. towards the Texel IJsselmeer High, and in the direction of increasing thickness and permeability of the Slochteren Formation.
The available data on overpressures in pre-Tertiary reservoir units indicate a clear distinction between the northern overpressured area and a southern normal to slightly overpressured area. A possible explanation for this is the combined effect of differences in magnitude and duration of Cenozoic or Late Cenozoic sedimentary loading and in distribution and sealing capacity of the Zechstein Group.
Figure 32 includes the geographical distribution of the pressure that is thought to be related to burial-induced groundwater flow systems.
The identified forces and processes operating during Tertiary and Quaternary times may all have contributed to present-day pressure and fluid flow conditions: sedimentary loading, the topography of the water table, tectonic forces and ice loading (Table 2).
Fluid density differences, gas generation and, most recently, human activities, are additional factors of influence. Present-day fluid flow systems and the associated groundwater potential and pressure distributions will reflect these relatively recent driving forces and processes to a greater or lesser extent. The available data and information alone were not sufficient to evaluate the importance of each of these forces and processes on present-day fluid flow conditions.
The foregoing showed that from the Miocene onward, an increasing part of the southeastern Southern North Sea Basin emerged above sea level. It was considered plausible that since that time the Dutch part of the basin has contained at least two major types of groundwater flow system: a topography-induced groundwater flow system in which water of meteoric origin is driven by the relief of the water table and the flow system activated by sedimentary loading and associated processes related to ongoing burial of the sediments. Sedimentary loading influenced groundwater flow and pressure conditions throughout onshore and offshore Netherlands to a greater or lesser extent. These natural groundwater flow systems are presented in Figure 32 together with the artificial flow systems. The figure shows the widely different temporal and spatial scales associated with this present-day hydrodynamic setting of onshore and offshore Netherlands.