Elsevier

Journal of Hydrology

Volume 378, Issues 3–4, 30 November 2009, Pages 205-217
Journal of Hydrology

Palaeo-hydrological reconstruction of a managed fen area in The Netherlands

https://doi.org/10.1016/j.jhydrol.2009.09.014Get rights and content

Summary

Knowledge of the anthropogenic impact on the hydrology of low-productive fens that are subject to environmental degradation is essential to improve currently utilized hydrological fen restoration strategies. We analyse the naturally and anthropogenically driven evolution of groundwater systems in an intensively managed fen area in The Netherlands using a series of three-dimensional palaeo-groundwater models at a high spatial and temporal resolution. These palaeo-models are representative for five time slices of the time frame 0–2000 AD (Anno Domini), which are defined according to the timing of the natural and anthropogenic developments that had major impacts on the groundwater system configuration. For each time slice, palaeo-geohydrological conditions are reconstructed, which allowed for the calculation of groundwater discharge patterns, water balances and groundwater flow patterns.

Contrary to former studies on the evolution of groundwater systems, our palaeo-hydrological reconstruction indicates that current groundwater discharge flux into managed fens may exceed the late-natural groundwater discharge flux. The increased groundwater discharge flux relates to the development of groundwater systems in the river valley with the establishment of polders since 1350 AD. Notably, more recent redirections of groundwater flow due to the reclamation of lakes and the establishment of abstractions wells, as well as the decreased groundwater recharge by anthropogenic land cover change, reduced the groundwater discharge flux only to a minor extent. This finding opposes the hypothesis that a decreased groundwater flux to fens underlies the environmental degradation of fens in intensively managed regions. The palaeo-hydrological reconstruction provides evidence that it is mainly the changes in the spatial configuration, and the shift in the predominant groundwater discharge mechanism, that underlies the environmental degradation of managed low-productive fens. We discuss the consequences of these hydrological changes for the suitability and availability of fen habitat sites.

Introduction

Many fen plant species have become endangered due to worldwide land reclamation and environmental degradation of fens (Millennium Ecosystem Assessment, 2005). Environmental degradation of fens consists of acidification, eutrophication and desiccation. These processes are often induced by anthropogenic interferences into the regional hydrology of fens (Lamers et al., 2002). Counteraction of these interferences by active water management aims at sustainable conservation of the remaining populations of endangered fen plant species, and the re-establishment of fen plants in areas where they have disappeared. However, hydrological restoration strategies have not been particularly effective so far, possibly, due to insufficient knowledge of the human impact on the hydrology of managed fens. The present study aims at providing new insights into the long-term changes of the groundwater supply of a managed fen area in The Netherlands.

Most endangered fen plants are typically found in nutrient-poor fens, which have a low biomass production (Wheeler and Shaw, 1991). These so-called low-productive fens develop at minerotrophic, acid-buffered, and moderately nutrient-rich sites (Bedford et al., 1999, Sjörs and Gunnarsson, 2002). These sites are usually associated with areas of groundwater discharge for two reasons. Firstly, areas of groundwater discharge receive excessive amounts of water, i.e., groundwater and precipitation, which results in shallow groundwater tables and reduced conditions in the shallow subsurface (Boomer and Bedford, 2008, De Mars and Wassen, 1999). Secondly, weathering products dissolved during flow are transported with the groundwater to the fen surface. Both reduced conditions and the presence of weathering products in the shallow subsurface, are essential for buffering the acidity at a near-neutral pH level (Almendinger and Leete, 1998) and limiting nutrient availability for plant growth (Boyer and Wheeler, 1989, Olde Venterink et al., 2002).

Many of the processes underlying the environmental degradation of low-productive fens are reinforced or induced by a decrease of the groundwater supply of fens (Barendregt et al., 1995, Fojt and Harding, 1995, Lamers et al., 2002). A decrease of the groundwater supply of fens may cause desiccation if the consequent water deficits are not counterbalanced by an increased supply of precipitation (Schot et al., 2004) or surface water (Van Wirdum, 1991). However, these shifts in the origin of water supply affect the biogeochemical processes determining the acidity and nutrient availability in fens (Almendinger and Leete, 1998), and may cause acidification or eutrophication of fens (Beltman et al., 2000, Smolders et al., 2006). Therefore, the restoration of groundwater flow to low-productive fens is thought to be a prerequisite for the preservation of threatened fen plant species in intensively managed areas (Fojt and Harding, 1995, Wassen et al., 1990).

The design of effective hydrological fen restoration strategies requires insight into the causal relation between water management and the evolution of groundwater systems. Numerous studies have been performed on the evolution of groundwater systems in the past by analysing (Lamentowicz et al., 2007) or reconstructing (Pons and Oosten, 1974) botanical peat deposits, performing groundwater model exercises (Schot and Molenaar, 1992) and comparing intensively managed areas with near-natural areas (Wassen et al., 1996). These studies suggest that the groundwater supply of managed fens decreased with anthropogenic developments. However, none of these studies provided a quantitative analysis on the evolution of groundwater systems driven by natural and anthropogenic developments. Obtaining this lacking knowledge may improve the current insights into the anthropogenic impacts on the hydrological key-processes that underlie the environmental degradation of low-productive fens in intensively managed regions like The Netherlands.

In this paper, we analyse the naturally and anthropogenically driven evolution of groundwater systems that discharge into an intensively managed fen in The Netherlands using a series of three-dimensional palaeo-groundwater models at a high spatial and temporal resolution. These palaeo-groundwater models are based on a thoroughly underpinned and complete reconstruction of geohydrological conditions in the past. The purpose of this model exercise is to analyse past shifts in the groundwater supply of fens, and to disentangle the effects of subsequent natural and anthropogenic developments on the groundwater supply of fens.

Section snippets

Study area and its historical development

The palaeo-hydrological reconstruction was performed for the Gooi- and Vechtstreek area in The Netherlands (52°03′N–52°20′N and 5°00′E–5°18′E; Fig. 1). The study area was selected because (1) sufficient geo-scientific knowledge and data is available to perform a reconstruction of past geohydrological conditions on a high spatial resolution and (2) human domination of the current hydrology is large, due to a series of hydrological changes that have occurred in the past, many of which are

General

Five time slices were defined according to the timing of natural and anthropogenic developments that were expected to underlie the major transformations of the groundwater system configuration in the past (Fig. 2). From a comparison of these five time slices, insight into the evolution of the groundwater systems in response to natural and anthropogenic developments was deduced.

For each time slice, groundwater flow was simulated with a stationary groundwater model based on the MODFLOW-code (

Results

The results of the palaeo-hydrological reconstruction are depicted in Table 5 and Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8. We distinguish three phases of landscape development according to the evolution of geo-hydrological properties of the river valley (Fig. 4): the accumulation phase, the stagnation phase and the degradation phase. Below we describe the co-evolution of geo-hydrological properties and groundwater systems during these phases.

Methodological approach

In this study, we analyzed the naturally and anthropogenically driven evolution of groundwater systems using a series of three-dimensional palaeo-groundwater models. Similar to many palaeo-groundwater models such as presented by Sanford and Buapeng, 1996, Piotrowski, 1997 and Van Weert et al. (1997), the palaeo-models presented in this paper could not be validated, because quantitative reference data is only available for the last few decades, and not for the past time slices. Nevertheless, we

Conclusion

The results of the present study provide new insights into the impact of anthropogenic hydrological interferences on the groundwater supply of fens on a regional scale. The finding of an increased groundwater discharge flux into managed fens after the initiation of water management opposes the hypothesis that a decreased groundwater flux to fens underlies the environmental degradation of fens in intensively managed regions (Barendregt et al., 1995, Fojt, 1994). Instead, the palaeo-hydrological

Acknowledgements

The authors would like to thank Henk Weerts and Jan Gunnink for their contribution to the palaeo-geographical reconstruction of the studied fen during natural conditions, Ype van der Velde and Ruth Heerdink for their technical support with the construction of the groundwater models, Henk Kramer for providing the historical land use map of The Netherlands, Rebecca Elkington for proofreading the manuscript, and two anonymous reviewers for their suggestions to improve the manuscript.

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