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Journal of Ecological Engineering Received: 2018.10.04 Revised: 2018.11.13 Volume 20, Issue 3, March 2019, pages 112–125 Accepted: 2019.01.10 Available online: 2019.01.20 https://doi.org/10.12911/22998993/99743 Natural Isotopes and Ion Compositions Identify Changes in Groundwater Flows Affecting Wetland Vegetation in the Drentsche Aa Brook Valley, The Netherlands Samer Elshehawi1,2,*, Enno Bregman3,4, Paul Schot5, Ab Grootjans1,6 1 Centre for Energy and Environmental Studies, University of Groningen, The Netherlands. 2 Centre for Isotope Research, University of Groningen, The Netherlands. 3 Province of Drenthe, The Netherlands. 4 Physical Geography Department, University of Utrecht, The Netherlands. 5 Copernicus Institute of Sustainable Development, University of Utrecht, The Netherlands. 6 Institute of Water and Wetland Research, Radboud University Nijmegen, The Netherlands. * Corresponding author’s e-mail: [email protected] ABSTRACT This study uses groundwater isotopes and ion composition to verify model simulations and ecohydrological stud- ies in the Drentsche Aa nature reserve in The Netherlands, which is representative for the northwestern wetland areas in the Ice Marginal Landscape zone. At eight field sites, a total of 24 samples were analysed for their 13C, 14C, 2H, and 18O isotopes and ionic composition. The isotopes indicate that most of the fen peatlands in the area depend on the exfiltration of sub-regional groundwater flows, which confirmed the previous model simulations and ecohydrological studies. At three sites, isotopes and ionic composition indicate that the groundwater from the sub-regional system has been replaced by local infiltrated rainwater, due to nearby groundwater abstractions for drinking water, which influenced the success rates of the restoration measures. Furthermore, the evidence from chloride and 14C contents was found to indicate the presence of more saline groundwater, which are influenced by the groundwater flows near salt diapirs. Groundwater abstractions may enhance the upward flow of the saline groundwater to eventually exfiltrate at the wetlands, affecting the biodiversity of the nature reserve. Keywords: ecohydrology, groundwater modelling, nature conservation, radiocarbon dating, groundwater abstraction. INTRODUCTION types of wetlands, agricultural fields, heathlands and small villages. The heathlands, forests and The wetland vegetation is strongly infleunced wetlands are all part of a protected nature re- by hydrology [Wheeler & Shaw, 1995; Gilvear serve [Van Diggelen et al., 1995]. Different types & Bradley, 2009], notably by the interactions be- of wetland vegetation are dependent on various tween local, sub-regional and regional groundwa- water sources, with the biodiversity-rich fen peat- ter flow systems [Tóth, 1963; Schot & Molenaar, lands primarily depending on the groundwater 1992; Dahl et al., 2007; Van Loon et al., 2009]. flows from phreatic and semi-confined ground- Such interactions among the groundwater flows water aquifers [Grootjans et al., 1993; Van Dig- lead to different types of groundwater-dependent gelen et al., 1995]. Everts & De Vries [1991] il- ecosystems [Wassen et al., 1990]. An example of lustrated the hypothetical groundwater systems in such systems is the Drentsche Aa Brook Valley, Drentsche Aa based on the vegetation gradients which is a nature reserve in the north of the Neth- using the theoretical framework from the ground- erlands. This valley is characterized by various water systems by Toth, [1963] (Figure 1). These 112 Journal of Ecological Engineering Vol. 20(3), 2019 local and sub-regional groundwater systems have regarding the reliability of the simulated flow pat- been intensively studied and the results have been terns and possible up-coning of salt plumes. The used for nature and landscape policy plans. accuracy of the groundwater simulation models However, the area is part of the Ice Marginal depends on the underpinning assumptions and the Landscape zone, and geological proccesses such availability of adequate data. The ecohydrologi- as the pro- and postglacial differential Saalian and cal studies using plant species occurrence as in- Weichselian rebound and the sub-glacial deeply dicators for groundwater flow systems may disre- eroded Elsterian channel systems are not well gard the time lag in the response of plant species understood, especially in relation to the ground- following groundwater flow changes due to e.g. water flows (Figure 2a) [Smit et al., 2015]. On buffering of soil water quality due to the buffering the basis of better geological input, Magri & processes in the soil. Bregman [2011] simulated the groundwater Simulated groundwater flows may be validat- flows in the area using particle tracking, where ed by independent hydrological tracers, such as they indicated the areas of infiltration and exfil- chloride and natural isotopes [Schot & Molenaar, tration of groundwater on sub-regional and local 1992; Gibson et al., 2005; Mayer et al., 2014]. scales. One of the model outcomes was that salt The natural isotopes used as tracers in the ecohy- plumes might diffuse into the groundwater ex- drological research related to groundwater, have filtrating into the wetlands, due to the influence been mostly limited to the stable isotopes of oxy- of double diffusive convection (DDC) processes gen and hydrogen [e.g. Schot & Wassen, 1993; [Diersch & Kolditz, 1998], and compromise the Isokangas et al., 2017]. Radioactive isotopes, wetland biodiversity under increased groundwa- especially radiocarbon, are less commonly used ter abstraction rates (Figure 2b). These salt plume due to practical reasons, e.g. sample volumes and formations originate from the intrusive salt dia- analysis costs [Mook, 2006]. In the study area, pirs of the Zechstein formation [De Vries, 2007; the radiocarbon age dating of groundwater may Magri & Bregman, 2011]. provide insight into the residence times which re- Another threat to the nature reserve involves flects the groundwater flow systems affecting the the groundwater abstractions for drinking wa- wetland vegetation. Young groundwater will gen- ter production from the semi-confined aquifers erally reflect the local flow systems with recently [Mendizabal & Stuyfzand, 2009; Mendizabal et infiltrated water from the vicinity and possibly al., 2011] at two locations adjacent to the brook showing the human pollution signs in their ionic valley (Figure 2). These may change the natural composition. Old groundwater may, however, groundwater flow patterns, which in turn may af- indicate the deep groundwater flow from sub- fect the wetland vegetation and biodiversity. Such regional to regional systems, which would have impacts have been reported for European wet- recharged long ago showing enhanced mineral lands [e.g. Wassen et al., 1990; Schot & Van der dissolution and free from human pollution. Ad- Wal, 1992; Grootjans et al., 1993]. ditionally, deep groundwater affected by the DDC Although the Drentsche Aa nature reserve has near the salt domes would show lower 14C activ- been widely studied, uncertainties remain, i.e. ity, and enriched δ18O values and/or high salinity Figure 1. An illustration of the hypothetical groundwater flow systems conditioning the ecohydrological systems in Drentsche Aa Brook Valley [Source: Everts and De Vries, 1991] 113 Journal of Ecological Engineering Vol. 20(3), 2019 a) b) Figure 2. Groundwater sampling sites and geological features that control the flow patterns in the groundwater (a), simulation model results show the effect of the double diffusion convection along the X-Y transect in (a), which affects groundwater reaching the surface (b) from the evaporites present in the salt diapirs. different rain water isotope characteristics in Stable isotopes may also indicate increased stable ancient recharge water. isotope values as a result of increased evapora- This study uses groundwater isotopes in the tion during slow infiltration through peat layers, Drentsche Aa brook valley to validate (i) the ori- as compared to present-day rain water, or signal gin of groundwater flows to the fen peatlands as 114 Journal of Ecological Engineering Vol. 20(3), 2019 indicated by the model simulations by Magri and cultural aspects of the landscape are still in good Bregman [2011] and by ecohydrological studies condition and were partly restored to a former [e.g. Everts & De Vries, 1991; Grootjans et al., state [Bakker et al., 1980]. The total catchment 1993 and Van Diggelen et al., 1995], (ii) the influ- area of the Drentsche Aa is about 30,000 ha. Out ence of groundwater abstractions on the ground- of these, only 3,500 ha are managed to restore the water flow, and (iii) the possible salination of semi-natural landscape of c. 1900. The topogra- shallow groundwater in the nature reserve by up- phy of the landscape consists of a plateau at a coning water from salt diapir evaporites. Further- height of about 28 m AMSL in the upper reaches, more, the study aims to assess restoration success with the brook valley following the topographic in light of these investigations. relief to reach the height of about 0 m AMSL in the lower reaches at Kappersbult. According to De Vries [2007], the rainfall av- STUDY AREA erages in the Netherlands are relatively constant with a precipitation surplus of 250–300 mm a The Drentsche Aa brook valley is the best- year that mostly falls during winters. The fresh preserved brook valley landscape in the Nether- groundwater flows in the eastern parts of the lands (53°1’52.18”N.

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