Preface To preserve and restore rainwater fed wetlands (ombrotrophic bogs) and their characteristic biodiversity, the Dutch Survival Plan for Forest a

Transcription

1 Preface To preserve and restore rainwater fed wetlands (ombrotrophic bogs) and their characteristic biodiversity, the Dutch Survival Plan for Forest and Nature (OBN) has financed a pilot study on hydrological relationships between bogs and the surrounding landscape. Such knowledge is vital for the conservation and restoration of such wetlands. In the pilot study well developed bog with abundant Sphagnum growth will be compared with hydrological affected systems with little Sphagnum growth. Four research areas throughout the Netherlands have been selected (Groote Heide, Haaksbergerveen, Gasselterveld and Lheebroekerzand). This master thesis focuses on the Gasselterveld area and the Lheebroekerzand area, both in the province of Drenthe (NL).

2

3 Summary Shallow bogs and pingo remnants are two different types of mires that have existed for a long time already in peat filled depressions in the cover sand landscape of Drenthe. Bogs are often small and shallow. Pingo remnants can be very deep and are often influenced by base rich groundwater now or in the past. The present vegetation usually consists of living Sphagnum carpets or, in case of desiccation, Molinia caerulea tussocks are dominating. In the Gasselterveld (near Gieten) and the Lheebroekerzand (near Dwingeloo) both pingo remnants and bogs are present. In both areas we selected some bogs and pingo remnants to study soil development and hydrological conditions in order to obtain a better understanding of the (hydrological) boundaries of bogs and pingo remnants systems. The bogs studied in the Lheebroekerzand were regenerating very well after peat abstraction and drainage in the past. They are fed by both precipitation water and local groundwater. The bogs in the Gasselterveld originated in the same way as the bogs in the Lheebroekerzand but the bogs in the Gasselterveld have dried out and the top layers of the peat is decomposing. In the Pingo remnants in this area a rainwater lens has been formed on top of layers of Braun moss peat that was influenced by base-rich groundwater in the past. This rainwater lens has created an acidic, ombrotrophic environment were Sphagnum mosses could grow. The reconstructions of the bog and ping remnants in both areas show that in Lheebroekerzand the peat levels were higher in the past. The reconstruction in the Gasselterveld shows that there has been a considerable drawdown of the groundwater level due to sand abstraction nearby and also due to high evaporation of pine plantations. This has led to severe desiccation of the shallow bogs and to drying out of the top layer of the pingo remnants.

4

5 Table of Content Introduction...7 Methods and Materials...11 Results...13 Soil profile descriptions...13 Temperature profiles...18 Chemical analysis of peat profiles...19 Water composition...23 Water levels and water quality...24 Determination of plant remains in peat profiles...25 Discussion...27 Lheebroekerzand...27 Gasselterveld...31 Overall Discussion...34 Conclusion...35 Acknowledgements...35 Literature...37 Appendix 1: Peat analyses (ICP)...39 Appendix 2 Water quality Barkmansveen and Poort Appendix 3 Water quality Veentje 1 and Veentje

6

7 Introduction Rainwater fed wetlands (ombrotrophic bogs) are very sensitive to acidification and desiccate easily when changes occur in the local hydrology (Streefkerk et al. 1987). Leaking water to its environment through the rather thin organic layers underneath the bogs seems to be the biggest problem here. Dekker et al. (1986), studied desiccation bogs near Sellingen and found that change in local hydrology, in this case caused by groundwater abstraction increased water losses through the B-horizon underneath the bogs. Due to low water levels the peat starts decomposing and as a result, typical bog species are declining rapidly or become extinct. In North West Europe such small heath land bogs are usually formed in wet depressions where the wind has blown away the dry sand. Underneath such depressions we usually find boulder clay (Bakker et al. 1986) and these depressions are quite shallow (1-2 m). Their primary water source is precipitation water. Fine organic material can be transported downwards from the surface of the bog to deeper layers. Clogging of the pores in the mineral soil occurs and an almost impervious layer is formed in the subsoil. This soil layer is called a B-horizon. The more organic material is percolated the more the pores get clogged and less permeable to water the layer is. The B-horizons with much disperse humus in them have a black color (found underneath thick organic layers); the ones with less disperse humus in it, a red/brown color. The forming of a B-horizon is part of the podsolisation process. This soil forming process occurs in sand soils where water can percolate. Typical of a podsol soil is the ABC structure were organic material, iron and other ions can be incorporated into the B-horizon. The C layer is the undisturbed parent material. Different podsolic soils can be distinguished based on the thickness and the content of the B- horizon (Pape 1976, Stobbe & Wright 1959). In some cases also Waterhard layer can be formed. These are thin but very hard layers in the mineral soil where organic material binds with sand particles like a pearl necklace (Koopman 2004). When permanent water is present above the B-horizon or Water hard layers a small Gliede layer is formed on top of the B-horizon. This organic mud formed by decaying water plants adds considerably to the low permeability of the organic soil layers (Dekker et al 1986). When such a pool terrestrializes ombrotrophic peat is formed (Bakker et al 1986). The peat formation of most Drenthian heath land bogs goes back to the Holocene (Bakker et al 1986). In natural wet depressions of sand landscapes, organic matter accumulated. Because of the wet and ombrotrophic conditions, peat forming mosses of the genus Sphagnum dominate the vegetation in these depressions. In eastern-europe a similar type of bog system is found which are not formed in deflated depressions but by melting ice remnants after the last glaciation. Timmerman (1999), Succow (1986, 1988) and Succow & Joosten (2001) described such kettle hole mires in Germany and Poland. These depressions were formed in the Saalien ice age. Large chunks of glacier ice were left behind in the sediments. When the ice melted, a steep depression was left behind. Later peat forming started in these depressions. De Gans (1981) described landforms in Drenthe called pingo remnants. These landforms are very similar to the kettle hole mires described by Timmerman (1999) and Succow (2001), but they were formed in a much earlier period. Pingo formation can only occur under permafrost conditions. When groundwater enters the permafrost layer it freezes. Because of the groundwater pressure the ice forms a hill beneath the surface. A hill with a core of ice is called a pingo (de Gans, 1981). When the core of the hill melts, the hill will collapse. What is left is a typical landform that is similar to a kettle hole: a round and deep depression in the landscape with a wall around it (figure 1). Usually clogging of organic material also occurs in the walls of such depressions and peat formation can proceed during thousands of years. 7

8 Because pingo remnants are formed under permafrost conditions, they could only be formed in Drenthe during the two ice ages Saalien and Weichselien (de Gans 1981). Pingos that were formed in the Saalien period penetrate the boulder clay layers that were formed during the same period (de Gans 1981). Pingos that were formed in the Weichselien era are situated in places with thin or no boulder clay layers (de Gans 1981). In Drenthe the pingo remnants originated between and BP, which corresponds with the Weichelian permafrost era (de Gans 1981). Pingos that originated in the Saalien era may not be recognized as pingo remnant nowadays. Figure 1. The development of a pingo and pingo remnant (De Gans 1981). In this figure no clay layer is displayed. The Drenthian geomorphology was primarily formed in the second of last ice age, Saalien. In this era the north of the Netherlands was covered with ice. Glaciers from the north of Europe reached al the way to the line Haarlem-Utrecht-Nijmegen. These glaciers brought, much sand, large rocks, and other debris from Scandinavia. Beneath the glacier, material was compressed and packed in to a layer of fine material combined with debris; the boulder clay layer. When the temperature rose the glacier retreated and all the debris and the boulder clay layer were left behind. The melting water from the glacier created rivers that cut into the clay and deposited fine material. In the warmer period after the Saalien ice age (Eemien), the melting water flattened the surface area. The river gullies were still present. In the last ice age (Weichselien) the glaciers did not reach Drenthe, nevertheless the temperature decreased. In northern Europe large quantities of water turned into ice and consequently sea levels dropped. As a result groundwater levels in Drenthe also declined. Because of the cold and dry climate the vegetation was also declining and erosion increased rapidly. Sand became easily mobilized by wind and sand dunes began to form, especially in wet depressions. Wet depressions were filled with sand and became the highest point in the landscape. Bakker et al. (1986) describes this phenomenon as reversing the land topography (figure 2) 8

9 Figure 2: reversing the land topography (obtained from Verschoor et al. 2003) A) Erosion gully with water flow B) Erosion gully with poor or no water flow C) Reversing the land topography by catching sand in wet areas D) Peat and B-horizon forming. At the end of the Weichselien the temperature rose and the land ice in the northern Europe retreated. Sea levels and groundwater levels in Drenthe rose considerably. This warmer period after the Weichselien is defined as the Holocene. In this period peat forming started to accelerate. At present many heath land bogs are suffering from desiccation. One of the main reasons is the changing hydrology in the surroundings of the bogs (Streefkerk et al. 1987). The reasons of these changes are primarily anthropogenic (extraction of groundwater for drinking water production and the planting of dense pine forests). Undisturbed Sphagnum growth in small bogs requires permanently wet conditions and a relatively high CO 2 content of the surface water (Smolders 2001). Nature conservation organizations need more knowledge about local hydrology of local bogs to successfully restore and protect these ecosystems. The main question in this research project is: Are small heath lands bogs and pingo remnants responding differently on hydrological changes in their environment? Sub questions are: How are bogs and pingo remnants influenced by the local a regional hydrological systems? How did the bogs in the Lheebroekerzand and Gasselterveld area originate? 9

10 10

11 Methods and Materials Two areas in Drenthe have been selected for this research project: 1) The Lheebroekerzand area (part of the national park Dwingelerveld) nearby the village of Dwingeloo and 2) the Gasselterveld area near the village of Gieten. In the Lheebroekerzand area well developed bogs are present. The bogs selected are Barkmansveen, Zandveen, Wolfsklauwtjesveen, Poort 1, and Poort 2. Schurenberg fits the description of a pingo remnant (Vrielink 1976, Castel 1984, Bakker 1986) and is also studied (figure 3). All altitude measurements are done relatively to the Spieregerweg. Figure 3. Map of the Lheebroekerzand area. The white lines represent transects, where the soil development is described. Altitude measurements are set to zero at the red point Bogs in the Gasselterveld area are in a bad condition. Many have dried out and there is almost no Sphagnum growth. Four of these bogs have been selected. All altitude measurements are done relative to the bottom of the Anderse Diep, which is part of the upper course of the Drentsche Aa. Figure 4. Map of the Gasselterveld area. The white lines represent transects, where the soil development is described. Altitude measurements are set to zero at the red point 11

12 The bogs that have been selected in the Lheebroekerzand area are all shallow wetlands with poorly permeable ground layers beneath it (Bakker et al. 1986, Verschoor 2003). Baaijens (1984) describes that the bogs are possibly laying in erosion gullies made by the glaciers in ice age Saalien. Verschoor et al (2003) and Rooke (2002) proved there is a water flow between some of the selected bogs in the same erosion gully. It is possible that the poorly permeable ground layer is not only present beneath one bog but also spreads to a second bog (Verschoor et al. 2003). Schurenberg is classified as a pingo remnant (Vrielink et al. 1976, Castel 1984, Bakker 1986). Because of its depth (4 meters interpreted of Vrielink et al 1976), it could be influenced by base rich groundwater form underneath the boulder clay layer. Most bogs in the Gasselterveld have been affected by desiccation. The cause of the down fall of the groundwater level in this area can be the massive planting of trees in the forest. Also commercial sand extraction may have influenced the groundwater level. De Gans (1981) studied pingo remnants in the Gasselterveld area and did not find poorly permeable B- horizons that are common in the Lheebroekerzand area. It is possible that Veentje 1 and Veentje 2 can be marked as pingo remnants. The cause of the down fall of the groundwater level in this area can be the massive planting of trees in the forest. Also commercial sand extraction may have influenced the groundwater level. All measurements were carried out in the summer and autumn of These measurements have all been done within transects. All available hydrological, geological and soil maps were studied and several peizometers (ø 32mm) were installed. The peizometers where installed using a small 32mm Edelman corer. Soil descriptions were made using an Edelman corer. We also used a chamber corer to sample peat for detailed macroscopic analyses. Profile descriptions where done in transects (figure 3 and 4), usually 1-4 meters deep. Soils have been described according to de Bakker et al. (1989), Locher et al. (1990a), Locher et al. (1990b), Kuipers (1976), van Breemen et al. (2002) and de Bakker et al. (1989). B-horizons were classified into two types (i) black b-horizons with a totally black color and (ii) brown b-horizons with a light to dark brown color (less disperse humus). Leveling was done with a theodolite. In the Lheebroekerzand area, the road junction with the sand path to Barkmansveen and the Spieregerweg (N855) (figure 3) was set to zero. In the Gasselterveld the bottom of the Anderse Diep was set to zero. The vegetation of three bogs in the Lheebroekerzand was described (Barkmansveen-Zandveen- Wolfsklauwtjesveen) in plots along a transect 50 cm on either side of the transect. Percentage cover and frequency of plants were noted using the method of Baas Becking et al (1934). The nomenclature of the plant species follows van der Meijden (1996) and Bouman (2002). In order to detect flow patterns in addition to the work of Rooke (2001), temperature were measured in transects through Barkmansveen and Poort 2. This was done by using an EC/temperature probe with a length of two meters. Soil water in the peat was sampled at different depths using rhizons, which prevent oxidation of the water. The samples were analyzed for ph, alkalinity, CO 2, HCO 3-, NO 3, NH 4, PO 4, K Na, Cl, Al, Ca, Fe, K, Mg, Mn, P, Si and Zn by the Radboud University of Nijmegen using ICP. Also cores from Veentje 1, Barkmansveen, and a decomposed peat section between Barkmansveen and Zandveen were analyzed. For the analysis all organic material of 200 mg of soil was completely destructed (with the result that elements that were attached to the organic matter are not bonded anymore) and analyzed using ICP. The amount of organic material from the same core samples was also measured. The samples dried ((105 C) and burned at 550 C for four hours and again at 850 C also for four hours. After each step the sample was weighted. 12

13 Results Soil profile descriptions Data from field coring combined with information of soil maps were used to make the following figures. Transect 1 Lheebroekerzand: Barkmansveen Zandveen-Wolfsklauwtjesveen Figure 5: Lheebroekerzand transect 1: Barkmansveen Zandveen-Wolfsklauwtjesveen, black dots indicates the coring locations. Data was also information obtained from Verschoor et al. (2003). A boulder clay layer is underneath the whole transect (Fig. 5). Above the boulder clay, B-horizons are present underneath the bogs (dark brown), but they do not form a continuous layer. Between the bogs the B-horizons are lacking. Gliede layers are also only present underneath the bogs. 13

14 Vegetation description Transect 1: Lheebroekerzand Barkmansveen-Zandveen Figure 6: Vegetation description Lheebroekerzand: Barkmansveen-Zandveen In figure 6 the vegetation descriptions are added to the figure 5. 14

15 Profile Barkmansveen east-west Figure 7 Profile Barkmansveen east-west Transect 2 Lheebroekerzand: Schurenberg-Poort 1-Poort 2 Figure 8: Lheebroekerzand transect 2 Schurenberg-Poort 1-Poort 2. Also information obtained from Vrielink et al. (1976) 15

16 Figure 7 shows the profile of Barkmansveen at a 90 degrees angle on transect 1. The B-horizon stretches much further than the peat it self. The boulder clay layer is lower at the west side. Between the 120 and 160 point no soil descriptions were carried out. Only at point 175 a soil description was made, which shows that the B-horizon and decomposed peat is present far away from the present bog. Typical in figure 7 are the sharp angles of elevation between 50 and 150 meters. In this figure the boulder clay is missing underneath Schurenberg because soil descriptions could not be carried out. It is possible that the boulder clay layer is penetrated by Schurenberg. The black B-horizons are only present underneath Poort 1 and Poort 2. On the slope of the hill between Poort 1 and Poort 2 a decomposed peat section was found. Profile Poort 2 West-east Figure 9: Profile Poort 2 west to east Figure 9 shows a profile of Poort 2 at a 90 degrees angle. No detailed information of the boulder clay layer could be obtained except for the coring in the peat. The black B-horizon and the Gliede layer are only present at the places were peat is present. 16

17 Transect 1 Gasselterveld: Veentje 1-Veentje 2 Figure 10: Gasselterveld transect 1 Veentje 1 Veentje 2 Figure 10 (transect 1 Veentje 1-Veentje 2 ) shows that the bottom of Veentje 1 as well as Veentje 2 is situated lower than the fluvial loam layer. This picture shows that these depressions are much deeper than the small Sphagnum bogs, up to 2, 70 meters. In these depressions gyttja layers are found. Also Braun moss peat in various stages of decomposition was found. In Veentje 1 also a layer of black decomposed peat was found within a Braun moss peat layer. Course sand was found on the east side of Veentje 1. On this coring location no fluvial loam was found but aeolian loam. Within the transect some course sand was found. The points in the profile were course sand was found are too scattered to interpolate a plausible layer. 17

18 Transect 2 Veentje 3-Veentje 4 Figure 11: Gasselterveld Veentje 3- Veentje 4 Figure 11 shows that both bogs are shallow and that the peat is almost completely decomposed. A black B-horizon is situated underneath the bogs. A brown B-horizon is found between the bogs. Square depressions within the bog are man made in the early nineties (Ouwerkerk 2004). On two places it was possible to core into the fluvial loam (vertical blue lines). Coarse sand was found directly underneath the loam layer. Temperature profiles Temperature profiles from Barkmansveen and Poort 2 were made in addition of the work of Rooke (2001). Temperature profile Barkmansveen In this figure, layers of the same temperature class can be determined. The layers are mostly horizontal. On the south side of Barkmansveen (left in the figure) the temperature is lower than on the north side. 18

19 Z N Depth (cm) /distance(m) ,3 11,2 10,8 11,3 11,5 11, ,3 11,7 14,2 12,1 12,3 12,8 13,1 12,8 12,2 11,8 11, ,1 10,3 11,1 11,2 11,8 11,1 11, ,2 12,6 13,2 12,2 12,7 12, ,8 11,8 11, ,1 10,8 11,2 11, ,3 11,8 12,2 12,5 12,5 12,2 12,3 12,2 12,7 11,2 11,5 11,6 60 9,8 10, ,7 10,8 11,2 11,8 11,8 11,8 11,6 11,8 11,6 11, ,2 10,9 80 9,7 9,7 10,1 10,5 10,5 10,5 10,5 11,5 11,2 10,5 10,8 11,1 11, ,7 10,8 10, ,4 9,4 9,7 9,9 10,1 10, ,5 10,6 10,1 10,3 10,7 10,8 10,5 10, ,2 9,7 9,8 9,8 9,5 9,9 10,1 9, ,2 10,5 10, , Figure 12: Temperature profile Barkmansveen south to north. The arrow shows the direction of the water flow found by Rooke (2001). At point 60 it was too wet to measure. Temperature profile Poort 2 In the temperature profile of Poort 2 is no clear vertical pattern can be determined. The temperatures in the top layer are higher than the ones measured in the deeper parts of the bog, indicating a horizontal pattern. Z N Depth (cm)/distance(m) , ,5 10,8 10,1 11,3 11,3 10,9 14, ,2 11,1 11,2 11,2 12,2 10, , ,7 12, ,5 10,8 11,2 10,9 11,9 10,5 10,1 10,7 10,8 10,7 12, ,2 10,5 10,8 10,5 11,2 10 9,7 10,1 10,5 10,2 11,4 80 9,7 10,1 9,8 10,7 9,5 9,2 9,7 9,8 9,7 10, ,3 9,6 9,5 9,9 9 8,8 9,1 9,2 9, ,9 9,1 9,1 9,5 8,6 8,6 8,6 8, ,5 8, Figure 13: Temperature profile Poort 2. The arrow shows direction of water flow by Rooke (2001). Chemical analysis of peat profiles Samples from Veentje 1, Barkmansveen and the decomposed peat section between Barkmansveen and Zandveen were analysed. Veentje 1 Barkmansveen decomposed peat Ca - - No trend Mg - - No trend Al Fe K + - No trend Mn Zn + same - Si + no trend same Na same - same S same - No trend P same - No trend Organic matter - - No trend Legenda 12,1- hoog 11, ,1-11,5 10, ,1-10,5 9,6-10 9,1-9,5 8,5-9 Table 1: summary of chemical peat anlyses. + = concentration of element increases with the depth, - = concentration of element decreases with the depth. Same= concentration remains the same at all depths. No trend = data is too scattered to make a plausible trend line 19

20 An overview of the results of the chemical analysis is shown in figure 12. In this figure the relation with the depth of the mire is displayed. A selection of the results of the ICP and organic matter analysis are displayed in the following figures. All results are shown in appendix 1. Calcium and organic matter Calcium concentration and organic matter content in soils can be linked while calcium can bind to organic matter (Benedetti et al, 1995). Therefore calcium concentrations and organic matter contents of each bog are shown separately. Calcium veentje Depth (cm) Ca (µmol/g) Figure 14: Calcium concentrations per gram dry peat in Veentje 1 Organic matter Veentje 1 Depth (cm) % Organic matter Figure 15: percentage organic matter in Veentje 1 In Veentje 1 the amount of organic matter in the top soil is less than the amount at 50 cm beneath the surface (figure 15). At a depth of 2 meters the amount of organic matter drops severely indicating more mineral components. Calcium follows the same trend as the organic matter but decreases rapidly starting at 50 cm below the surface (figure 14). While organic matter icontent remain constant in the top 2 meters. 20

21 Calcium Barkmansveen Depth (cm) Ca (µmol/g) Figure 16: Calcium concentrations per gram dry peat in Barkmansveen Organic mat t er Barkmansveen Depth % organic matter Figure 17: percentage organic matter in Barkmansveen In Barkmansveen calcium concentrations are low and organic matter content is high. Both are decreasing in the bottom part of the bog. Calcium Lheeboekerzand decomposed peat Depth (cm) Ca (µmol/g) Figure 18: Calcium concentrations per gram dry peat in decomposed peat section at point 160 figure 5 in the Lheebroekerzand area 21

22 Organic mat ter Lheebroekerzand decomposed peat Depth (cm) % Organic matter Figure 19: percentage humus in decomposed peat section at point 160 figure 5 in the Lheebroekerzand area In the decomposed peat section found between Barkmansveen and Zandveen, higher calcium concentrations are found in contrast to Barkmansveen. The amount of organic matter is lower in contrast to Barkmansveen. Iron Iron is often found in high concentrations in deep groundwater. Peat that is influenced by deep groundwater now or in the past is likely to have also high iron concentrations. Iron concentrations in Veentje 1 increase with depth. Also in Barkmansveen the iron concentration increases in the deeper regions and are as high as in Veentje 1. The decomposed peat section in the Lheebroekerzand area has a lower iron concentration. Fe veentje Fe ( µmol / g) Figure 20: Iron concentrations per gram dry peat in Veentje 1 Fe Barkmansveen Fe ( µmol/ g) Figure 21: iron concentrations per gram dry peat in Barkmansveen. 22

23 Fe Lheebroekerzand decomposed peat Depth (cm) Fe (µmol/g) Figure 22: iron concentrations per gram dry peat in decomposed peat section at point 160 figure 5 in the Lheebroekerzand area Water composition Water samples from Barkmansveen and Poort 2 in the Lheebroekerzand and Veentje 1 and Veentje 3 in the Gasselterveld area were taken on different dates. A selection of the results is displayed in the following tables (all results appendix 2 and 3). Barkmansveen Date Depth ph - HCO 3 µmol/l Ca µmol/l Fe µmol/l , ,57 11, , ,96 21, , ,17 48, , ,73 85, , ,36 17, , ,32 22,95 Table 2. Water composition of Barkmansveen, Lheebroekerzand. Poort 2 Date Depth ph - HCO 3 µmol/l Ca µmol/l Fe µmol/l ,65 9 9,19 2, ,91 1 4,84 1, , ,44 5, , ,49 7, ,00. 43,33 6, , ,04 1,55 Table 3. Water composition of Poort 2, Lheebroekerzand 23

24 Veentje 1 Date Depth ph - HCO 3 µmol/l Ca µmol/l Fe µmol/l , ,41 6, , ,41 2, , ,80 20, , ,21 15, , ,64 95, , ,54 18,28 Table 4. Water composition of Veentje 1, Gasselterveld Veentje 3 Date Depth ph HCO 3 - µmol/l Ca µmol/l Fe µmol/l , ,53 2, , ,38 7, ,57. 60,48 4, , ,42 8, , ,58 35, , ,64 50,35 Table 5. Water composition of Veentje 3 Gasselterveld. The water samples of the Lheebroekerzand and the Gasselterveld show differences in the bog water composition. In general calcium concentrations are higher in the Gasselterveld. Also iron concentrations are higher in the Gasselterveld except for the elevated iron concentrations at 50 cm of depth in Barkmansveen. Bicarbonate concentrations are low in all bogs except in the deeper regions of Barkmansveen and Veentje 1. The ph is lower at the surface and higher in deeper regions in all of the four bogs. Water levels and water quality Water tubes were placed but because of problems with the data loggers little data have been obtained. Because of the lack of data, the figures from Verschoor et al. (2003) were used. 24

25 Figure 23: Map of transect Verschoor et al. (2003) in October 2001 (dry situation). Blue is water saturated sand, Orange is dry sand, brown is peat. Figure 24: Map of transect Verschoor et al. (2003) February 2002 (Wet situation). Blue is water saturated sand, Orange is dry sand, brown is peat. These figures illustrate the high and the low water level situation in the Lheebroekerzand area. Determination of plant remains in peat profiles Peat samples from Veentje 1 Veentje 2 in the Gasselterveld and the decomposed peat area in the Lheebroekerzand were sent to the university of Szczecin Poland. Prof. Leslaw Wolejko identified plant remains in the samples. He made the following pictures through the microscope. Veentje 1 is a brown moss peat with small sedges, little decomposed (H2-3), remnants of Drepanocladus and Calliergon. The bottom part of Veentje 1 is a mineral material (fine sand) with some sedge remnants and humus - maybe an in wash; Veentje 2 m is a brown moss peat little decomposed (H2), remnants of Drepanocladus. 25

26 Figure Microscope photographs of peat samples from Veentje 1 and Veentje 2. 26

27 Discussion The Lheebroekerzand and Gasselterveld area are both Pleistocene sand landscapes with peat filled depressions. The poorly permeable ground layers and the availability of water play important roles in the present state of the landscape. Dekker et al. (1986) studied water losses from bogs in the surrounding of Sellingen (also in the northern of the Netherlands). They performed percolation experiments in bogs to determine how much water is leaking trough the poorly permeable ground layer (table 6). The black B-horizon has a longer percolation time in contrast to Gliede layers (although some Waterhard layers are not permeable at all). Although the two layers are both present in most bogs, the b-horizon can be interpreted as the main poorly permeable ground layer underneath bogs. Table 6 also shows that desiccated Gliede is far more permeable to water than Gliede that is still hydrated. The boulder clay is poorly permeable but has a longer percolation time than a black b-horizon or Gliede. In the percolation experiment the brown B-horizon that is mostly found not under but near the bogs was not taken into account. This layer has less disperse humus in it than a black b-horizon, and is therefore more permeable to water than a black b-horizon.an important conclusion from Dekker et al. (1986) is that the lowering of the groundwater level underneath a bog accelerates the percolation speed with 10 to 50%. In the dry season, this may result in a situation where more water leaks away then it enters the bog. This means that in the dry season the bog can desiccate and de peat gets oxidized. This could well be the case in the Lheebroekerzand and the Gasselterveld area. Ground layer Saturated percolation time (cm/day) Thick kazige B-horizon (black) Gliede Desiccated Gliede 5-8 Waterhard (20cm) Fine sand ( µm), little loam Boulder clay (50 cm) 2 Boulder clay (200 cm) 0.4 Medium fine sand ( µm), little loam Medium fine sand ( µm), much loam Table 6: percolation time (after Dekker et al (1986), Bakker 1994 Castel (1984) and Verschoor et al (2003) Lheebroekerzand The boulder clay layer in the Lheebroekerzand area (Barkmansveen Zandveen Wolfsklauwtjesveen) is present underneath the whole transect. Therefore no base rich groundwater form underneath the boulder clay layer can enter the bogs. The infiltration though the boulder clay layer is determined by the groundwater potential underneath the boulder clay layer. As a result of lowering the water levels of the nearby Oude Vaart and Wold Aa valleys the potential underneath the boulder clay is lowered. This means that the infiltration through the boulder clay layer is elevated (Bakker 1986). The boulder clay layer tilts downwards at the direction of North West (figure 5 and 7). The decomposed peat sections near Barkmansveen indicate that groundwater levels once where higher. The decomposed peat section next to Zandveen seems to be the result of the sand path (on the 200 point figure 5). This path was made from sandy clay material that is apparently poorly permeable to water. The depression from next to the road ( meters figure 5) is not filled with water and the peat beneath it is decomposed. This road is now functioning as a man made threshold. The road between Zandveen and Wolfsklauwtjesveen is built on a sand ridge with no decomposed peat or black B-horizons underneath it. 27

28 Also an erosion ditch is present on the north side of Barkmansveen. This ditch probably drains the woodland section between Barkmansveen and the sand path and therefore the water is flowing into Barkmansveen. Sand has blown into Wolfsklauwtjesveen when all the peat was cut out of this bog. Because of the B- horizon and the Gliede layer that were not cut out a perched water table is present. This allows Sphagnum mosses to grow on top of the sand and form a new peat layer. The decomposed peat section at the northern side of the transect indicates that the peat level in Wolfsklauwtjesveen was at least as high as the decomposed peat level. The presence of sand ridges between the bogs with no decomposed peat or black B-horizons indicates that the bogs were never linked to each other. These ridges are the only places in the transect where water can infiltrate directly to the boulder clay layer. Therefore subsurface flow between Barkmansveen Zandveen and Wolfsklauwtjesveen is unlikely. The presence or absence of a B-horizon can also be linked to the vegetation. Vegetation descriptions show that the Deschampsia flexuosa is only dominating the vegetation on places where no black B-horizon is present (between Barkmansveen and Zandveen). This species is associated with dry areas where water can flow away (van der Meijden 1996) which fits the field observation. Molinia caerulea is only dominant on places where a B-horizon is present. It is known that Molinia is a plant that grows well on wet places where water can stagnate (van der Meijden 1996). In bog boundaries Molinia caerulea is a good indicator for poorly permeable ground layers in the top soil. The second transect in the Lheebroekerzand area (Schurenberg, Poort 1, Poort 2) has also a continuous boulder clay layer underneath it, with Schurenberg area as an exception. In this pingo remnant we were not able to make a boring. Soil maps of STIBOKA (Vrielink 1976) also does not give enough information to determine whether Schurenberg penetrates the boulder clay layer ore not. The sharp differences in altitude of the land topography next to Schurenberg are the result of aeolian sediments (Vrielink 1976). This also explains the odd form of the side of Poort 1. The wind has blown sand into Poort 1 when peat levels where low. When more peat was formed the sand was overgrown. Decomposed peat layers indicate the peat level of Poort 1 was once at least 50cm meter higher. The present peat level is probably the result of peat cutting. In this transect also sand ridges are present between the bogs. Groundwater flow Because of the earlier mentioned sand ridges between the bog in the Lheebroekerzand with no poorly permeable ground layers in it, it is not likely that water from one bog can enter an other and therefore do not explain the water flow patterns found by Rooke (2002) Rooke (2002) found no water flow in Barkmansveen. The results from this project also show no water flow patterns. Baaijens (pers.observ 1991) reports that in the wet season water is flowing from Zandveen to Barkmansveen over the sand path. It is likely that when the water levels are high, water from Zandveen is flowing over the edge and fills up the depression nearby the road. Because of the absent of poorly permeable ground layers (at point 150 meter, figure 4) water from Zandveen can not reach Barkmansveen. Baaijens (1984) also reports water that is flowing over sand path Wolfsklauwtjesveen to Zandveen (north to south). The slight angle downwards of the boulder clay to the North-West suggests that water that is flowing over the boulder clay layer flows at a different direction (South-East to North-West). In Poort 2, Rooke (2002) reported water flow (from south to north) in spring It is likely that subsurface water from the hill (490 meter figure 5) is the source of this pattern. The pattern that Rooke (2002) found can not be discriminated from the results in this research. A temperature profile was carried out but, as in Barkmansveen, in Poort 2 no flow patterns were present. In the dry season probably not enough water isavailable to create a flow patterns. 28

29 Figure 29: Temperature profile Poort from Rooke (2002) The water that is present in Barkmansveen and Poort 2 probably originates from precipitation, runoff and subsurface water. The subsurface water contains probably more dissolved ions and can cause an elevation of ph and concentration of ions in the bogs. The results of the water quality analysis and the analysis of elements in the peat show an elevated level of iron in Barkmansveen. This is probably caused by subsurface water that has dissolved iron from the sand and brown b-horizons in the hills around the bog. There is also a difference in concentration of ions on different dates. Probably after a rainy period, there is an increase of subsurface flow into the bog and elevates the ion concentration and ph. The subsurface water flow with slightly more dissolved ions can also explain the elevated iron contents in the peat samples from Barkmansveen that were analyzed. To conclude that the enrichment of the bogs is caused by the subsurface water flow, the water must be analyzed (in following research) and compared with the water that is in the bog. General conclusion Lheebroekerzand The system boundaries of the bog in the Lheebroekerzand area are in the first place the B-horizons and Gliede layers that are present underneath the bogs. The water that stagnates on these layers is the water that hydrates the peat in the bogs. Water levels below these layers, the water that is stagnated on the boulder clay layer, can have an effect on the permeability of the B-horizon and Gliede layer (table 7) and therefore is also important. Subsurface water flows originated from sandy hill next to a bog can flow into a bog because of the black B-horizons that stretch in some cases further that the bog that is present now. These black B- horizons do not stretch form one bog to another (figure 5). Subsurface water flow between the bogs is therefore ruled out. When the water levels in the bogs are extremely high the bogs can flow over. In this case water is flowing over the surface area of the sand ridges (there is probably on that moment to much water to infiltrate) and can reach other bogs as run off water or, as it infiltrate, as subsurface water that is flowing over the b-horizon from the next bog. Schurenberg has a different origin (pingo) than the bogs and possibly penetrates the boulder clay layer and may be influenced by ground water from underneath the boulder clay layer. This research has not succeeded in determining the depth and whether Schurenberg is directly fed by groundwater originated form under the boulder clay layer. 29

30 Figure 30: Reconstruction of Lheebroekerzand transect 1 Figure 30 shows a possible reconstruction based on the B-horizons and the Gliede layers that are described in this research (figure 5). The red line shows the missing interpreted information before the peat cutting from the last centuries and the aeolian sediments of the middle ages. Comparative to figure 5 peat levels are almost the same except for Wolfsklauwtjesveen it is much higher. This is based on the decomposed peat sections found on the sides of Wolfsklauwtjesveen. The main conclusion is that the bogs are separated by sand ridges. Figure 31: reconstruction of Lheebroekerzand transect 2 Schurenberg-Poort 1-Poort 2 Figure 31 shows a reconstruction of the bogs based on the B-horizons and decomposed peat section described in soil descriptions (figure 8). The red line is the interpreted missing information. Schurenberg is estimated on a depth of four meters witch means it possible penetrates the bolder clay layer. The typical pingo wall is also displayed. The peat level of Poort 1 is displayed at a higher level than the present state based on decomposed peat sections at the side of the bog. The situation in Poort 1 remains more or less the same. The elevation differences next to Schurenberg in figure 8 are the result of aeolian sediments from the middle ages and are not displayed in this figure. The decomposed peat section at the hill between Poort 1 and Poort 2 cannot be the remains of a bog because of the high place in the land topography. More plausible is that this decomposed peat is the result of an Ericetum tetralicis. 30

31 Gasselterveld No B-horizons are present underneath Veentje 1 and Veentje 2 (figure 10). This means that in these depressions there was never a situation that water was flowing into the soil. The presence of a gyttja layer suggests that Veentje 1 and Veentje 2 started off as lakes and later became filled in with peat. The layer of Braun moss peat indicates groundwater influence when the peat was formed. Only the upper 50 cm of the peat consists of decomposed Sphagnum peat. The (decomposed) braun moss peat layers and the gyttja are probably functioning as poorly permeable ground layers. This is probably the reason that the water level on the surface in the peat can be higher than the surrounding water levels (perched water table) (van Straten et al. 2003). Under such conditions precipitation water forms a lens on top of the groundwater influenced peat with meso- to ombrotrophic conditions and low ph as in shallow bogs (as the results of the water analysis show for Barkmansveen and Poort 2, Lheebroekerzand and Veentje 3 Gasselterveld, appendix 2). The water analysis from Veentje 1 shows also that this lens is present nowadays at a depth of 110cm (Appendix 2). Sphagnum species can grow in this environment and form a peat layer on top of the braun moss peat. Groundwater influence is also shown by the photographs and the core taken from Veentje 1. Table 1 shows the trends of the different elements and the organic matter in Veentje 1. Elements that are present in high concentrations in groundwater for example iron have a positive trend (the deeper the sample the higher the concentration). The positive tends indicates groundwater influence the lower part of Veentje 1 when the peat was formed. The concentrations calcium and magnesium in the peat samples have a negative trend. These elements are probably associated with organic matter, which has a negative trend. In the first 60 cm of Veentje 1 (figure 14, table 1) the calcium and magnesium levels are rising. From 60 cm down the concentrations are decreasing. Probably this first 60 cm increase in calcium and magnesium is the result contamination. Most likely construction waste containing high calcium and magnesium levels have been dumped in Veentje 1. The percentages of organic matter in Veentje 1 (figure 15) are mostly between %. Only at the top and bottom of Veentje 1 the organic matter levels are lower (probably due to decomposition. In the lower regions (between 200 and 250 cm) the percentage organic matter is lower. The gyttja layer found at bottom of Veentje 1 has more mineral material in it. This can explain the lower percentage of organic matter.due to percolation and binding to organic matter no boundary can be distinguished between groundwater influenced peat and precipitation water influenced peat on the peat sample analysis alone. Veentje 3 and Veentje 4 are much shallower than the other two bogs and consist mainly of black decomposed peat. There are also B-horizons present underneath them. This indicates that these bogs have originated in wet depressions in the landscape. Base rich groundwater could not have entered these bogs because of a poorly permeable fluvial loam layer that is present underneath the bogs. The water that can influence the bog can be precipitation water run off and subsurface water. The results of the water analysis show that, as in Barkmansveen Lheebroekerzand, elevated iron levels. This can be the result of subsurface water that dissolved iron in the sandy hills. General conclusion Gasselterveld De Gans (1981, 1984) describes Veentje 2 as a pingo remnant. In the RIN report (De Gans 1984) also three other pingo remnants are mentioned which are situated near Veentje 2. In combination with the information from de Gans (1981) Veentje 1 is mentioned in the RIN report (De Gans, 1984) as one of the 3 pingo remnants in Veentje 2 s surrounding. This information combined with the profiles peat identification and chemical analysis from this research, leads to the conclusion that Veentje 1 and Veentje 2 are indeed pingo remnants, although they appear to look more like dried out shallow Sphagnum bogs. 31

32 Because of the decomposed peat that functions as a poorly permeable layers, the water levels in these pingo remnants can be higher than the groundwater levels in the surroundings. In contrast to van Straten et al (2003) our research suggest that lowering of the groundwater levels underneath the loam layer has a direct effect on the surface water level within the pingo remnant. When the groundwater levels decrease the percolation speeds in poorly permeable ground layers can accelerate (Dekker 1986), eventually resulting in the lowering of the water levels at the surface area. This process can eventually result in the decomposition of the top peat layer peat caused by low water levels. Figure 32: possible reconstruction of Veentje 1 and Veentje 2. Figure 32 shows a possibly reconstruction of the two pingo remnants Veentje 1 and Veentje 2. The red lines display the interpreted data. The B-horizons and fluvial loam layer are obtained from soil descriptions made in this research. In contrast to the present situation (figure10), the walls around the pingo are higher in this reconstruction. Due to erosion they are poorly visible nowadays. A layer of black decomposed peat displayed in figure 10 shows that the ground water level was once as low as 150 cm above the present bottom of the Anderse diep (set as zero). After that period the Groundwater levels rose, and the decomposed peat was overgrown with Braun moss peat. The groundwater level probably rose at the level of cm above the present bottom of the Anderse diep because this level is the highest level that groundwater influenced peat was found (figure 32). Figure 33 was obtained from De Gans (1983). This figure shows a profile in a west to east orientation. De Gans shows that Veentje 2 has a depth of 7 meter. Much more than our research shows. It could well be the case that the soil description made in our research was not at the most deepest place. De Gans also found some humic loam form river deposits. In this research also loam from river deposits was found. Also aeolian loam was found. Outside the transects. It is possible that the fluvial loam layer that is displayed in the figures in this research also contains aeolian loam which can explain the changeable shape of this layer. 32

33 Figure 33: Pingo remnant Papenvoort (Veentje 2), West to East. From De Gans (1983) The peat layer in Veentje 3 and 4 are relatively thin and therefore it is likely that the peat formation started of later in contrast to Veentje 1 and 2. Although no carbon dating is available there is 50 cm peat present nowadays in these bogs which suggest an age of 5000 years. De Gans (1981) estimated the age of Veentje 1 and 2 to be years BP. Veentje 3 and 4 originated in a situation where the infiltration of precipitation water stagnated. In this situation the Sphagnum mosses could grow and the B-horizons were formed. Nowadays the peat layer has almost completely dried out for a long time already as a result of less subsurface and run off water flow into the bogs. Decomposed Braun moss peat layers suggest that the water level dropped severely in the pingo remnants (Veentje 1 and 2). At the time the Braun mosses were dominating the vegetation in Veentje 1 and 2 it is likely that there was a major influence of base rich groundwater. Probably this influence came to an end and the top Braun moss peat layer became desiccated and decomposed. On top of this layer the top Sphagnum peat layer could form that was influenced by precipitation, run off and sub surface water. This Sphagnum peat layer has become desiccated and is nowadays completely decomposed. 33

34 Resent developments as the drainage of the Anderse diep, massive planting of trees, erosion ditches and the sand extraction area results is a further declining of the amount of available water for the bogs and pingo remnants. The massive planting of trees in the Gasselterveld bog s area causes a high evapotranspiration. Trees take up subsurface water and evaporate water from the leaves. Therefore less water is available for bogs or pingo remnant. A second factor is the drainage ditches that were dug to make the area more useful for agriculture and foresting. All these gullies are eventually drain in the Anderse diep which is part of the of the Drenthschse Aa valley system. How much water is actually is drained out of the Gasselterveld and what the relation is with the water level in the Anderse diep has not become clear in this research project. More hydrological research has to be done. On the western side of the research area a large sand extraction area is present. At the time the pit was dug the water level in the hole rose (Ouwerkerk et al, 2004). This confirms the idea that a lot of sub surface water from the surrounding area has drained into the sand extraction area and not into the bogs and ping remnants. Also the surface water has a higher evaporation rate (Ouwerkerk et al, 2004) which means even more water is lost. Overall Discussion Nijland et al (1982) gives a short review on the possible origin of small bogs in depressions in the Pleistocene sand landscape with the available knowledge of that time. He suggests that possibly all depressions deeper than 2 meters are pingo remnants (after De Gans, (1981)). Also the possibility of kettle hole mires is not ruled out at that time. De Gans (1983), working on pingo remnants, suggest that pingo remnants are associated with periglacial gullies. Baaijens (1984, 1992) discusses several types of heath land pools and their possible origin. He points out that most bogs are associated with gullies, not periglacial ones but abandoned braided river valleys which were partly dammed up by aeolian sand deposits and superficial gullies in the boulder clay layer. Later Kleijberg et al. (1985) studied dobben (round depressions in the landscape filled with peat) near Twijzel and Buitenpost and concluded that dobben in their area were pingo remnants. Jager (1985, 1986), also working on dobben in northern Drenthe, made a distinction between pingo remnants, aeolian depressions and depression with an unknown origin. Factors that are considered important for a possible typology of small bogs are the depth of the depression and its position in relation to poorly permeable ground layers. This leads to the conclusion that small depressions in the Pleistocene landscape in the north of the Netherlands can be either aeolian depressions or pingo remnants. The boundaries of these peat filled depressions in sand soils are related to the lowest water level in the time that peat was formed. Peat could only be formed in the depressions where water was permanently available. In the case of aeolian depressions in the sand landscape, the run off and subsurface flow must stagnate into the depression to create an ombrotrophic wet situation. This means that there must be some kind of resistance to the infiltration of this water in the sand. High levels of groundwater above the boulder clay or fluvial loam layer is an explanation. When these levels are just below the surface of a depression, precipitation, runoff and subsurface water can stagnate. This creates an environment where Sphagnum peat can be formed. When the peat layer increases more disperse humus is available to form a Gliede layer. Due to fluctuations of the groundwater level an infiltration situation (as mentioned earlier) can occur. In this situation a b-horizon can be formed. Once a B-horizon is formed a drop in groundwater level has no immediate effect on the water level within the bog but, as Dekker (1986) proved, the poorly permeable layers underneath a bog are indeed poorly permeable, not impermeable. This means that although there are poorly permeable ground layers present it is still necessary to keep groundwater levels above the boulder clay or fluvial loam layer high. In the situation of a nearby river valley (this is the case in the Lheebroekerzand and Gasselterveld) it means that also the water level in the river must be high to minimize drainage. It is also 34