NOTE A MODIFIED METHOD FOR MEASURING SATURATED HYDRAULIC CONDUCTIVITY AND ANISOTROPY OF FEN PEAT SAMPLES

Size: px

Start display at page:

Download "NOTE A MODIFIED METHOD FOR MEASURING SATURATED HYDRAULIC CONDUCTIVITY AND ANISOTROPY OF FEN PEAT SAMPLES"

Lynette Gray
5 years ago
Views:

1 WETLANDS, Vol. 28, No. 2, June 2008, pp , The Society of Wetland Scientists NOTE A MODIFIED METHOD FOR MEASURING SATURATED HYDRAULIC CONDUCTIVITY AND ANISOTROPY OF FEN PEAT SAMPLES Jens Kruse 1, Bernd Lennartz, and Peter Leinweber University of Rostock Faculty of Agricultural and Environmental Sciences, Institute for Land Use Justus-von-Liebig Weg 6, Rostock, Germany 1 jens.kruse@uni-rostock.de Abstract: The determination of the anisotropic and heterogeneous character of the saturated hydraulic conductivity (K s ) of wetland soils is technically challenging, but is crucial for the accurate quantification of flow and transport processes. We modified a laboratory method to determine K s both in the vertical (K s,v ) and horizontal directions (K s,h ), and tested it on the same undisturbed peat samples using a constant head upward flow permeameter. The first results showed that K s,v was greater than K s,h in the majority of samples from two profiles of a degraded fen peat, indicating that K s was anisotropic. In conclusion, the described method was suitable to determine K s,v and K s,h and can be recommended to estimate the anisotropy of K s in wetland soils. Key Words: cube, restoration, undisturbed samples, wetlands INTRODUCTION The saturated hydraulic conductivity (K s )ofsoil is generally determined in the laboratory using 100 to 250 cm 3 core samples. However, boundary effects at the cylinder wall-sample-interface may lead to short-circuiting water flow (Chason and Siegel 1986); this is particularly likely for peat samples and may lead to a significant overestimation of K s. Generally, peat can be very heterogeneous and anisotropic (Mitsch and Gosselink 2000). The anisotropy can be measured with vertically and horizontally collected core samples within the same peat layer, but these results may partly reflect heterogeneity of the peat and not just anisotropy. To overcome these problems, Beckwith et al. (2003) modified a method developed by Bouma and Dekker (1981) who proposed to measure K s in both directions, horizontally (K s,h ) and vertically (K s,v ) within the same peat sample so as to minimize boundary effects. Beckwith et al. (2003) used bog peat cube samples (7.5 cm cm cm) that they encased completely with gypsum. After removing the gypsum from two opposing sides, they measured K s between the two surfaces, described as follows. A thin layer of water was allowed to pond on the top of an exposed face of the peat block. The water that then flowed through the core was collected with a funnel that drained into a graduated cylinder. The hydraulic conductivity was calculated from the downward water flow through the peat sample using Darcy s law. After the first measurement, the faces were sealed again with gypsum, the cube was turned 90u, two more opposing faces were exposed, and K s was measured again by following the same flow-through procedure. Our own pre-tests using gypsum to encase the peat cube, as described by Beckwith et al. (2003), showed that during the drying of the gypsum, water was removed from the wet peat sample. This removal of water led to the development of distinctive shrinkage-cracks across the cube. These cracks are known to have a large influence on the flow characteristics of peat samples (Schwärzel et al. 2002). Furthermore, the required prolonged contact with water partly dissolved the gypsum during both pre-saturation of the sample and K s measurement. To overcome the problems associated with the partial dissolution and destabilization of the gypsum coating, Surridge et al. (2005) and Rosa and Larocque (2007) used paraffin wax to encase their cube samples. Beckwith et al. (2003), Surridge et al. (2005), and Rosa and Larocque (2007) all used the downward water flux method to measure K s.thismethoddoes not allow adjusting K s for gradients, 1 and may therefore cause fast flow situations with possible internal erosion and structure deformation. By contrast, the upward flow method as described below enables the arrangement of any desired 527

2 528 WETLANDS, Volume 28, No. 2, 2008 gradient by adjusting the suction tube. In this case, gradients could be adjusted individually for each sample to obtain and maintain approximately the same flux rate in all samples (one drop per second). Another advantage of the upward flow method is the improved avoidance of air entrapment. It is well known that air entrapment alters K s measurements (e.g., Beckwith and Baird 2001). The downward flow method requires removal of the saturated sample from the water bath and placement on a funnel to drain. Since the sample already starts to drain from the moment the sample is removed from the water bath, new air can become entrapped before the measurement starts. Furthermore, by having the bottom of the sample exposed to atmosphere pressure, the effect of drop resistance can also alter the results of measurements. Using the upward flow method the saturated sample remains undisturbed in the water bath during the measurements and thus the above shortcomings are avoided. Therefore, we modified the cube method to enable K s measurements with an upward flow, constant head method in which both faces of the sample remain in contact with water. This short, technical note describes the excavation techniques, sample preparation methodology, and results measured on two fen peat profiles. MATERIALS AND METHODS Site and Peat Type The peat samples were collected in the Trebel valley near Tribsees (Northeast Germany 54u059 N, 12u449 E). For a detailed site description, see Rupp et al. (2007). Phragmites and tall sedge were the major peat forming plants. The fen was intensively drained in the 1960s, and subsequently has been managed as improved grassland; since 1988, the fen has been partly rewetted. The samples consisted of moderately decomposed peat without visible shrinkage cracks (degree of humification [H] 5 to 7 according to the scale of von Post; Puustjärivi 1970). Peat Core Excavation To determine valid and representative hydraulic soil properties in the laboratory it is necessary to take undisturbed peat samples. To this end, various mechanical systems have been developed for collecting these samples (e.g., Cuttle and Malcolm 1979, Buttler et al. 1998, Seaby 2001, Beckwith et al. 2003, Caldwell et al. 2005). Our sampler consisted of a 1.5 m-long PVC pipe (0.15 m I.D.) with a sharpened edge. First, the intensively rooted zone (approximately 0.2 m) was removed with a spade to prevent roots from being displaced down with the pipe, which could damage the structure of deeper peat layers. Then, the PVC pipe was carefully driven into the peat to a depth of 1.3 m below ground surface. The difference between the peat surface level inside and outside the inserted pipe was only 2 3 cm; hence, compression was negligible. The pipe was excavated by pulling on a metal rod attached to the pipe using a manually pulled hauling system. This pulling was done very slowly to prevent an expansion or a slipping of the peat core. A piezometer pipe was inserted next to the PVC pipe to reduce the suction created during the excavation (Cuttle and Malcolm 1979). After excavation, both ends of the pipe were sealed with plugs and waterproof tape for transport to the laboratory. Cube Cutting and Coating In the laboratory, the pipe was opened using a thin angle grinder. Once the PVC pipe was removed, the peat core was checked for cracks. A thin piece of fishing line was used to cut cube samples of 9 cm 3 9cm3 9 cm out of the peat core. Our own pre-tests showed that unlike knifes, using a fishing line minimizes smearing of pores and dragging of peat fibers during cutting. The upward flow approach to measuring K s, through a constant head permeameter, requires a waterproof and pressure tight connection between the sample and the permeameter. Gypsum, as proposed by Bouma and Dekker (1981) for the downward water flow permeameters, did not fulfill this requirement in our pre-tests. Therefore, we used an unfilled casting resin (P30GB, BreMod Modelbau D Kamen). Some cotton fibers were added to increase the viscosity of the resin. Additionally, a degassing additive (BEL 51, BreMod Modelbau D Kamen) was added to reduce air bubbles in the resin. Casting moulds (11 cm 3 11 cm 3 13 cm) were constructed to encase the peat sample. The whole coating process in the moulds was performed in three steps. 1) To start, the bottom of the cube (approx. 1 cm thick layer resin) was cast. 2) Once the resin had set (60 min), the peat cube was placed on top of the bottom layer. Then, resin was cast into the mould until half of the cube sample was encased. This prevented the sample from buoying upwards within the liquid resin. 3) After the resin started to set (20 min), the entire mould was filled with resin until the peat cube was encased completely within a 1 cm-thick resin coat (Figure 1a). The mould was then left for 24 hours to ensure complete hardening

The next day the coated peat cube was extracted from the casting mould (Figure 1b). The coating was removed from two opposing faces of the cube using a thin angle grinder (Figure 1c d).

3 Kruse et al., SATURATED HYDRAULIC CONDUCTIVITY AND ANISOTROPY 529 Figure 1. A) Casting sequence, B) peat sample completely encased with polyester resin, C) exposed sample face, D) lid with some attachments, and E) cube sample with adapter. of the resin. The next day the coated peat cube was extracted from the casting mould (Figure 1b). The coating was removed from two opposing faces of the cube using a thin angle grinder (Figure 1c d). Faces were exposed carefully because on occasion the coarse and loose nature of peat structures and the partial infiltration of resin into pores caused peat fragments to become attached to the resin coating. An adapter was glued to the 1 cm-wide coating edge to ensure a waterproof and pressure-tight connection to the measuring device (Figure 1e). Then, the peat sample cube was saturated slowly from the bottom for at least 12 hours. where K s is the saturated hydraulic conductivity (m d 21 ), V is the percolated water volume (m 3 ), A is the surface area of the peat cube ( m 2 ), DH is the hydraulic head difference (m), L is the height of the sample (m), and t is the duration of the measurement (d). All readings were corrected for temperature effects by normalizing the values to 10uC. After the experiment, the exposed surfaces were re-sealed with resin. The cube was turned 90u and two more opposing faces of the cube were removed. The sample was re-saturated for another 12 h and K s determined again. Determination of the Saturated Hydraulic Conductivity (K s ) K s was determined by the constant head method (Figure 2). The chosen upward flow method allowed an exact adjustment of the hydraulic gradient to obtain small flow rates and thereby avoiding internal erosion effects within the soil cube. The gradient was regulated by sliding the capillary glass tube until a constant flow rate of around one drop per second was reached. The pressure head needed to overcome the capillarity of the glass tube was measured separately (, m) and subtracted from the hydraulic head acting on the soil sample. The experimental setup allowed us to simultaneously percolate 10 samples placed in a 1.4 m m water tank. The water level in the tank was kept constant at 7 cm by means of a Mariotte s bottle. K s was calculated using Darcy s law: K s ~ V L DH A t Figure 2. Experimental setup for saturated hydraulic conductivity (K s ) determination with the bottom-up, constant head method.

4 530 WETLANDS, Volume 28, No. 2, 2008 Figure 3. A) Saturated hydraulic conductivity in vertical (K s,v ) and horizontal (K s,h ) direction determined on the same peat sample; B) Anisotropy of saturated hydraulic conductivity (i.e., log 10 (K s,h /K s,v )) of both sampled profiles. RESULTS AND DISCUSSION Results obtained from 15 samples (five from profile 1, 10 from profile 2) are shown in Figure 3a. The hydraulic conductivities, K s in vertical (K s,v ) and horizontal (K s,h ) directions, from the two profiles were similar. The similarity in K s among the two profiles was not expected because distinct spatial heterogeneity of K s in peat soils has been well documented in other investigations (e.g., Rycroft et al. 1975, Baird 1995). In both profiles, K s,h and particularly K s,v slightly increased with depth. The distinctively high K s,h values at soil depths of 0.94 m (profile 1) and 1.15 m (profile 2) could be attributed to preferential flow paths in well preserved plant residues, mainly Phragmites australis, within the cube. The majority of the samples (n 5 15, 80%) had greater K s,v - than K s,h -values indicating that the hydraulic conductivity in the peat was anisotropic. This anisotropy was expressed as log 10 (K s,h /K s,v )as proposed by Chason and Siegel (1986), allowing a simple graphical comparison of the two peat profiles (Figure 3b). The pattern of anisotropy agreed with results of Surridge et al. (2005) who also found K s,v larger than K s,h in a fen peat. In contrast, others have found K s,h greater than K s,v for bog or fen peat (Beckwith et al. 2003, Rosa and Larocque 2007). Other studies investigating anisotropy in peat soils, albeit measuring K s,h and K s,v on separate samples, found anisotropy (K s,h. K s,v or K s,h, K s,v )aswell as isotropy (K s,h 5 K s,v ) of saturated hydraulic conductivity (Childs et al. 1957, Chason and Siegel 1986, Scholtzhauer and Price 1999). These contrasting results suggest that relationships between K s,h and K s,v vary among different peat soils. This can be attributed to the different origin and history of the peat soils studied. Furthermore, it has to be considered that K s, and consequently, anisotropy of hydraulic conductivity, is scale-dependent. In other words, values of anisotropy are a function of sample size. Therefore, the scale (e.g., 729 cm 3 in the present study) must be considered when comparing values of anisotropy among different studies. In our study, anisotropy varied at both sites with depth and was lowest in the top soil samples (0.33 m profile 2 and 0.36 m profile 1). These low values of anisotropy in the upper soil layer can be explained by the advanced peat degradation from H5 at 1.2 m to H7 at 0.3 m. Depending on the peat-forming plant community and its specific accumulation, preferred pore orientations may have been developed during the peat-forming process. Preferred pore orientation, in addition to a layering structure and the remnants of root material within the peat matrix, can explain the anisotropic character of most peat soils. The artificial drainage of fen peat soils caused massive structural changes (e.g., pore size distribution and porosity) (Schwärzel et al. 2002). Thus, peat degradation can result in a total loss of the preferred pore orientation, and the anisotropic character may diminish. There are some indications in Figure 3b that anisotropy is depthdependent. Whether this is a property that is specific to the degraded fen peat under study, or applies to other peat soils as well, will be a topic for forthcoming investigations that will involve measurements of more samples to account for the known heterogeneity of fen peat soils. In conclusion, the described method was useful for quantifying the anisotropic character of saturated hydraulic conductivity in peat soils. ACKNOWLEDGMENTS The authors express their appreciation to A. Fröhlich and H. Knoll for their advice and technical help. We also thank U. Grunzel and M. Kietzmann for assistance in the laboratory. Thanks to Dr. Bing Cheng Si, Department of Soil Science, University of Saskatchewan, Dr. Andrew Baird, Queen Mary University London, Dr. Steve Robinson, The University of Reading, and another anonymous reviewer for helpful comments and language editing on earlier versions of the manuscript. LITERATURE CITED Baird, A. J Hydrological investigations of soil water and groundwater processes in wetlands. p In J. Hughes and A. L. Heathwaite (eds.) Hydrology and Hydrochemistry of British Wetlands. John Wiley & Sons, Chichester, NY, USA. Beckwith, C. W. and A. Baird Effect of biogenic gas bubbles on water flow through poorly decomposed blanket peat. Water Resources Research 37:

5 Kruse et al., SATURATED HYDRAULIC CONDUCTIVITY AND ANISOTROPY 531 Beckwith, C. W., A. Baird, and A. L. Heathwaite Anisotropy and depth-related heterogeneity of hydraulic conductivity in a bog peat. I: laboratory measurements. Hydrological Processes 17: Bouma, J. and L. Dekker A method for measuring the vertical and horizontal K sat of clay soils with macropores. Soil Science Society America Journal 45: Buttler, A., P. Grosvernier, and Y. Matthey A new sampler for extracting undisturbed surface peat cores for growth pot experiments. New Phytologist 140: Caldwell, P. V., A. A. Adams, C. P. Niewoehner, M. J. Vepraskas, and J. D. Gregory Sampling device to extract intact cores in saturated organic soils. Soil Science Society America Journal 69: Chason, D. B. and D. I. Siegel Hydraulic conductivity and related physical properties of peat, Lost River Peatland, northern Minnesota. Soil Science 142: Cuttle, S. P. and D. C. Malcolm Corer for taking undisturbed peat samples. Plant and Soil 51: Mitsch, W. J. and J. G. Gosselink Wetlands, third edition. John Wiley & Sons, Inc., New York, NY, USA. Puustjärivi, V Degree of humification. Peat and Plant News 3: Rosa, E. and M. Larocque Investigating peat hydrological properties using field and laboratory methods: application to the Lanoraie peatland complex (southern Quebec, Canada). Hydrological Processes DOI: /hyp Rupp, H., R. Meissner, P. Leinweber, B. Lennartz, and M. Seyfarth Design and operatability of a large weightable lysimeter. Water, Air and Soil Pollution 186: Rycroft, D. W., D. J. A. Williams, and H. A. P. Ingram The transmission of water trough peat. I. Review. The Journal of Ecology 63: Schwärzel, K., M. Renger, R. Sauerbrey, and G. Wessolek Soil physical characteristics of peat soils. Journal of Plant Nutrition and Soil Science 165: Seaby, D Designs for one-man, two-stage, samplers for obtaining undisturbed cores of peat over 1 m long. Forestry 74: Surridge, B. W. J., A. J. Baird, and A. L. Heathwaite Evaluating the quality of hydraulic conductivity estimates from piezometer slug tests in peat. Hydrological Processes 19: Manuscript received 16 August 2007; accepted 30 January 2008.

Water Related Soil Properties

Water Related Soil Properties Water is essential for plant growth. Plants extract mineral laden water from the soil. For effective irrigation and drainage it is important that the processes governing soil