Published online 27 October 2005
Published in Soil Sci Soc Am J 69:2071-2075 (2005)
DOI: 10.2136/sssaj2005.0150
© 2005 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
Wetland Soils Note
Sampling Device to Extract Intact Cores in Saturated Organic Soils
P. V. Caldwella,*,
A. A. Adamsb,
C. P. Niewoehnerb,
M. J. Vepraskasb and
J. D. Gregorya
a Dep. of Forestry, Box 8008, North Carolina State University, Raleigh, NC 27695
b Dep. of Soil Science, Box 7619, North Carolina State University, Raleigh, NC 27695
* Corresponding author (pvcaldwe{at}unity.ncsu.edu)
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ABSTRACT
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Physical property data on organic soils are lacking due to difficulty in collecting undisturbed samples from these frequently saturated and weakly consolidated soils. A sampling device was constructed to extract undisturbed cores from saturated organic soils in a forested setting. The sampler consists of a 100-cm-long, 7.6-cm-diam. schedule 40 PVC pipe that was fitted with female threaded adapters on either end. A cutting head was constructed to cut through the fibric root mat and other woody debris in the profile by gluing a 7.6-cm-diam. hole-saw to a male threaded adaptor that was attached to the PVC pipe. The sampler was rotated by hand into the organic soil with gentle downward pressure. When the desired depth was reached, the remaining air space in the PVC pipe was filled with water and a threaded cap was used to seal the top of the sampler. A 1.3-cm-diam. galvanized pipe was inserted next to the sampler to add water to the bottom of the core, relieving the suction created as the core was pulled from the soil. The sampler and vent pipe were pulled from the soil either by hand or with a tripod–winch arrangement. Before the cutting head was raised above the water table, it was removed and replaced with another threaded PVC cap. The 100-cm-long pipe containing the soil core was then cut into 7.6-cm-long sections using a wheel-type PVC pipe cutter. Saturated hydraulic conductivity and soil water characteristics were then measured in the laboratory using the resulting 7.6-cm-long samples encased in the PVC cylinders.
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INTRODUCTION
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HYDROLOGIC COMPUTER MODELS are powerful tools for predicting water table levels in soils. Most models require soil property data such as saturated hydraulic conductivity and soil water characteristic curves to simulate the movement and storage of water in the soil profile (Skaggs, 1978; McDonald and Harbaugh, 1988). These properties are frequently measured on undisturbed soil cores in the laboratory. Various mechanical devices have been developed for the collection of undisturbed soil cores. Many samplers are attached to tractors and use hydraulic pressure to either pound or push the sampler into the soil (Abu-Hamdeh and Al-Jalil, 1999; Janssen et al., 1998; Vepraskas et al., 1990). Others have used more portable, manual means to push or pound the sampler into the soil profile (Chong et al., 1982; Jackson, 1987; Seaby, 2000; Swanson, 1950; Stolt et al., 1991). Most of these methods work well only in soil of sufficient strength to withstand compaction or disturbance during sample collection.
Little data are available for the saturated hydraulic conductivity or soil water characteristic of saturated organic soils under forest vegetation, because it is difficult to collect undisturbed samples from these soils. Organic soils in forested settings have a dense, interconnected mat of roots at the surface that must be severed when collecting core samples. These organic soil materials are usually saturated and have very low soil strength. Collecting undisturbed cores using traditional methods virtually destroys the sample during the collection process. Additional problems are encountered when collecting samples in flooded conditions. When a sampler is successfully inserted into a saturated organic soil, the action of lifting it out of the ground induces suction at the bottom of the sampler that causes the soil core to be pulled from the sampler.
The objective of this study was to develop a sampler that could be used in saturated organic soils to collect undisturbed soil cores suitable for laboratory measurements of saturated hydraulic conductivity and the soil water characteristic curves.
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SAMPLER DESIGN AND CONSTRUCTION
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The sampling device was made from a section of 7.6-cm-diam. schedule 40 PVC pipe that was 100 cm long (Fig. 1)
. Diameters of all parts of the apparatus are 7.6 cm (3.0 in.) unless otherwise noted. Two female threaded adapters were glued to the ends of the PVC pipe using PVC cement. A cutting head was fashioned from a threaded male PVC adaptor with a short piece of PVC pipe glued into the slip-fit end (Fig. 2)
. The edge of the male adaptor and PVC pipe were tapered to a 45° angle using a belt sander to facilitate easier penetration into the soil. The arbor end of a hole-saw was removed using a pneumatic cut-off wheel, resulting in a 7.6-cm-diam. saw-toothed ring that was 3.8 cm long. This ring was glued into the PVC pipe in the male adaptor using epoxy cement such that the saw teeth extended approximately 1.3 cm from the end of the PVC pipe. Additional supplies included a 120-cm length of 1.3-cm-diam. galvanized-steel tubing, a plastic funnel that fit into the galvanized tube, and two PVC male threaded caps.
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Fig. 1. Schematic diagram of the core sampler. Components are (A) a 100-cm-long, 7.6-cm-diam. schedule 40 PVC pipe with female threaded adapters on each end, (B) a sampler cutting head, (C) two 7.6-cm-diam. male threaded PVC caps, (D) a 120-cm-long, 1.3-cm-diam. galvanized pipe, and (E) a plastic funnel.
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Fig. 2. Drawing and relevant dimensions of the sampler cutting head. Components are (A) a 7.6-cm-diam. male threaded adaptor, (B) a 7.6-cm-diam. section of PVC pipe glued into (A) and (C) a 7.6-cm-diam. hole-saw with arbor end removed.
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SAMPLER OPERATION
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To collect samples, the sampler cutting head was threaded onto one end of the PVC pipe. The cutting head was then placed on the soil surface and slowly rotated by hand using gentle downward pressure. The saw cut through fibric organic soil material as well as the numerous roots encountered in the profile. Once the sampler reached the desired depth, the remaining air space in the top of the sampler was filled with water. The pipe was then capped on the top with one of the threaded PVC caps using sealant on the threads. Capping the top of the sampler helped maintain suction inside the sampler to prevent loss of the core when it was raised from the ground, similar to holding one's finger over the end of a drinking straw and lifting it out of the glass. Next, the galvanized tubing was pounded into the ground immediately adjacent to the sampler such that the end of the tube was at the same depth as the cutting head of the sampler. The tube was cleared of debris with a length of steel rod, and was then attached to the sampler using duct tape. A small hole was then dug into the soil next to the sampler with a spade to allow removal of the saw adaptor just before the bottom of the sampler was raised above the water table.
Before the sampler was raised from the soil, water was poured through the funnel into the galvanized tube until the tube was full. At this point, the sampler and galvanized tube were lifted from the soil either manually or with a tripod–winch arrangement. As the sampler was lifted, the void created beneath it was filled with water from the galvanized tube to relieve the suction that would otherwise draw the sample out of the pipe. Cuttle and Malcolm (1979) suggested inserting a tube adjacent to the sampler to allow air into the area below the sampler to relieve the suction. However, we found that due to the weight of the saturated organic soil sample and the low friction between the sample and the sampler tube, it was necessary to fill the void below with water to prevent loss of the core. Depending on the depth of sampling, it may be necessary to add water to the funnel as the sampler is being raised. When the sampler was lifted to within a few centimeters of the top of the water table, the cutting head was removed and a threaded cap installed in its place using sealant on the threads. By keeping the bottom of the sampler below the water table, pressure was maintained on the bottom of the core to minimize the possibility of losing the sample. With both ends capped, the sampler was completely raised from the soil surface and stored. The sampler could then be transported to the laboratory in a vertical orientation to prevent damage to the saturated soil material inside. The soil core can be removed from the sampler by either cutting the PVC pipe lengthwise to expose the sample, or by pushing it out of the PVC pipe using a plunger-type device. Short core samples (e.g., 7.6 cm long) can also be prepared by cutting the PVC pipe crosswise and leaving the soil surrounded by a PVC ring.
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EXAMPLE OF USE
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Samples were collected from organic soils using this device in three Carolina Bays in NC located approximately at 34°40'59'' N and 78°34'54'' W. The organic soils were members of the Croatan series (loamy, siliceous, dysic, thermic Terric Haplosaprists) and Pamlico series (sandy or sandy-skeletal, siliceous, dysic, thermic Terric Haplosaprists). The dominant vegetation at these sites consist of pocosin plant species, with trees such as pond pine (Pinus serotina L.), swamp red bay (Persea palustris L.), loblolly bay (Gordonia lasianthus L.), and sweetbay (Magnolia virginiana L.), and shrub species including fetterbush (Lyonia lucida L.), ti-ti (Cyrilla racemiflora L.), inkberry (Ilex glabra L.), and blaspheme vine (Smilax laurifolia L.). Hydrologic models of the three bays were to be developed, requiring soil data such as saturated hydraulic conductivity and soil water characteristic curves to be measured from intact soil cores. Three cores were taken in each of the three bays.
The apparatus that was used to measure hydraulic conductivity and soil water characteristic required samples of approximately 7.6-cm diam. and 7.6 cm in height. The 100-cm-long soil core was cut into 7.6-cm-long sections by first setting the PVC pipe into a bucket while maintaining its vertical orientation. The top threaded cap was removed, and the water above the core surface was siphoned out. The location of the upper core surface was marked on the outside of the sampler, and marks were made at 7.6-cm intervals to the bottom of the pipe. A wheel-type PVC pipe cutter was used to gently cut through the pipe at the marked locations while leaving the soil sample intact.
When the PVC pipe was cut completely through, a 10.2-cm-wide, sharpened spatula was inserted into the cut and gently worked across the sample. Any roots in the sample were cut using razor blades or thin saws to minimize damage to the sample. The PVC section containing the sample was then removed while being supported on the bottom by a spatula. The sample was then inverted onto another spatula, and a piece of cheesecloth was secured to the bottom of the sample using a rubber band. The sample was then flipped back over and placed in a pan of water to maintain saturation.
The saturated hydraulic conductivity was measured using the constant head method (Klute and Dirksen, 1986). The soil water characteristic curve was also determined using a pressure cell apparatus (Klute, 1986). At the completion of these tests, the samples were placed in cans, weighed, oven-dried at 105°C for 24 h, and reweighed to determine water content. Bulk density was computed using the oven-dry weight and inner volume of the PVC ring.
The saturated hydraulic conductivity and bulk density for the Oi, Oe, and Oa horizons in the organic soils sampled are shown in Table 1. The organic soil horizons had very low bulk densities (<0.2 g cm–3) due to the lack of mineral soil material in the matrix. The surface (Oi) horizons, composed mostly of undecomposed leaves and fibrous roots, generally had saturated hydraulic conductivities too high to be measured with the laboratory apparatus (Table 1). The soil water characteristic data (Fig. 3) showed that the organic soils had a porosity of approximately 0.9 cm3 cm–3. Most of the water drained from the cores at soil water pressures > –50 cm. These pores had equivalent cylindrical diameters > 0.1 mm as estimated from the capillary rise equation (Hillel, 2004).
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Table 1. Laboratory bulk density and hydraulic conductivity measurements conducted on organic soils across all three Carolina Bays.
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Fig. 3. Mean soil water characteristic curves for the Oi, Oe, and Oa horizons of organic soils. Error bars show one standard deviation from the mean.
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DISCUSSION
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The sampler successfully removed intact cores from the sites in most cases. The surface Oi horizon, which consisted of loose leaves and some fibrous roots, was occasionally compressed during the coring process. Extreme care should be taken not to exert excessive downward force when taking the sample, which can destroy the fragile structure of the organic surface horizons. Samples collected from Oe and Oa horizons showed no apparent compaction in that the surface of the samples remained at a constant elevation as the sampler was pushed into the ground. Despite the slightly smaller (2 mm) inner diameter of the hole saw, no gaps between the sample and core wall were visible to the naked eye.
This sampling device was also applied to saturated sandy mineral soils at some sites. For these soils, 7.6-cm-diam. thin-walled steel tubing replaced the PVC piping. No cutting head was necessary in these soils, but the leading edge of the steel tubing was sharpened using a grinder to facilitate cutting through roots during the sampling process. The steel tubing was pounded into the soil using a fence-post driver until it reached the desired depth. In this case, rubber end caps with gear clamps were used on either end of the sampler to secure the core inside. No apparent compaction of samples was observed in these soils. Extraction of the cores from the soil and sample preparation was performed as described previously for the organic soils.
The sampler has proven reliable in collecting virtually undisturbed samples from saturated Oe and Oa horizons in more than 30 plots. The undisturbed samples can be used to determine soil bulk density, saturated and unsaturated hydraulic conductivity, soil water characteristic curves, among others. The samples can also be used to evaluate root growth and soil micromorphology. The same cutting head apparatus was used for all organic soil cores without the need to replace or reglue the hole-saw. Soil data collected by this method have successfully been used in hydrologic models predicting water table levels with a high degree of accuracy, with typical average absolute deviations between simulated and measured water table depths of less than 5 cm. Distinct advantages of this sampler are its low cost, ease of construction, lightweight, and the ability to section the core into samples that can be tested in the laboratory.
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ACKNOWLEDGMENTS
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Funding for this research was provided by the North Carolina Department of Transportation (Research Project No. HWY-2004-19) and is gratefully acknowledged.
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REFERENCES
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ABSTRACT
INTRODUCTION
