Infiltrometer Tests
by: Sebastien Fortin, E.I.T., M.Sc.
Ponded Infiltrometer
The Ponded Infiltrometer is a variant of a single ring infiltrometer. Figure 8 presents a schematic diagram of the ponded infiltrometer. Bouwer (1986) stated that cylinder infiltrometers are typically 0.30m in diameter but that infiltrometers of 1m in diameter or greater should be used to obtain meaningful results. However, driving large cylinders into most soils may disrupt soil macropores and other structural features affecting infiltration. Soil variability necessitates infiltration measurements at many locations to characterize infiltration accurately on a field scale. Because of size and set-up time required for existing cylinder infiltrometers, infiltration measurements at multiple sites are difficult to obtain in a reasonable length of time.
Photo 6 presents a ponded infiltrometer used for testing the permeability of an alluvial cover installed on mine tailings. The ponded infiltrometer is a self-regulating, single-ring infiltrometer which is simple in design and operation and can be used with a variety of water containment rings. The device allows (i) accurate control of ponded water height, (ii) precise measurement of water flow, (iii) direct delivery of water into the containment ring, and (iv) rapid setup and transport.
Obtaining data from these permeameters is a lot easier than with other single-ring devices or with the double ring infiltrometer, although it is a lot more complicated to analyze due to the flow being in three dimensions. When analyzing this data, absorption and capillary forces, which act in all directions, and the geometry of the water source have to be considered (White et al, 1992). When using this device, a good intimate contact between the disc and the soil surface needs to be established, e.g. fine sand. A drawback of using such a material is that it will interfere with the measurements especially in the early stages of infiltration giving inaccurate sorptivity values. Another disadvantage when using the ponded infiltrometer is that if there is a large macropore in the site the water tower may not be able to supply water quick enough, also causing inaccurate results.
Figure 8. Schematic diagram of a ponded infiltrometer (after Ankeny, M.D., 1992) .
Photo 6.Ponded Infiltrometer used for testing the permeability of an alluvial cover installed on mine tailings (courtesy of Robertson GeoConsultants Inc., 2002) .
Principles
The major components of the ponded infiltrometer are a Mariotte reservoir, a valved base, a containment ring and a tripod (see Figure 8, adapted after Prieksat et al., 1992). Optionally, a datalogger connected to two pressure transducers at the top and base of the water column can be used for automating the water flow measurements. Prieksat et al., (1992) describe the design for an automated, self-regulating ponded (single-ring) infiltrometer. Commonly, the water reservoir and the base are constructed of plastic polycarbonate. A rubber stopper is used to seal the top of the reservoir after filling. Pressure, created by pushing the stopper into the reservoir, starts water flow out of the base when the base valve is opened.
The base consists of a bubble chamber, and bubbling tube, a high-flow air-impermeable nylon membrane, two ports and a two-port valve. The bubble tube regulates the height of water ponded on the soil to +/-1mm. The bubble tube is adjusted up or down within the bubble chamber to raise or lower, respectively, the height of the ponded water in the ring from 0.5 to 1.0cm. This means that the water level in the containment ring can be adjusted without having to raise or lower the entire Mariotte reservoir as is required by previous designs. Because water flow from the device is partly determined by the ponded water height, water heights of < 1.0cm will minimize the size of the water reservoir required to make infiltration measurements.
Two ports connect the Mariotte reservoir to the bubble chamber. The two-port valve is opened during measurement and is closed for movement between sites. The bubble chamber is design to funnel air bubbles up through only one of the two ports. Thus, only water flows through the other port and air bubbles do not limit water flow. Having a second port with unrestricted water flow reduces water-height fluctuations in the containment ring and thus increases measurement precision.
A low-impedence nylon filter covers the bottom of the base, which helps to disperse water flowing from the Mariotte reservoir and limit disturbance of the soil surface. The nylon filter also prevents air from entering the device, except through the bubble tube. The membrane provides a direct link between water ponded in the containment ring and water contained in the Mariotte reservoir without allowing air to enter the system.
Much discussion has occurred about the size of the containment ring that is required to obtain accurate infiltration data. This question remains unanswered, but scaling the dimensions of the device to fit the desired conditions of specific studies will allow the device to be used with a variety of containment ring sizes.
A unit change in water height in the Mariotte reservoir causes a unit change in air pressure above the water (Constantz and Murphy, 1987). Thus, water flow from the reservoir can be calculated from the change in air pressure in the reservoir with time.
Summary of Field Procedures
The typical approach to infiltration measurements using the ponded infiltrometer is briefly summarized here (after Ankeny, 1992):
Soil surface preparation: The soil surface crust or top 10 or 20mm is carefully removed ub a 150mm diameter area, unless the crust itself is being tested.
Ring insertion: A sharpened ring is pushed a short distance (~1cm) into the soil to define the area of the infiltration surface and prevent lateral surface flow of ponded water, and prevent disruption of the soil structure. Cheesecloth (or geotextile) is normally placed in the ring to act as a separation medium.
Measurements: The ponded infiltrometer is set over the ring and ponded measurements are made.
Infiltration can be measured with or without removal of any soil crust. A pointing trowel works well to prepare the surface. If the soil is too wet to avoid smearing, the measurement should wait. The skirted ring is gently pressed into the prepared surface up to the stop ring (the larger diameter outer ring). Next, layers of cheesecloth are placed on the soil surface in the ring to reduce soil slaking into macropores. Initially, the infiltrometer is centered and leveled above the containment ring by adjusting the angle of each tripod legs with the leveling screws. The pointed tripod legs can also be pushed into the ground to stabilize the device. After leveling and centering, the water reservoir and the base are lowered until the base makes contact with the containment ring.
The water reservoir tube and base are then locked in place with the collar lock so that the weight of the infiltrometer is supported by the tripod and not by the containment ring. Using this procedure allows the base and the bubbling tube to be placed at the same relative height above the soil surface each time the device is set up.The water valve can then be open, and water level adjusted prior to starting infiltration test proper.
Analysis of Field Data
During infiltration events, the water enters the soil in response to potential gradients of water potential and gravitational potential. The water potential term is governed by the dryness of the soil and the pore structure of the soil. These two factors combine to form a sorptivity factor which is made up of the combined influences of capillary action and adhesive forces to soil solid surfaces. The sorptivity of the soil is often expressed as "S". The gravity term is a constant for different soils and is due to the impact the pore size, continuity and distribution on the rate of water flow through soil under the influence of gravity. This term is known as "A".
Infiltrometer tests are useful for measuring the rate of infiltration but do not provide a direct measure of field-saturated hydraulic conductivity. Since entrapped air exists within the wetting front, true saturated conditions do not form during infiltration tests. Experience indicates that field saturated Kfs is approximately 50-75% less than Ks (Reynolds and Elrick, 1986).
The initial water infiltration rate is largely governed by the sorptive forces of the dry soil, this is then replaced once the soil wets up by the gravitational term. Equations describing infiltration include the Green et al. empirical model (described by Bouwer, 1986):
Consult the reference list on Ponded Infiltrometers.
Forward to Double-Ring Infiltrometer.
Return to Infiltrometer Methods.
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