Numerical Analysis to Investigate the Effects of the Design and Installation of Equilibrium Tension Plate Lysimeters on Leachate Volume

steel porous plates. Various control options for the tension applied to these porous plates have been advocated. The composition and quantity of leachate as it moves down through The simplest being zero tension, where no capacity to the vadose zone is seldom measured directly because sampling in this unsaturated zone at the depths required has proven to be extremely exert a tension on the plates exists. The zero-tension difficult. A promising technique is the use of large porous plates lysimeters rely on the formation of a saturated zone known as equilibrium tension plate lysimeters (ETPLs), which have above the plate before water can be collected and sama controlled suction exerted on them that mimics the soil matric pled. The requirement of this saturated zone for sampotential measured in the surrounding undisturbed soil profile. In the pling is an artifact of the measurement technique and design phase for the installation of 15 ETPLs at five different depths is an unlikely condition to exist in most deep and reason(three replicates) around a central access chamber in the vadose ably drained soils (Brye et al., 1999). Additionally, the zone in the Lake Taupo catchment of New Zealand, questions arose saturated area above the plate has been shown to cause regarding the effects of the design and installation layout of the ETPLs on the measured leachate fluxes. To investigate the important design divergent water flow away from the lysimeter and genercriteria and the spacing of the ETPLs, a numerical investigation using ally results in an underestimation of the natural water the HYDRUS-2D software was conducted. This analysis showed that flux (Chiu and Shackelford, 2000). In a numerical evaluthe distance between the tension plate and the base (lower boundary) ation by Flury et al. (1999), it was shown that the satuof the ETPL needs to be large enough to ensure that the effect of rated zone not only has a large effect on the estimated the dry zone (rain shadow effect) created below the ETPL on the leachate flux but it also influences the chemical concensoil zone being sampled above the plate is minimized. The boundary tration of the leachate sampled. condition between the ETPL and the central access chamber was One of the most frequent control options is the conalso shown to be critical, requiring a free-drainage condition in this stant tension plate lysimeter. Under these conditions location. The sampling efficiency of the ETPLs increases when the horizontal distance between adjacent ETPLs increases. Less obvious the applied suction on the plate is fixed at a preselected was the result that the sampling efficiency decreases with increasing value. This value is generally somewhere between 100 vertical distance. This work demonstrates that the design and installaand 300 cm. As a result, the soil water conditions close tion configuration of the ETPLs can significantly impact sampling to the sampler are a function of the applied suction efficiency. The relationships demonstrated in this work must be recogpossible, resulting in the sampled leachate being dissiminized and implemented in the design process, particularly when multilar to the soil water flux occurring under undisturbed ple ETPLs are installed at various depths through the vadose zone conditions. Because the pore sizes from which water is at other locations. sampled are related to the sampling suction applied, and recognizing that different concentrations of solute are known to exist in the differing pore sizes, the solute M effort has recently been put into developing concentration that is sampled will depend on the applied rigorous and defensible in situ measurement constant suction (Rhoades and Oster, 1986; Kosugi and techniques capable of determining the quantity and Katsuyama, 2004). To try and overcome these problems quality of leachate draining from the active root zone with fixed tension systems, confined lysimeter systems (Brye et al., 1999; Goyne et al., 2000; Gee et al., 2002; have been used that have side walls extending above Zhu et al., 2002; Lentz and Kincaid, 2003; Siemens et the tension plates. The soil volume confined by the al., 2003; Foley et al., 2003; Barzegar et al., 2004; Kosugi sidewalls reduces the influence of the applied constant and Katsuyama, 2004; Masarik et al., 2004). In most situations, the zone beneath the plant roots is unsatusuction on the undisturbed water flow through the top rated and this places significant constraints on the type boundary (at the top of the sidewalls) of the sampler of measurement devices that are suitable for sampling (Lentz and Kincaid, 2003). However, the sidewalls introin this zone. The most promising technique has been duce the risk of saturation and preferential flow along the development of in situ tension plate lysimeters, the walls. Additionally, the walls make the installation which use relatively large ceramic or sintered stainlessof plates at depths in the profile more difficult. In contrast to these more traditional methods, recent studies have recommended controlling the suction on J. Mertens and R. Stenger, Lincoln Environmental, Ruakura Research Centre, Private Bag 3062, Hamilton, New Zealand. G.F. Barkle, Aquathe plate for extracting the soil water to be at the same linc Research Limited, P.O. Box 14-041, Enderley, Hamilton, New tension as the soil matric pressure in the soil profile Zealand. Received 14 Nov. 2004. *Corresponding author (mertensja@ surrounding the tension plate (Brye et al., 1999, 2001; yahoo.co.nz). Siemens et al., 2003; Foley et al., 2003; Pelger et al., Published in Vadose Zone Journal 4:488–499 (2005). 2003; Lentz and Kincaid, 2003; Barzegar et al., 2004; Original Research Paper Kosugi and Katsuyama, 2004). In these so called equilibdoi:10.2136/vzj2004.0161 rium-tension systems, the suction applied to the plate © Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA is either manually or automatically adjusted to the target 488 Published online June 13, 2005