Suction lysimeters

Suction lysimeters are required for some field-scale groundwater monitoring studies to monitor the transport of compounds of interest through the unsaturated zone. Unlike monitoring wells or water supply wells that sample water from the saturated zone, suction lysimeters sample water from the unsaturated zone. This section provides a summary of the installation and sampling procedures for pressure-vacuum suction lysimeters. A detailed discussion of unsaturated zone sampling devices is available elsewhere. 

The reservoir materials may be PVC, stainless steel, or a fluorocarbon polymer, and the porous cup may be constructed of ceramic, stainless steel, or fluorocarbon polymer. Ceramic cups have a smaller pore size, a greater bubble pressure (pressure under which the cup produces bubbles), and a greater operational suction range, and are preferred to other porous cup materials. All materials used for the construction of the suction lysimeter should be tested in the laboratory to determine if any bias in the sample analysis will result from their use. 

The sampling of a suction lysimeter is initiated by applying a vacuum (approximately 40-50 cm of mercury) through the vacuum/pressure line with a hand pump or electric pump. The valve on the sampling line must be closed. A constant vacuum may be maintained on the lysimeter using an electric pump. The time required before collecting a sample from a lysimeter will depend on the method of vacuum application, the moisture content of the soil, and the soil type. 

Figure 6 Example of a suction lysimeter sampling form...

A detailed look at the evolution of soil-moisture chemistry was reported by Sears (1976). In his study Sears assumed the average composition of precipitation shown in Table 8.7. Table 8.7 also lists analyses of the soil moisture he collected from suction lysimeters at 1- and 3-m depths in respective B- and C-horizon soils formed by the weathering of underlying sandy dolomite. The 1-m sample is chiefly a Na -NOj water, with the nitrate probably from fertilizer. The TDS is about 70 mg/L at 1 m and has increased to 500 mg/L at the 3-m depth. In order to explain changes occurring between the 1- and 3-m depth, it is useful to select a solute we can assume to be practically un-reactive in the soil. The best common species for this purpose is probably Cl, with which we can then compare other species concentrations. Relative increases from 1- to 3-m depth are shown in the third column. Increases compared to chloride are given in the fourth column. 

You sample shallow infiltrating waters from suction lysimeters installed in holes drilled by a truck-mounted auger in a humid climate soil at depths of 1 m and 3 m. Hie following mg/L concentrations are obtained at the two depths. 

Detailed fractionation studies of water extracted from acid soils by suction lysimeters in part of the Hubbard Brook Experimental Forest were reported by Driscoll et al (1985). Unfortunately they quote only Al, but of this fraction concentrations around 30)jM were found in various horizons of a podzolic soil in a high elevation site at pH 3.6-4.7, while around lO iM was found in horizons at lower sites at pH 4.5-5.1. In every case the majority of this Al was in organic form. The total salt concentrations were low, <100 M. 

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