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4. Form additional slurry of silica flour and distilled water and tremie the <br /> mixture to the base of the hole to ensure embedding the unit to a depth of <br /> about one foot above the cup. <br /> 5. Hold the unit in place within the hole until the water drains from the slurry. <br /> 6. Test the lysimeter to ensure that it is capable of holding a vacuum of about <br /> 400 centimeters water pressure (0.4 bar or 6 psi); if the unit fails, remove it <br /> and replace it with a backup unit. <br /> 7. Using a second, dry tremie pipe, add a small amount of sieved native <br /> material to the hole, then add about 12 kilograms of 3/8 or 1/4 inch <br /> bentonite tablets (Volclay tablets or equivalent).A dry tremie pipe should be <br /> used for adding the bentonite to prevent swelling of the bentonite inside the <br /> tremie pipe. <br /> Add distilled water through the first (wet) tremie pipe to the bentonite <br /> tablets, causing them to swell and form a seal above the lysimeter unit. <br /> 8. Gently back out the auger flight, adding backfill of native material. Tamp the <br /> backfill between additions of backfill. <br /> 9. On the surface, the PTFE sampling tubing should be connected to the <br /> collection bottle. If vacuum is used to collect samples from the lysimeters <br /> (can only be done for shallow lysimeters), a tube with valve or tubing clamp <br /> is installed between the collection bottle and an overflow bottle. The <br /> overflow bottle is in turn connected with a tube to the vacuum pump. <br /> The overflow bottle prevents fluid from entering the vacuum pump. <br /> Sample collection: <br /> Sample collection can be done in two ways. <br /> The most common method is to apply vacuum to the vacuum/pressure line <br /> while keeping the fluid return line closed. This draws the pore water into the <br /> lysimeter. After a given time, often 24 hours, the fluid return line is opened and <br /> pressure is applied to the vacuum/pressure line. This forces the fluid up to the <br /> soil surface and into the collection bottle. In very wet soils the time it takes to <br /> bring sufficient water into the lysimeter can be significantly shorter than 24 <br /> hours. In dry soil it may be difficult or impossible, depending on how dry the <br /> soil is, to draw sufficient water into the lysimeter <br />