Suction Lysimeter Operation at Hazardous Waste Sites

<jats:p>Four questions relative to suction lysimeter performance have been raised by operators of land treatment systems. These questions deal with plugging of the porous segments, soil suction operational ranges, loss of volatile organics under negative pressure, and adsorption and screening of the various lysimeter parts. This paper describes the physical operation of a lysimeter, the characteristics of the lysimeters tested [high- and low-flow lysimeters with porous ceramic cups and lysimeters with porous polytetrafluoroethylene (PTFE) cups], and procedures followed in preparing the lysimeters for testing. It then presents the results of experiments dealing with the first three questions—plugging, operational ranges, and volatile organics loss. It was found that suction lysimeters placed in the vadose zone of most types of soils will initially drop off rapidly in intake rate, but will stabilize after about 15 L of moisture has been drawn through the porous cups. Packing a crystalline silica flour slurry around the cups of PTFE lysimeters negates most of the plugging associated with finer particles in soils. The effective operating range of ceramic lysimeters is between 0 and 60 centibars of suction, independent of the use of silica flour. The operating range of PTFE lysimeters without silica flour is extremely narrow, but with the use of silica flour is extended to about 7 centibars of suction. Volatile organics are lost from suction lysimeters, but the amount of the loss is difficult to estimate. The testing program indicates that before field installation all lysimeters should be checked for leaks using pressure techniques, and that lysimeters have “dead” spaces, that is, reservoirs of moisture, ranging from 34 to 80 mL, that cannot be extracted from the cups, and that must be taken into account when determining moisture collection rates. Information gathered during the test program is planned for inclusion in an Environmental Protection Agency (EPA) guideline document entitled “Unsaturated Zone Monitoring at Hazardous Waste Land Treatment Units.”</jats:p>