relevant constituent properties, and examination of the machine's performance records should indicate how effective the system will be in treating the soil. The proven effectiveness of a particular system for a specific site or waste does not ensure that it will be effective at all sites or that the treatment efficiencies achieved will be acceptable at other sites. If a test burn is conducted, it is important to ensure that the soil tested is representative of average conditions and that enough samples are analyzed before and after treatment to confidently determine whether LTTD will be effective. Operation of LTTD units requires various permits and demonstration of compliance with permit requirements. Monitoring requirements for LTTD systems are by their nature different from monitoring required at a UST site. Monitoring of LTTD system waste streams (e.g. concentrations of particulates, volatiles, and carbon monoxide in stack gas) are required by the agency or agencies issuing the permits for operation of the facility. The LTTD facility owner/operator is responsible for complying with limits specified by the permits and for other LTTD system operating parameters (e.g. desorber temperature, soil feed rate, afterburner temperature). The decision as to whether or not LTTD is a practical remedial alternative depends upon site-specific characteristics (e.g. the location and volume of contaminated soils, site layout). Practicability is also determined by regulatory, logistical, and economic considerations. The economics of LTTD as a remedial option are highly site-specific. Economic factors include:- Site usage (because excavation and onsite soil treatment at a retail site (e.g. gasoline station, convenience store) will most likely prevent the business from operating for a longish time). The cost of LTTD per unit volume of soil relative to other remedial options. The location of the nearest applicable LTTD system (because transportation costs are a function of distance). Operation principles Thermal desorption systems fall into two general classes—stationary facilities and mobile units. Contaminated soils are excavated and transported to stationary facilities; mobile units can be operated directly onsite. Desorption units are available in a variety of process configurations including rotary desorbers, asphalt plant aggregate dryers, thermal screws, and conveyor furnaces. The plasticity of the soil is a measure of its ability to deform without shearing and is to some extent a function of water content. Plastic soils tend to stick to screens and other equipment, and agglomerate into large clumps. In addition to slowing down the feed rate, plastic soils