Vadose Zone Monitoring: An Early Warning System

Currently, vadose zone monitoring is required under the Resource Conservation and Recovery Act (RCRA) only at land treatment facilities. Contaminant leak detection through ground water monitoring is very important, but it is considered to be after the fact. Remedial action costs can be reduced considerably by monitoring the vadose zone for compounds that exhibit high rates of movement. Volatile organic compounds (VOCs) exhibit this property and are present at many municipal landfills, recycling facilities, and treatment storage and disposal facilities (TSDFs). Through the authors'personal experience, it has been noted that gaseous phase transport of VOCs through the vadose zone is at least an order of magnitude greater than advective transport of VOCs in ground water. Therefore, VOCs in soil gas are an effective early warning leak detection parameter. Downward movement of leachate can be intercepted by porous cup lysimeters. Attenuation in the vadose zone slows the apparent movement of contaminants; however, it is only a matter of time before leachate reaches the water table. The authors believe that soil-gas and pore-water monitoring should and eventually will be required at all RCRA sites. If vadose zone monitoring becomes an additional requirement under RCRA, both the facility owner and the taxpayer will benefit. During the interim, facility owners can benefit by employing vadose zone monitoring techniques coupled with either qualitative or quantitative chemical analyses.