FIELD SYSTEM FOR CONTINUOUS MEASUREMENT OF LANDFILL GAS PRESSURES AND TEMPERATURES

Abstract

The driving forces for gas movement in the subsurface include both concentration gradients (diffusion) and pressure gradients (convection). Near the top of the landfill, small soil gas pressure differences with respect to atmospheric pressure have been typically disregarded but may be important to considerations of gas flux into and out of landfill cover materials. The authors have developed a portable, inexpensive system using off-the-shelf components to sensitively monitor pressures, temperatures and meteorological variables on a continuous basis. Previous experience has indicated that continuous monitoring of pressure changes is necessary to understand the dynamics of convection in the shallow subsurface. This application relies on a distributed network of commercially available Z-180 microprocessor-based integrated controllers to condition signals from electronic pressure transducers, meteorological sensors, and various temperature-sensing devices: thermocouples, thermistors and resistance temperature detectors (RTDs). Output is recorded continuously over an Appletalk™ network. The sensitivity of the authors' current system exceeds 4 Pa for pressure and 0.01°C for temperature (thermistor sensors). This paper will describe the basic system components. The various choices for both temperature and pressure sensors will be discussed with respect to their sensitivity, adaptability and resolution. In addition, sample data output will be presented to illustrate the dynamics of shallow subsurface pressure and temperature changes.