A Microcosm for Soil‐Atmosphere Gas Transfer Investigations

Abstract

Proposed disposal of high‐level nuclear wastes containing primarily inorganic 14C may provide a source of 14CO2 to the atmosphere. Laboratory experiments show that the loss of inorganic 14C is driven primarily by gaseous diffusion and is affected by the pH, porosity, temperature, moisture, and organic matter content of the soil. Two soil microcosms were designed to determine the degassing rate of volatile contaminants for the top 0.3 m of soil, the depth generally defined as the root zone in environmental assessment models. The performance of the system was evaluated in an investigation of the major factors affecting the transport of 14CO2 across the soft‐atmosphere boundary. These microcosms were used in a flow‐through system that allowed for manipulation and monitoring of parameters such as airflow, air moisture levels, air‐CO2 concentration and soil temperature and soil moisture with soils of different pH and organic matter content. The performance of the microcosms was assessed for 14CO2 using soil slurries and soil columns. The evolved 14CO2 was trapped in a series of NaOH gas traps, which were frequently sampled and analyzed. Profiles of soil moisture content, soil temperature and 14C activity could be established. This laboratory system was useful in estimating the rate at which 14CO2 is released from soil. Other traps could be tested for use with other soil contaminants.