Design and Evaluation of a Mesoscale Model Vadose Zone and Ground‐Water System

Abstract

Abstract A mesoscale model soil and ground‐water system was constructed and instrumented to study physical, chemical, and microbial processes, including transport and degradation of contaminants. The model system was 4.6 m in height by 2.4 m in diameter and contained up to 65 tonnes of soil and geologic materials. The model was filled with Ap (11 cm), B (20 cm), C1 and C2 (369 cm) horizons; the water table was 3.3 m from the soil surface. A rain simulation system applied uniform coverage of rain and/or chloride tracer to the model. Sampling of soil, sediment, pore waters, and gas phase was carried out using a series of in situ collectors and sampling ports. Major anions, metals, pH, alkalinity, CO 2 , N 2 , O 2 , CH 4 , microbial numbers, microbial biomass and heterotrophic potential ( 14 C‐amino‐acid uptake) were monitored over time (120 days) and depth. The system approached steady state with respect to solution chemistry and gases after approximately 60–70 days. During this interval, carbon dioxide rose from atmospheric to 3–5% of total gas volume measured at each sampling depth above the water table. Physical, chemical, and biological conditions in the model system after the initial 120 days were comparable to those reported for vadose and saturated zones during field studies on shallow phreatic aquifers. These results indicated that the mesoscale model preserved many of the features of natural systems while providing controlled conditions for studies such as evaluating the fate, and ground‐water contamination potential, of agricultural and industrial chemicals.