2-D Simulation of Non-isothermal Fate and Transport of a Drip-applied Fumigant in Plastic-mulched Soil Beds. I. Model Development and Performance Investigation

Pre-plant application of toxic fumigants to soil beds covered by plastic film is commonly used in agriculture to control soil-borne pathogens. Plastic mulch covers tend to physically suppress the emissive loss of gaseous fumigant to the atmosphere. When liquid fumigant metham sodium (MS) is applied in irrigation water to field soil, it is rapidly transformed to the gaseous methyl isothiocyanate (MITC). The gaseous MITC is a potential atmospheric contaminant, and any untransformed MS is a potential contaminant of underlying groundwater due to the high water solubility of MS. A finite element numerical model was developed to investigate two-dimensional MITC fate/transport under non-isothermal soil conditions. Directional solar heating on soil beds, coupled heat and water flow in the soil, and non-isothermal chemical transport were included in the model. Field soil data for MITC distribution, soil water content, meteorological data, and laboratory data were used to verify the model for soil beds covered with plastic mulch. Four possible scenarios were considered: low and high drip-irrigation rates and low and high water contents. The movement of the center of MITC mass in the soil profile was effectively simulated. The lower drip-irrigation rate of MS yielded more extensive coverage of MITC in the plastic-covered soil bed. The lower soil air contents due to higher soil water contents for the higher irrigation rate resulted in high concentrations of soil MITC. NRMSE (normalized root mean square error) calculations further verified that the model predicted fumigant fate/transport well under these non-isothermal field conditions.     

Filtration in a Porous Granular Medium: 2. Application of Bubble Model to 1-D Column Experiments

This paper describes the formulation of a quasi-1-D network model, referred to as the ‘bubble model’, and its application for simulating particle transport and filtration through a granular filter bed. The model comprises a series of homogeneous sites linked through bundles of cylindrical bonds that represent flow pathways through distributions of pores and pore throats. This model incorporates pore scale processes of particle sieving and infiltration are based on numerical simulations described in a companion paper. The modeling of infiltration is further refined based on detailed experimental observations and measurements of the filtration of a dilute suspension of acrylic particles through a column of glass beads reported by Yoon et al. (2005 Water Resour. Res., to appear). Their data distinguish (a) between the collection of particles on grain surfaces and at grain-to-grain contact points, and (b) between particles that are fully entrapped and those that are hindered (temporarily collected) and can later become detached. These effects are represented by two parameters that characterize the probability of attachment and are linked to the surface roughness of the grains; one that describes the minimum particle size that can be fully entrapped, and one that describes the detachment rate. These parameters can be readily calibrated from conventional measurements of effluent concentration and effluent particle size distribution. Detailed comparisons with the data reported by Yoon et al. show that the proposed bubble model is able to achieve reliable predictions of the spatial distribution of particles within the filter bed following phases of particle injection and washing.