Using Soil Physics Algorithms to Determine VOC Source Strengths for Regional Air Dispersion Calculations.

Soil fumigants are specific VOC compounds that are applied to agricultural land to control nematode populations, weeds, and crop diseases.  Field studies help quantify the transient flux profile observed under typical agronomic conditions.  Unfortunately, field trials only represent a small proportion of the near semi-infinite parameter combinations of environmental, agronomic, and meteorological conditions and often offer little quantification for mechanisms responsible to reduce fumigant flux losses.  Source strength in and of itself is simply a mandatory precursor to the larger problem of air concentration predictions and non-target organism risk assessment.  Fumigant loss from soil is estimated using the USDA soil physics program CHAIN_2D, which expands field knowledge and provides estimates of transient source strength for use in air dispersion modeling.  CHAIN_2D has largely been underutilized by the agricultural community due to lack of pre and post processing tools, slow CPU execution times, and lack of simulation of physical constraints often associated with agriculture.  CHAIN_2D was modified to address many of these shortcomings while accounting for soil fumigation practices such as an agricultural film at the soil surface, and the ability to simulate film tearing and removal at user specified time intervals post application.  Additionally, transient stagnant boundary layer thickness at the air/soil interface, as a function of meteorological conditions, was incorporated into the model, along with direct coupling to air dispersion modeling.  A sensitivity analysis for CHAIN_2D, comparison to field observations for the fumigant 1-3-dichloropropene, and several best management practices (BMP) examples are provided to illustrate the impact of source strength and resulting near-field air concentrations.  Discussion on coupling heterogeneous soil properties with fumigant flux predictions and the regional air dispersion models ISCST3 and CALPUFF are provided.