Development of a Dust Hazard Rating System for California Benchmark Soils.

Agricultural tillage in California contributes significantly to airborne particulate matter of diameter <10μm (PM₁₀), which has been associated with respiratory and cardiac illness in farm workers. A dust hazard rating system for California soils under cultivation will be useful in establishing regulations or monitoring practices. Soil characteristics found in previous studies to correlate with increased PM₁₀ generation from soil in a laboratory setting include low water content and higher %silt:%clay ratio. Few other studies have investigated correlations between soil characteristics and laboratory-generated dust; this study expanded upon previous investigations by including quantification of citrate-dithionite extractable iron (Fe_d) content, carbonate content, % organic carbon (OC), wet aggregate stability, and texture, and included dust generation values both in the PM₁₀ and PM_2.5 fractions. The study was conducted using samples collected from a variety of soil series, many of which are benchmark soils and thus widely distributed or commonly used in California agriculture. For dust generation, soils were each kept at equivalent gravimetric water content (EGWC) of 33% vol. water held at 15-bar pressure, the driest a soil can be in the field. Rindge muck (19% OC) was found to have the lowest PM₁₀ generation, producing 102.1 mg m^-3. Andisols and other soils with high Fe_d (Aiken, Auburn, Forward, and Jiggs) were found to have the highest PM₁₀ generation, overloading the Teflon filters when a 300 g sample was used. Solano had notably high PM₁₀ generation and the highest PM_2.5 generation, possibly due to its exchangeable sodium content and deflocculation. Quantities of generated PM₁₀ and PM_2.5 were not found to correlate directly. Characteristics found to correlate with higher PM₁₀ generation were: higher Fe_d, lower %OC, lower wet aggregate stability, and higher %silt:%clay ratio. Further investigation is needed to clarify the relationship between PM₁₀ and PM_2.5 generation.