Mineralogy of agricultural source soil and respirable dust in California

Inhalation of respirable dust (RD), defined as particles less than or equal to 4 mu m in aerodynamic diameter, is a health concern. Soil disturbance by farming produces a range of airborne soil minerals of various sizes, but little is known about RD mineralogy compared with source soil or about mineralogy of RD as a function of particle size, General assumptions are that dust mineralogy reflects the source soil and that there is no sorting of RD mineralogy by particle size. We tested these assumptions by determining the size and mineralogy of RD particles from specific farming activities and comparing them to the source soil mineralogy, Samples were taken from plots growing furrow-irrigated crops near Davis, CA. Source-soil was analyzed by x-ray diffraction (XRD), Respirable dust was analyzed by scanning electron microscopy and transmission electron microscopy (TEM) with selected area electron diffraction, Mineralogy and size data of mineral groups from the two techniques were different. The TEM method was more reliable because it included crystallographic data as well as elemental composition and particle morphology. The same major mineral groups (quartz, mica, feldspar, and days) were found in the RD and the source soil, but minor constituents of the RD such as serpentine minerals and zeolites were not detected in the source soil because they were not sufficiently abundant to be detected by standard XRD techniques. No significant sorting of source soil mineralogy occurred over the short distance between soil and RD sampler.