BLOWING DUST AND A DANGEROUS HIGHWAY: EVALUATING THE SOURCES OF DUST ALONG INTERSTATE 10 BETWEEN PHOENIX AND TUCSON, ARIZONA

A 145 km stretch of Interstate 10 between Phoenix and Tucson is considered one of the most-dangerous sections of highway in the U.S. primarily due to dust brownout conditions leading to a loss of driver visibility. Between about 1960 and 2015, blowing dust was a leading factor in over 400 vehicle accidents (many multiple vehicles) with nearly 100 fatalities. Windblown dust events are primarily related to haboobs created by summer convective storms (June-September) and synoptically driven frontal storms (November-April). Dust is primarily attributed to wind erosion of extensive areas of active and abandoned cropland and secondarily to natural (undisturbed) soil surfaces; however, few studies have characterized or identified actual sources of dust. We characterized the physical soil surface and measured dust emission potential using the Portable in Situ Wind Erosion Lab (PI-SWERL) at 42 sites, consisting of abandoned cropland and undisturbed soil surfaces, marginal to I-10 between Phoenix and Tucson. All soil surfaces are primarily fine-textured (silt-loam to clay-loam), gravel-poor (<5 % volume), and occur on low-gradient (< 2%) distal alluvium fans or plains. Dust from active, recently tilled crop land has declined in recent years due to mitigation. Abandoned cropland surfaces commonly have a hard, 1-2 cm thick physical crust (massive), with the plow-line furrows still present (many last plowed before 1997). Undisturbed soil surfaces primarily consist of a smooth, hard, 1-5 cm thick soil-crust (platy to blocky structure). A discontinuous cover of sand including coppice dunes commonly occurs on the undisturbed surfaces. In many places small ripples consisting of fine to coarse sand and granules (~2-4 mm) are present on the undisturbed soil surfaces. PI-SWERL data indicate that at high shear velocities (0.7-1.4 m/s), fluxes of PM-10 for abandoned cropland remain low, whereas fluxes from undisturbed surfaces are much higher. These higher dust fluxes are associated with the presence of loose sand (e.g. ripples) on the soil surface. These results indicate that, at least presently, the undisturbed soils are a primary source of windblown dust events. PI-SWERL shear velocities greater than 1.0 m/s equates to an estimated 10-m wind speed of 25 m/s which overlaps reported wind speeds associated with storms in Arizona.