Dust Deposition in Southern Nevada and California

Dust samples taken annually for five years from 55 sites in southern Nevada and
California provide an unparalleled source of information on modern rates of
dust deposition, grain size, and mineralogical and chemical composition. The
relations of modern dust to climatic factors, type and lithology of dust
source, and regional wind patterns shed new light on the processes of dust
entrainment and deposition. The average silt-plus-clay flux in southern Nevada
and southeastern California ranges from 4.3 to 15.7 g/m2/yr, but in
southwestern California the average flux is as high is 30 g/m2/yr. These rates
are generally less than those of previous studies in the arid southwestern
United States, probably due to differences in measurement techniques (other
studies mostly used traps at lower heights and did not exclude bird- derived
sediment).

The climatic factors that affect dust flux interact with each other and with
the factors of source type, source lithology, geographic area, and human
disturbance. For example, average dust flux increases with mean annual
temperature but is only weakly related to decreases in mean annual
precipitation, because the prevailing winds bring dust to relatively wet areas.
In contrast, annual dust flux mostly reflects changes in annual precipitation
rather than temperature. Although playa and alluvial sources emit about the
same amount of dust per unit area, the volume of dust from the more extensive
alluvial sources is much larger. In addition, playa and alluvial sources
respond differently to annual changes in precipitation. Most playas emit dust
that is richer in soluble salts and carbonate than that from alluvial sources
(except carbonate-rich alluvial fans), but the dust-deposition rates do not
reflect this trend: salt flux tends to be larger in mountain ranges, and gypsum
flux parallels carbonate flux. Gypsum dust may be produced by the interaction
of carbonate dust and anthropogenic sulfates. Cultivated areas generally yield
about 20 percent more dust than uncultivated areas. The dust flux in an arid
urbanizing area may be as much as twice that before disturbance, but decreases
when construction stops.

The mineralogic and major-oxide composition of the dust samples indicate that
sand and some silt is locally derived and deposited, whereas clay and some silt
from different sources can be far-travelled. Dust deposited in the Transverse
Ranges of California by the Santa Ana winds appears to be mainly derived from
sources to the north and east.

The sampling design for this study was not statistically based; rather, sites
were chosen to provide data on dust influx at soil-study sites and to answer
specific questions about the relations of dust to local source lithology and
type, distance from source, and climate. Some sites were chosen for their
proximity to potential dust sources of different lithologic composition (for
example, playas versus granitic, calcic, or mafic alluvial fans). Other sites
were placed along transects crossing topographic barriers downwind from a dust
source. These transects include sites east of Tonopah (43-46) crossing the
rhyolitic Kawich Range, sites downwind of northern (40, 35, 36) and central
Death Valley (38, 39, 11-14) crossing the mixed-lithology Grapevine and Funeral
Mountains, respectively, and sites downwind of Desert Dry Lake crossing the
calcareous Sheep Range (47-50) north of Las Vegas. In addition, some sites were
chosen for their proximity to weather stations. Specific locations for dust
traps were chosen on the basis of the above criteria plus accessibility,
absence of dirt roads or other artificially disturbed areas upwind, and
inconspicuousness. The last factor is important because the sites are not
protected or monitored; hence, most sites are at least 0.5 mile from a road or
trail. Despite these precautions, dust traps are sometimes tampered with, often
violently. This is a particular problem in areas close to population centers,
and most of these sites (52-55 near Los Angeles and 17-19 and 22 near Las
Vegas) have been abandoned. A few other sites, mostly those that appeared to be
greatly influenced by nearby farming (20, 21, and 41), were eliminated in 1989.
Dust traps were also generally placed in flat, relatively open areas to
mitigate wind-eddy effects created by tall vegetation or topographic
irregularities.

The 55 sites established in 1984 and 1985 were sampled annually through 1989 in
order to establish an adequate statistical basis to calculate annual dust flux.
Sampling continues at 37 of these sites (many sites now have two or more dust
traps) every two or three years as opportunity and funding permit.