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No. 94-15 |
| September 1994 | |
| RESEARCH NOTES |
| California Environmental Protection Agency | Brief Reports to the Scientific and Technical |
| Air Resources Board | Community |
| Research Division, John R. Holmes, Ph.D., Chief | P.O. Box 2815, Sacramento CA 98512 |
This study investigated the sources and generation mechanisms of windblown dust on the dry bed of Owens Lake. The dust is transported to nearby populated areas, including the towns of Keeler and Lone Pine, and to the Naval Air Weapons Center at China Lake. The levels of dust in this area are among the highest ever measured in the United States. The largest dust storms are generated primarily by abrasion of the surface by fine sand particles. This suggests that the intense nearby dust storms could be reduced by effectively controlling the movement of sand on the lake bed playa. This study was performed by the University of California, Davis.
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Background: |
The Los Angeles Department of Water and Power began diverting Owens River water from Inyo county to Los Angeles county in 1913. This and other diversions left Owens Lake without its primary source of water. Within a few years, the desert climate caused evaporation of the lake, leaving behind a vast alkaline bed (playa). Frequent high winds in the Owens Valley generate intense dust storms that transport high concentrations of PM10 (particulate matter of 10 µm or smaller aerodynamic diameter) north or south over great distances, and which have caused the Owens Valley to violate the PM10 standard. In order to design cost-effective measures to reduce their concentrations, it is crucial to understand the mechanisms that generate these dusts.
A number of approaches to reducing the severity of dust storms have been tried and/or discussed. Sand fences have been constructed to halt the progress of blowing sand, large areas of the lake bed have been sprinkled with water during windy periods, and smaller areas have been subjected to chemical stabilizers. The concept of covering the lake bed with gravel, railroad ties, or old tires has been discussed and/or tried and discarded. Some of these approaches were unsuccessful or only partially successful, some violated the public trust values of the lake bed, and others were too expensive. Refilling the lake with water is not practical: The areas that would be covered first do not generate dust and it would take ten to twenty years to cover the most severe dust sources. The State Lands Commission, as owner of the lake bed, has received complaints about dust coming from the lake bed and desires to undertake mitigation measures as soon as possible. The project described here provided research support to a large-scale mitigation study funded by the Commission that involves constructing sand fence arrays on the lake bed to channel water and support vegetation. The research conducted here examined details of the dust generation mechanisms to provide information on the optimal design of sand fence arrays. |
| Methods: | Saltating sand flux, meteorological parameters, and PM10 concentrations were measured during several high wind dust storm events. Sand transport was measured using BSNE (Big Springs Number Eight) samplers and a modified Weaver sampler recommended by the Great Basin Unified Air Pollution Control District. In some locations, BSNE samplers were arranged in vertical "towers" to assess the vertical extent of sand transport. The modified Weaver samplers were limited to a fixed position above the sand surface. Sand flux was measured at several locations using a Sensit monitor, which detects the impact of sand grains. The Sensit monitors were located adjacent to meteorological towers to measure the characteristics of the surface boundary layer. Additional meteorological towers provided more information on the surface boundary layer at other locations during dust storms. PM10 concentrations were measured on the lake bed using the DRUM eight-stage impactor and a new portable PM10 sampler designed by researchers at the University of California, Davis. Detailed particle size and chemistry at locations away from the lake bed were measured using DRUM impactors, stacked filter unit samplers, SMART samplers, an aerodynamic particle sizer, an integrating nephelometer, and a laser aerosol counter. The transport of PM10 dust was assessed by measuring the concentrations at downwind locations using both fixed monitoring sites and a mobile van, and by tracking the dust plumes using satellite photographs. |
| Results: |
The primary dust generation mechanism on the south sand sheet involves the saltation of sand grains across the lake playa and subsequent pulverization of the protective crust. The source of saltating particles on the south sand sheet appears to include the Olancha Dunes and the Dirty Socks Springs dunes. In saltation, the wind starts the sand particles rolling along the surface (surface creep) and, as the wind speed increases, the particles are lofted into the air, either as a result of striking an obstruction or by aerodynamic forces on the spinning sand particle. Once airborne, a particle travels an elongated trajectory until it strikes the surface, dislodging PM10 particles. The sand particle may rebound immediately into the air, repeating the saltation process as it moves downwind. As the crust is destroyed by saltating sand particles, the generation of dust becomes more efficient.
Three types of crust were observed on the lake bed; a fragile efflorescent crust that forms in late winter after the lake bed has been completely wetted, a hard salt-silt-clay crust that underlies the fragile efflorescent crust, and a cemented crust that forms after rain storms in spring, summer, or fall. The efflorescent crust is easily suspended by wind, but the crust is soon blown away and generates little dust afterwards. The salt-silt-clay crust generates moderate dust as it is abraded by saltating particles, and can be broken by thermal heaving in the hot desert environment. The broken material generates large amounts of dust. The cemented crust is very stable; it resists dust formation until it is abraded by sand transported into the area by high winds. |
| Significance and Application: | This research indicates that the intensity of major dust storms can be diminished by reducing the movement of sand particles across the surface. Various measures would accomplish this, but the use of sand fences, in combination with water and vegetation in sustainable amounts, is recommended. Specifically, fence arrays should be constructed to reduce sand movement in the dust generation areas and vegetation should be established using drip irrigation to stabilize the resulting dunes. Eventually, small amounts of water should be introduced between the dunes to establish a riparian corridor microenvironment that would support permanent vegetation and wildlife. The research reported here will aid in the design of sand fence arrays for maximum effectiveness at the lowest cost. |
| Related Projects: | The ARB has funded these related studies (ARB contract number in parentheses): A Study of Ambient Aerosols in the Owens Valley Area (A7-178-30) and The Effect of Mono Lake on Air Quality in the Mono Lake Region (A9-167-31). |
| This research was conducted under contract with the University of California, Davis (ARB contract no. A132-105). Comments or questions can be directed to the contract manager, Tony VanCuren, by mail, FAX (916) 322-4357, phone (916) 327-1511, or e-mail: rvancure@arb.ca.gov . For an index of Research Notes, call (916) 445-0753 or FAX (916) 322-4357. |
| Copies of the research report upon which this Note is based can be ordered from: |
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National Technical Information Service
5285 Port Royal Rd Springfield VA 22161 Request NTIS No. PB95-171799 |
| Title: Generation, Characterization, and Transport of Owens Lake Dust |
| Authors: Thomas A. Cahill, Dale A. Gillette, Thomas E. Gill, Elizabeth A. Gearhart, Jeffrey S. Reid, and Mee-Ling Yau |
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