Summary

Owens Lake (Patsiata) is located at the southern end of the Owens Valley (Payahuunadü) in California. After water was diverted into the Los Angeles Aqueduct starting in 1913, the dry lakebed became one of the largest sources of airborne particulate matter in the United States, including particulate matter with an aerodynamic diameter of 10 micrometers or less (PM₁₀). These small particles can penetrate into the lungs and cause or worsen a variety of health problems like asthma, bronchitis, chronic obstructive pulmonary disease, and respiratory infections. Since 2001, the Los Angeles Department of Water and Power (LADWP), at the direction of the Great Basin Unified Air Pollution Control District (which this report will refer to as “the District”), has implemented dust control measures on the lakebed. These on-lake sources are defined as those that lie below the 3,600-ft elevation regulatory shoreline that defines the Owens Lake bed (Figure S-1). The District has also implemented approximately 140 acres of dust control measures on off-lake areas (above the 3,600-ft regulatory shoreline) at the Keeler Dunes. Together, these controls have made substantial progress toward reducing the frequency and intensity of exceedances of the PM₁₀ standard. However, both on-lake and off-lake sources continue to cause PM₁₀ exceedances that prevent the Owens Valley Planning Area (OVPA) from attaining the PM₁₀ National Ambient Air Quality Standard (NAAQS).

The National Academies of Sciences, Engineering, and Medicine’s Owens Lake Scientific Advisory Panel (OLSAP) was established following a 2014 stipulated judgment between the District and LADWP to foster understanding and collaboration on the scientific and technical approaches to dust control in the OVPA. OLSAP’s first task focused on the effectiveness of on-lake dust control measures, and they released a report on the topic in 2020 titled Effectiveness and Impacts of Dust Control Measures for Owens Lake. For its second task, the panel was asked to summarize the impact of off-lake sources on PM₁₀ exceedances, as well as their distribution, origin, and how they might change over time. The panel was asked to recommend better methods for characterizing and monitoring PM₁₀ contributions by off-lake sources. Additionally, the panel was asked to discuss possible dust control measures that could be applied to off-lake sources, as well as the applicability of the U.S. Environmental Protection Agency’s (EPA’s) Exceptional Events Rule for excluding air quality monitoring data from unusual or naturally occurring events that are not reasonably controllable. To gather the necessary information to address the statement of task, the panel held several information-gathering sessions, including a 2-day meeting at Owens Lake.

The panel found that off-lake sources are, and will remain, a major source of PM₁₀ exceedances in the OVPA. Local off-lake sources in the OVPA include flood deposits,¹ Keeler Dunes, Olancha Dunes, alluvial fans, up-valley sources, and anthropogenic disturbances, among others. PM₁₀ emissions from some of these sources, like certain alluvial fans and flood deposits not impacted by highway infrastructure, are judged with high certainty to be natural in origin. It would be reasonable to apply the Exceptional Events Rule to applicable high-wind events from these naturally occurring and not-reasonably controllable sources so that these data would be excluded from regulatory decisions. Exceedances from other sources—Keeler Dunes, Olancha Dunes, and flood deposits influenced by highway infrastructure—are at least partially anthropogenic in origin and thus dust control measures may be required. Further monitoring and modeling would define more clearly the sources of PM₁₀ emissions and their natural or anthropogenic origin. If dust control measures are required, the panel found that the establishment of native vegetation would be the most stable and sustainable dust control measure across all off-lake surfaces emitting particulate matter. On sand sheets and dunes, natural roughness features such as straw bales or straw checkerboards would be necessary components of revegetation efforts. On small, impounded flood deposits, gravel cover could provide near-term dust control, but longer-term control would require improved drainage infrastructure to reduce accumulation of fine-grained sediments. In all cases, early engagement of local Tribal Nations in the planning and design of dust control methods and consideration of the resilience of these dust control methods under a changing climate are essential for reducing PM₁₀ emissions in the OVPA.

SOURCES OF PM₁₀ EMISSIONS IN THE OWENS VALLEY PLANNING AREA

Dust control measures have made substantial progress toward reducing the frequency and intensity of on-lake exceedances, but both on-lake and off-lake sources continue to cause PM₁₀ exceedances in the OVPA. The relatively consistent number of PM₁₀ exceedances that the District has attributed to off-lake sources over the last 25 years, despite trends indicating a declining number of exceedances from on-lake sources, demonstrates the importance of these off-lake sources and suggests that these sources could hinder attainment with the PM₁₀ NAAQS in the region.

Since 2017, the District has used additional information to attribute PM₁₀ exceedances to specific sources within the OVPA. This information includes particulate and meteorological data, modeling, cameras, field observations, and media reports of dust storms. These data are compiled into the District’s exceedance database. Based on these data, the District classifies each exceedance as one of the following: 1) dust—primarily on-lake sources, 2) dust—primarily local off-lake sources, 3) dust—primarily regional event, 4) wildfire smoke, or 5) mixed—dust and wildfire sources. Each documented exceedance includes detailed comments on likely source areas. The panel supports the District’s general approach to source apportionment and used this information to identify a few specific local off-lake sources that cause a disproportionate impact on the PM₁₀ exceedances in the OVPA. These include flood deposits (including channelized, sheet/overland flow and impounded flood deposits), Keeler Dunes, Olancha Dunes, alluvial fans, up-valley sources, and anthropogenic disturbances.

This District’s method for source attribution is useful for assessing broad trends, but the classifications are nonquantitative, and some uncertainties remain in the identification of specific off-lake source areas. For example, the current methodology does not allow for the quantification of PM₁₀ contributions from different sources for a

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¹ Following the District’s methods for source attribution, “flood deposits” in this report is defined as including 1) channelized flood deposits; 2) sheet/overland flow deposits; and 3) deposits from impounded floodwaters. The term “alluvial fans,” in contrast, is used to refer to a general landscape feature and not a specific flood event.

single exceedance, and data are often not collected in locations that are ideal to capture detailed information about off-lake sources. Additional measurements and modeling would enable the District to more definitively identify how specific sources have contributed to exceedances and support future air quality management decisions.

ORIGIN AND EVOLUTION OF OFF-LAKE SOURCES IN OWENS VALLEY

The panel considered multiple lines of evidence to infer the origins and evolution of major PM₁₀ sources in the OVPA, or those that might become important sources in the future. One important process the panel considered was winnowing. This process suggested that PM₁₀ material from the dry lakebed deposited onto off-lake landforms would be expected to decrease over time due to its resuspension and removal by aeolian processes. This hypothesis was based on a correlation between on- and off-lake exceedances at the Dirty Socks monitor between 1999 and 2012, which has not continued in more recent estimated emissions trends and exceedance data. Most current PM₁₀ emissions from off-lake areas are likely not a result of resuspension of PM₁₀ material originating from the lakebed that was deposited on off-lake landforms. Instead, the presence and common replenishment of highly emissive flood deposits provide ample fine particulates that can be emitted as PM₁₀ as long as the horizontal flux of saltation-sized particles is sufficient to emit dust from the surface.

Northeast Side of the Lake

The northeastern side of Owens Lake is host to several landforms including the Keeler Dunes and the Slate Canyon/Keeler Alluvial Fan Complex that have undergone major changes over the last century and are substantial contributors to exceedances. During the 20th century, the Keeler Dunes transitioned from a largely vegetated dune system that was stabilized by greasewood (Sarcobatus vermiculatus) to an active dune field. The emergence of Keeler Dunes as an active dune field resulted in abundant saltation that can drive PM₁₀ emissions from flood deposits that are continually replenished from the alluvial fan. Therefore, ongoing PM₁₀ exceedances from this area are a direct result of the destabilization of the Keeler Dunes. The panel finds that the net transport direction, available imagery, and the evidence for groundwater-dependent vegetation currently present in the dunes support the conclusion that increased sand transport following the diversion of water from Owens Lake destabilized the Keeler Dunes. Changes to surface hydrology resulting from construction of berms designed to protect Highway 136 appear to have had an impact on upland (non-groundwater-dependent) vegetation but are unlikely to have led to the destabilization of groundwater-dependent vegetation in the Keeler Dunes.

Several constructed berms northeast of Highway 136 were intentionally designed to alter surface hydrology, directing overland flow to specific discharge points along the highways. The panel did not analyze each berm-related flood deposit but instead considered the berms on the Slate Canyon/Keeler Alluvial Fan Complex as potentially representative of similar features around Owens Lake. These berms have had appreciable, localized impacts on the distribution of flood deposits in the Keeler Dunes region, especially following impacts from the remnants of Hurricane Kay in September of 2022. Further investigation would be needed to determine the impacts that these berms have, if any, on potential off-lake PM₁₀ emissions from flood deposits.

Southern Side of the Lake

The southern side of Owens Lake is host to the Olancha Dunes and multiple alluvial systems that are important sources of PM₁₀ exceedances. The scientific literature on the origin and evolution of the Olancha Dunes is quite sparse, but available evidence indicates that the dunes formed prior to the diversion of water from Owens Lake. The panel’s analysis shows that the dunes experienced a slight southward extension (approximately 0.3 miles) from 1944 to the current day. This southward extension could be the result of increased sediment supply following the diversion of water from Owens Lake or from other natural or anthropogenic activities.

Olancha Dunes is also the location of an off-highway vehicle (OHV) and dispersed camping recreational area that makes up approximately 36 percent of the total dune area. There is little to no research on the impacts of decades of OHV activity on PM₁₀ emissions at Olancha Dunes, but research from other sites like the Oceano Dunes State Vehicular Recreation Area and the Imperial Sand Dunes Recreation Area shows clear associations between OHV activity, decreasing vegetation cover, and increased dust emissions. Additional study using aerial photography, PI-SWERL dust emission potential measurements, and the Bureau of Land Management’s records of impacts from recreational activity could provide information on the contribution of recreational activity to PM₁₀ emissions.

The southern side of Owens Lake also hosts multiple alluvial channel/wash systems that deliver and rework sediments from the neighboring Coso and Sierra Nevada ranges. These alluvial channel/wash systems supply sand

and PM₁₀ material, and they can only support low-density vegetation cover, which creates conditions ripe for high PM₁₀ emission. While the replenishment of these alluvial systems is a natural process that has been occurring for millennia, anthropogenic alteration of the flowpaths through the constructed infrastructure may change the amount and distribution of impounded water and sediment and thereby change its potential to contribute to PM₁₀ emissions. Climate change is projected to make extreme precipitation events more frequent and intense, which would more frequently replenish fine sediments in flood deposits that contribute to PM₁₀ emissions.

Potential Sources North of the Lake

Current data indicate a stable shallow groundwater table in the area around Owens Lake. However, there is substantial evidence that areas north of the lake have seen decreases in vegetation cover, which may have contributed to historical dust emission in the OVPA. A groundwater management plan and a number of revegetation projects were implemented in the 1990s to reduce blowing dust in affected areas. If this land is not managed carefully, dust emission from the area north of the lake could increase, especially under changing climate conditions.

Further Research to Establish the Origin and Evolution of OVPA Sources

More chronological research may reduce uncertainties surrounding the origin and evolution of dune fields and flood deposits. Collecting sediment samples across dunes and flood deposits by coring or auguring may be the best way to collect a comprehensive set of data. These methods may illuminate relatively recent processes that occurred after the diversion of water from Owens Lake, construction of berms, and the rerouting of Highway 190.

UTILIZATION OF THE EXCEPTIONAL EVENTS RULE IN THE OVPA

The EPA’s Exceptional Events Rule for high wind dust events can be used to exclude unusual or naturally occurring exceedance event data from consideration in regulatorily significant decisions, as long as other public health protections are met. Nevertheless, the way the Exceptional Events Rule is interpreted and implemented has consequences for a region’s air quality and associated health impacts. Based on current EPA guidance and the

panel’s evaluation of the origin and evolution of off-lake sources, some exceedances from off-lake sources in the OVPA may be considered natural events, as human activity played little or no direct causal role in their occurrences.

Some emissions from off-lake sources in the OVPA may be considered anthropogenically influenced. Since the OVPA has a State Implementation Plan that is over 5 years old, the Exceptional Events Rule requires an assessment and implementation of reasonable controls that could be applied to these anthropogenically influenced sources. It is beyond the charge to the panel to determine what controls are “reasonable” because that includes policy judgments rather than purely scientific assessments.

A thorough consideration of reasonable controls would include attention to resilience of the dust control measure under climate change. These considerations may include accounting for more intense flooding and prolonged drought.

INFORMING DUST CONTROL DECISIONS FOR OFF-LAKE SOURCES

If dust control measures are determined to be necessary and feasible for off-lake sources, implementation will require a systems-level landscape approach that considers cultural resources. Collaboration with local Tribal Nations will improve community investment in restoration efforts and outcomes.

Many areas around the OVPA are extremely dynamic settings, requiring different approaches over space and possible re-treatment over time (e.g., in flood deposits). Nevertheless, the panel found that the establishment and maintenance of vegetation offers the best chance for a natural, self-sustaining protection of the soil surface from wind. Additional methods that will support the eventual establishment of vegetation are expanded upon below.

A number of sand sheets and dune fields (e.g., Keeler and Olancha) are distributed along the eastern and southern shorelines. Efforts to partially stabilize Keeler Dunes using solid natural roughness elements of straw bales have resulted in the successful establishment of native shrub seedlings that are providing seed for additional colonization of the sand. Porous naturally sourced roughness elements such as straw checkerboards have also been used with great success in China to protect highways, rail lines, and villages from encroaching sand. These barriers are inexpensive to build and, although relatively short-lived, may be repaired or renewed as necessary until vegetation has successfully colonized the area.

Fine-grained flood deposits are scattered in topographic lows along the shoreline and within the sand sheets and dunes. These fine-grained deposits are extremely emissive and efforts to control these dust sources could reduce exceedances. Where current deposits of fine-grained material are small in size, they could be covered with unlined layers of gravel or cobbles. This system would allow for rapid infiltration of water into the flood deposits, which hold water very effectively. Such constructions would allow plants to find a hospitable root zone that would provide water and nutrients for growth and reproduction, further stabilizing the surface. Longevity of this dust control measure would depend on the interval between floods and would potentially need to be renewed following a major flood event barring changes to infrastructure or local topography.

Flood deposits around Owens Lake have been affected by both the construction of the highway and a number of upgradient berms. Drainage improvements along the highway with the addition of culverts or the elevation of roadways would reduce future ponding of floodwater and accumulation of material next to the highway leading to PM₁₀ emissions. Highway berms concentrate flow toward a limited number of culverts under the highway, which may contribute to large deposits of fine-grained material after major precipitation events (e.g., the remnants of Hurricane Kay in 2022). Modification of the berm structures, highway culverts, and water harvesting and spreading are large, intensive dust management options, but they have the potential to reduce floodwater velocity and the concentration of fine-grained sediment while enhancing water storage for vegetation establishment. Water harvesting from surface runoff and water spreading from drainage features could be used to encourage shrub growth in the future when climate change may make rainfall less predictable. Increased infiltration along the upper positions of the slope might also augment groundwater elevations such that the capillary fringe might be contacted by established shrub roots. Such a system may also limit erosion of gullies and ravines by reducing the velocity of floodwater.

Additionally, OHV recreation is known to be associated with landscape changes that lead to PM₁₀ exceedances. In the Owens Valley, the Bureau of Land Management manages an OHV recreation and primitive camping area of approximately 400 acres (1.6 km²) or 36 percent of the total area of the Olancha Dunes. Additionally, there have been reports of vehicle recreation on dry backwater lakes causing dust emissions.