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Introduction

Owens Lake, or Patsiata in the local Mono language, is located at the southern end of the Owens Valley and bordered by the Sierra Nevada Mountains to the west, the White-Inyo Mountains to the east, and the Coso Range to the south and east (Figure 1-1). Prior to the 20th century, the lake was a closed-basin saline lake of approximately 100 square miles (Holder 1997; NASEM 2020). The lake was and still is central to the way of life for the Nüümü and Newe (Paiute and Shoshone in English, respectively) who live in Owens Valley, or Payahuunadü, land of the flowing water (Owens Valley Indian Water Commission 2024). Beginning in 1913, water was diverted from the Owens River into the Los Angeles Aqueduct. This diversion eventually shrank Owens Lake to one-third of its former area, leaving a brine pool surrounded by a dry lakebed (NASEM 2020). Once exposed, the lakebed produced large amounts of airborne dust under high winds. This resulted in Owens Lake being one of the largest sources of airborne particulate matter in the United States (EPA 2017; Ono 2006), including particulate matter with an aerodynamic diameter of 10 micrometers or less (PM₁₀). These small particles can penetrate the lungs and can cause or worsen a variety of health problems such as asthma, bronchitis, chronic obstructive pulmonary disease, and respiratory infections (GBUAPCD 2016). In 1987, the U.S. Environmental Protection Agency (EPA) identified the southern Owens Valley (known as the Owens Valley Planning Area, or OVPA) as violating the health-based PM₁₀ National Ambient Air Quality Standard (NAAQS) (GBUAPCD 2016). The area also has been designated by the state of California as being in nonattainment of the corresponding state standards, the California Ambient Air Quality Standard (CAAQS) (NASEM 2020).¹

Over the last 25 years, the Los Angeles Department of Water and Power (LADWP), at the direction of the Great Basin Unified Air Pollution Control District (the District), has implemented dust control measures on the lakebed, defined as the land located below the regulatory shoreline, which was designated at an elevation of 3,600 ft (GBUAPCD 2016). These dust control methods were implemented with the objective of meeting the EPA NAAQS for PM₁₀, as well as the PM₁₀ standards set by the state of California. There have been four approved final State Implementation Plans (SIPs)—1998, 2003, 2008, 2016—and an amendment in 2013, that documented how PM₁₀ emissions would be reduced to comply with the NAAQS (GBUAPCD 1998, 2003, 2008, 2013, 2016). As of this publication, LADWP has spent approximately $2.5 billion on their dust mitigation program at Owens Lake and implemented controls on 49.18 square miles (including 1.21 square miles of deferred areas) with a

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¹ Attainment of the National Ambient Air Quality Standard (NAAQS) has precedence over attainment of the California Ambient Air Quality Standard (CAAQS). Unlike NAAQS, CAAQS does not require that standards be met by specified dates. Instead, the law requires incremental progress toward attainment (CARB 2024).

potential maximum buildout extent of 53.4 square miles (Figure 1-2) (LADWP 2024b). These controls will be maintained in perpetuity. The District has also implemented approximately 140 acres of dust control measures on off-lake areas (above the 3,600-ft-elevation regulatory shoreline) at the Keeler Dunes following a $10 million public benefit contribution from LADWP and $2.6 million from District funds (GBUAPCD 2013) (Grace Holder, GBUAPCD, personal communication, May 2024). Together, these controls have made substantial progress toward reducing the frequency and intensity of exceedances of the PM₁₀ standard, currently monitored at 9 locations surrounding the lakebed (Figures 1-2 and 1-3). Despite these efforts, both on-lake and off-lake sources continue to cause PM₁₀ exceedances.

STATEMENT OF TASK AND APPROACH

The Owens Lake Scientific Advisory Panel (OLSAP) was established following the 2014 stipulated judgment between the District and LADWP. The goal of the panel is to foster understanding and collaboration on the scientific and technical approaches to dust control. The panel’s first report (NASEM 2020) focused on the effectiveness of alternative dust control methods for on-lake sources of PM₁₀. Since that report, the OVPA has seen continued exceedances that have prevented attainment with the PM₁₀ NAAQS, which requires no more than one expected exceedance per year at each monitor, averaged over 3 years (40 C.F.R. § 50.6[a] 2023) (see also Chapter 3). As shown in Figure 1-3, these continued exceedances are increasingly dominated by off-lake sources. In 2022, the District sent a letter to LADWP and the National Academies of Sciences, Engineering, and Medicine requesting that OLSAP examine and evaluate PM₁₀ emissions from off-lake sources. The full statement of task is found in Box 1-1.

Given the focus of the statement of task on the PM₁₀ NAAQS and not the CAAQS, the panel focused much of its analysis on attainment of the NAAQS. However, the panel recognizes that attainment of the CAAQS is an eventual goal. Additionally, while both on-lake and off-lake sources can and do contribute to varying degrees to PM₁₀ exceedances on individual monitored exceedance days, the statement of task solely focuses on off-lake sources (i.e., those that are above the 3,600-ft-elevation regulatory shoreline). Although the District emphasized that the primary focus of the statement of task is local off-lake sources that are primarily within 2–3 km of Owens Lake (Table 1-1),² the panel also considered other sources in the Owens Valley Planning Area “for which available data indicate [as] potentially significant contributors to high PM₁₀ concentrations in the Owens Valley Planning Area (OVPA).” Additionally, the panel considered potential future sources as the task mandates that the panel consider “changes in off-lake dust sources over time” (Box 1-1). The District also noted that windblown sources of dust are the focus of the task, not local permitted sources of air pollution (e.g., landfills, mines) nor other local sources of dust from mechanical and combustion processes (e.g., woodburning devices, transportation, unpaved roads, paved road dust, tailpipe emissions, prescribed burning) (Table 1-1). Similarly, regional dust events originating from outside of the OVPA and wildfire smoke events are not the main focus of the task, although these ancillary sources of PM₁₀ may be relevant to the discussion, as they can contribute to exceedances in the OVPA (Table 1-1). Finally, as suggested by the statement of task, the panel only considered the potential application of the Exceptional Events Rule (40 C.F.R. § 50.14) to high wind dust events and not for wildfire smoke events.

OLSAP used numerous resources, including published literature, regulatory documents, presentations, and public comments in its information gathering. The panel held several information-gathering meetings, including a 2-day open session at Owens Lake May 29–30, 2024, where several science and policy experts, Tribal representatives, and environmental organizations presented to the panel. Another open information gathering session occurred in Los Angeles on July 30, which included presentations from experts on the hydrology, climatology, and emissions of Owens Valley. Another open information gathering session occurred on October 7 and focused on exceptional events in Imperial County, Clark County, and the South Coast Air Quality Management District. Virtual open

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² The focus on 2–3 km from the regulatory shoreline follows the 2016 State Implementation Plan (GBUAPCD 2016) that was based on an analysis by the Maricopa Association of Governments, which found source impacts decay by about a factor of “10 between 0 and 500 meters, between 500 and 2,800 meters, and between 2,800 and 30,000 meters.” This analysis is likely valid for smaller source areas, but may not be for more disperse sources, or when those emissions are channeled down the valley.

sessions were held on July 12, July 26, September 6, and December 16, 2024, to investigate air quality data, strategies for engaging with Tribal communities, and dispersion modeling results in more detail.

In a few cases, the panel completed original data analyses. The results of these analyses were replicated by fellow panel members to verify accuracy. This involved multiple panel members and project staff replicating the classification of data in the District’s exceedance database, and the panel also performed a comparative analysis of emission fluxes, including derivation of equations and code, shown in Appendix A.

OUTLINE OF REPORT

The report is organized into six chapters. Chapter 2 discusses the system-level context for off-lake PM₁₀ sources. Chapter 3 explores the sources of PM₁₀ exceedances in the OVPA and provides recommendations on how to improve monitoring and modeling. Chapter 4 provides several conclusions on the origin and evolution of

off-lake sources and recommendations for how to advance this research. Chapter 5 discusses the applicability of the Exceptional Events Rule to exceedances from off-lake sources, and Chapter 6 is a solution-focused exploration of potential dust control options that could be applied, if deemed necessary.