The overall goal of the Air Quality Mitigation Program is to keep playa emissions at low levels, even as playa exposure accelerates, through implementation of targeted, proactive dust control measures on priority playa areas. This approach provides flexibility for implementing effective dust control measures in the most cost-efficient manner and for facilitating dust control actions at the Salton Sea. The annual Proactive Dust Control Plan describes recommendations for site-specific dust control measures based on rigorous evaluation of the surface characteristics and emissions potential. This evaluation considers soil suitability, the availability of water resources, other planned stakeholder projects, and more.
The primary dust control measures recommended are surface roughening and vegetation. Surface roughening is recognized around the world as an effective dust control measure on exposed surfaces. It provides quick, waterless, and effective control. Vegetation is also widely recognized as an effective dust control measure, but requires irrigation for establishment. A series of plot and field studies inform how to tailor these dust control measures to Salton Sea soils and climate.
Surface Roughening with Irrigated Vegetation
Planning for implementation of dust control measures requires soil information, particularly texture and salinity. Due to the expansive area of exposed playa, a high-throughput method was developed to characterize soil conditions. Methods include an electromagnetic survey (EMI) to select soil coring locations, soil coring, and documentation and analysis of the soil cores. To date, soil conditions on over 7,500 acres of exposed playa have been characterized.
The playa is mapped with an EMI sensor to reveal sub-surface variability and inform soil core sampling locations. Over 1,140 soil cores have been collected with a Giddings soil core probe. Each soil core is characterized for soil texture, soil carbon, and soil moisture.
A high-resolution photo is taken using a custom-built photogrammetry workbench, including an automated camera station to take pictures every two inches. Individual photos are then stitched together to create a continuous, high-resolution soil core photo.
Spectroradiometer readings are collected at two-inch intervals along the core to inform soil texture, including percentage of sand and clay. Sub- samples of the cores are also taken at intervals that represented a single textural class and a single spectroradiometric measurement.
Water availability is another main driver of dust control measure suitability because vegetation requires irrigation for establishment. Surface water sources exist at the northern and southern areas of the Salton Sea, including agricultural drains, the Whitewater River (north), and the New and Alamo Rivers (south). Water supply from these agricultural drains and rivers supports stands of naturally-established vegetation. In the eastern and western areas of the Salton Sea, surface water flow occurs only with highly variable precipitation and is not a reliable source for vegetation establishment.
Vegetation establishment is challenging due to high salinity in playa soils, limited water availability for irrigation, and the desert climate. Results from a series of plot-based studies provide key insights into effective vegetation establishment at the Salton Sea. Key vegetation establishment strategies developed for the Salton Sea Air Quality Mitigation Program include Species Selection, Cultivation Strategies, and Irrigation.
Iodine bush is the primary species used due to its high salinity tolerance. Other species, notably saltbush species and greasewood, may play a future role in more upslope plantings due to their greater rooting depth relative to iodine bush.
Cultivation strategies increase the chances of successful plant establishment. Key strategies include proper siting relative to depth to groundwater, seed treatments to increase germination, irrigation to reduce soil salinity and support establishment, soil amendments to increase plant growth rate, and prevention of sand burial. Sand burial prevention and irrigation are critical to successful vegetation establishment.
The use of flooded furrows with seed spread onto the surface is effective for rapid vegetation establishment and works for most species, including iodine bush. However, the resulting hedgerow may contain some gaps and require infill plantings to enhance dust control. Scheduling seeding to occur after the high-wind months of March through May can reduce the amount of seed dispersion and reduce sand burial.
Plantings seeds in flooded furrow
In early 2017, a series of field scale pilot studies were implemented on approximately 800 acres around the Sea. Sites were selected based on known, priority dust source areas identified during the 2015/2016 dust season. The type and intensity of dust control measures were selected based on site specific characteristics, including emissions potential, topography, and soils data. The main planning and design steps are described below. Surface roughening occurred where soils have a higher silt and clay content and can produce larger, more stable clods resistant to erosion. In areas with medium-to-coarse soils, furrows or constructed beach ridges were seeded with native species. Dust control effectiveness is monitored with sand motion monitors, including Sensits and Cox Sand Catchers.
Step 1. Source areas were identified based on the evaluation of playa surface characteristics and emission potential, including a surface survey, visual observations and PI-SWERL sampling. The surface survey characterized several parameters, including salt crust type and thickness, soil moisture, crust relief, penetration resistance, and the percentage of free surface sand. PI-SWERL data were collected on an elevation profile that represents playa exposed for less than one year to greater than 5 years. As shown in the graph, PI-SWERL results are linked to the duration of exposure. Shorter exposure durations have a lower emissions potential than longer exposure durations because they have a higher soil moisture content.
Step 2. A soil survey was conducted to evaluate soil suitability for either surface roughening or vegetation enhancement. Methods included an electromagnetic induction (EMI) survey to map spatial soil variability, followed by the collection and analysis of soil core samples to correlate the EMI survey data to measured soil properties. A DUALEM421 meter was used to induce an electrical current into the subsurface and determine its conductivity. The DUALEM421 measures conductivity profiles from 0.5 meters and 6.4 meters in depth with overlap between the profiles. The DUALEM421 was towed behind an ATV and logged readings on a one second interval. Soil core locations were identified based on the EMI survey data.
Step 3. Soil cores analysis included photo documentation, spectral analysis, description and sub-sampling. After photo documentation, the cores were scanned with a spectroradiometer and described for horizons, color, structure, and oxidation status. Sub-samples were analyzed for particle size, soil fertility, soil quality, and salinity. These data were used to inform soil suitability for either surface roughening or vegetation enhancement.
Step 4. The site design is based on emissions, soils, and topographic data, as well as the results from SWEEP modeling. The Single-event Wind Erosion Evaluation Program, or SWEEP, is used to model potential sand motion, PM10 emissions, and dust control efficiency as a function of soil, surface, and unique design elements (roughness, vegetation barrier spacing, soil wetness patterns, etc.). SWEEP modeling results informed the design spacing for each furrow set. Topographic data informed design of the irrigation system, which is necessary to reclaim the soils and support vegetation establishment. Sand motion monitoring equipment will include sand catches and Sensits.