4.10 Air Stripping and Soil Vapor Extraction

Category 937

4.10.1 Introduction

This category accounts for organic emissions (TOG and ROG) from air stripping and soil vapor extraction processes. Soil and groundwater contamination are the result of leaking underground storage tanks, accidental spills, and landfill leachate. The air stripping process is commonly used for contaminated groundwater remediation. Contaminated groundwater is pumped into an air-stripping tower. A blower generally exhausts the effluent air stream from the stripping column. The contaminated air stream is then routed to an air pollution control device (i.e., carbon adsorbers). Soil vapor extraction is one of the techniques to extract volatile organic compounds (VOC) from contaminated soil by using a vacuum system. Fresh air is injected into the subsurface at locations around the contaminated area. The contaminated air is withdrawn under vacuum from extraction wells. This contaminated air is then vented directly to a VOC treatment system such as carbon adsorbers, thermal incinerators, catalytic incinerators, and condensers prior to being released to the atmosphere.

4.10.2 Methodology

Point Sources are operations that emit air pollution into the atmosphere at a fixed location within a facility, for which the Air District has issued a permit to operate, e.g., refinery cooling towers. These could also be a collection of similar equipment / sources located across multiple facilities, e.g., reciprocating engines.

During the permit to operate (PTO) issuance process, the BAAQMD collects information from the operating facility and/or determines from published literature, e.g., EPA’s AP-42, characteristics of a source including maximum throughput, emission factors for emitted pollutants, and control factors associated with downstream abatement devices. These characteristics are then stored for future use in the BAAQMD’s internal database. Facilities that hold a permit to operate are required to renew this permit periodically (this period varies based on facility and source type). Upon renewal, the facilities are requested to provide any updates to source characteristics as well as the source throughput for the last 12 months. This throughput, in combination with the emission factors and controls factors stored in the internal database, are used to estimate annual emissions at the source level. These source level emissions are then sorted and aggregated into categories.

The last part of the inventory development process includes forecasting and back casting, and aggregation into sub-sectors and sectors for documentation purposes. For those years where no data is available, emissions data are backcasted to year-1990, as well as forecasted to year-2040 using either interpolation or another mathematical approach (see Trends section). Finally, emissions trends spanning from year 1990-2040 for each category and pollutant are evaluated for anomalies that are then investigated and addressed.

Category 937 is considered a point source category and follows the above methodology for emissions estimates.

The District adopted Regulation 8, Rule 47 on December 20, 1989. This rule requires all facilities to control emissions by at least 90%; however, based on emissions data, the overall control efficiency is estimated at 97%.

The ROG/TOG ratios applied to this category or this group of related categories are based on an Air District internal speciation profile. Multiple data sources have been used for developing speciation profiles, such as Air District-approved source tests, TOG speciation ratios used by other regional air quality agencies, and relevant literature including latest speciation profiles developed by CARB68 and the US Environmental Protection Agency69. For this category or group of categories, ROG constitutes 100% of TOG.

4.10.3 Changes in Methodology

There are no changes to emission estimation methodology in this version of the base year emissions inventory.

4.10.4 Emissions

A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.

4.10.6 Uncertainties

When no specific emission factor is available, a generalized factor is used to determine emissions. Use of generalized emission factors can lead to inaccurate emissions estimate. Minimizing use of generalized emission factors would improve emissions estimates.

4.10.7 Contact

Author: Sukarn Claire

Reviewer: Ariana Husain

Last Updated: November 06, 2023

4.10.8 References & Footnotes

  1. ORGPROF. CARB. 2022. https://ww2.arb.ca.gov/speciation-profiles-used-carb-modeling↩︎

  2. SPECIATE. USEPA. 2022. https://www.epa.gov/air-emissions-modeling/speciate↩︎

  3. The Association of Bay Area Governments (ABAG). [accessed 2022, Dec 21]. https://abag.ca.gov/↩︎