4.9 Contaminated Soil Aeration
Category 37
4.9.1 Introduction
Category 37 accounts for organic (TOG and ROG) emissions from contaminated soil aeration events. Contaminated soil aeration occurs when soil containing organic compounds is excavated and allowed to vent into the open air. These events include underground gasoline tank removal, soil excavation during pipeline leak repairs, and removal of other organic and petroleum chemical spills.
4.9.2 Methodology
Category 37 is considered an area source category since it covers facilities / emission sources that are not directly permitted by the District, and hence not systematically cataloged. Emissions for area source categories are determined using the formula:
Current Year Emissions = Base Year Emission X Growth Profile, and,
Base Year Emission = Throughput X Control Factor X Emission Factor
where,
- throughput or activity data for applicable base year(s) is determined using a top-down approach (e.g. state-, national-level data);
- emission factor is derived from general literature, specific literature and reports, and/or source testing results provided by Air District staff;
- control factor (if applicable) is determined by District and state rules and regulations in effect;
- and, historical backcasting and forecasting of emissions is based on growth profiles as outlined in the Trends section of this chapter
More details on throughput, county distribution, emission factors and controls is provided in the following subsections.
(a) Activity Data / Throughput
The District requires a written report for any soil aeration activity. These reports were evaluated to estimate the total number of excavations.
On average, an estimated 250 excavation events take place annually in the San Francisco Bay Area.
(b) County Distribution / Fractions
The county activity distribution is based on population of each county within the Bay Area Air District boundary.
(c) Emission Factors
Soil aeration reports were evaluated to determine a generalized emission factor for each event as well as to estimate the total number of excavations. An emission factor of 2500 lbs of organic compound emissions per soil excavation event is used for this category.
District Rule 8, Regulation 40 allows up to 150 pounds per day of organic compound emissions per aeration event. The organic compound emissions factor was calculated assuming the aeration site starts emitting at 120 pounds per day, with emissions halving every ten days, and with emissions set at zero at the 90th day. Two tank removals or replacements are also assumed to occur per aeration event.
(d) Control Factors
The BAAQMD adopted Rule 8, Regulation 40 on February 15, 198962. The overall control afforded by this rule is 81%. This rule applies for soil aeration events only. Air stripping and soil excavation events are inventoried in category 937, under Rule 8, Regulation 47.
Rule 8, Regulation 40 sets the rate of uncontrolled aeration in addition to describing an acceptable soil aeration procedure. For an uncontrolled aeration, an operator may not aerate the soil at above certain rate per day; however, the soil aeration may be extended over many days. The soil aeration rate limit controls quantity of emissions per day but this does not reduce the total emissions from an aeration event.
(e) Speciation
The ROG/TOG ratios applied to this category or this group of related categories are based on the California Air Resources Board (CARB) Organic Speciation Profile number 600, “Species Unknown- All Category Composite”63. For this category or group of categories, ROG constitutes 100% of TOG.
The PM2.5/PM and the PM10/PM ratios applied to this category or this group of related categories are consistent with size fractions of speciation profiles developed by the California Air Resources Board (CARB) and published on their emissions inventory web-page64. For this category(s), CARB’s speciation profile number is 200; PM2.5 constitutes 92.5% of total PM and PM10 constitutes 96% of total PM.
(f) Sample Calculations
The following example shows TOG emission calculation for the Alameda county for 2015:
TOG Emissions (tons / day) = Throughput (Excavation Events/ year) x Emission Factor (lbs/ Excavation Events) x Almeda County Fraction x Growth Factor x Control Factor x (ton/ lbs)
= 269 x 2500 x 0.2202 x 1.0 x 0.19 x 1/2000 = 14.07 tons/ year
4.9.3 Changes in Methodology
There are no changes to emission estimation methodology for this category compared to previous base year emissions inventories.
4.9.4 Emissions
A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.
4.9.5 Trends
In 1990, the number of excavation events per year in the Bay Area totaled approximately 1254. The number of excavations per year remained at this level through 1998 and declined afterwards to an average of about 250 events annually. The TOG emissions follow the soil excavation activity trend based on an average population growth in the San Francisco Bay Area.
(a) Historical Emissions / History
State regulations governing the construction standards of gasoline holding tanks required many gas stations to replace their existing tanks. In the Source Inventory of 1987, the District had 152 excavation events reported. In 1990, this number had grown to 1254. The number of excavations remained above 1100 per year through the mid-1990s.
Much of the increase since 1987 had been due to the Porter and Cologne Act regulated by the California State Water Resources Control Board65 and California Regional Water Quality Control Boards. Parts of these underground tank regulations govern the replacement of underground storage tanks.
The California Code of Regulations, Title 23, Division 3, Chapter 1666 stated that by December 22, 1998, all underground storage tanks containing hazardous substances, other than those containing motor vehicle fuels, need to be retrofitted with secondary containment. By the same date, motor vehicle fuel tanks made of steel needed to be retrofitted with secondary containment or be provided with both interior lining and exterior cathodic protection.
The number of excavation events remained at close to the 1990 rate through 1998 and declined afterwards from 1254 to 250 by year 2005.
(b) Future Projections / Growth
Future growth for Category 37 is expected to follow average population growth in the San Francisco Bay Area. Population growth projections are in accordance with the Association of Bay Area Governments (ABAG)67.
4.9.6 Uncertainties
Uncertainties associated with the emission estimates from soil excavation events can be due to inaccurate process rate data available and generic emission factors used for all excavation events. The accuracy of emission estimates depends upon:
Accurate reporting of process rate data – such as, size and quantity of the soil excavation events during the year.
Use of specific emission factors for each soil excavation event – pounds of pollutant emitted per excavation event.
Therefore, ability to collect accurate process rate data and the use of site-specific source test data for emission factor development, rather than generic emission factors for all events, would improve the emissions estimate.
4.9.7 Contact
Author: Sukarn Claire
Reviewer: Ariana Husain
Last Update: November 06, 2023
4.9.8 References & Footnotes
BAAQMD. 1989. Rule 8, Regulation 40. https://www.baaqmd.gov/~/media/dotgov/files/rules/reg-8-rule-40-aeration-of-contaminated-soil-and-removal-of-underground-storage-tanks/documents/rg0840.pdf?la=en↩︎
ORGPROF. CARB 2022. https://ww2.arb.ca.gov/speciation-profiles-used-carb-modeling↩︎
PMSIZE. CARB 2022. https://ww2.arb.ca.gov/speciation-profiles-used-carb-modeling↩︎
The California State Water Resources Control Board. 2021. https://www.waterboards.ca.gov/ ↩︎
The California Code of Regulations. Title 23, Division 3, Chapter 16. 2021. https://www.waterboards.ca.gov/ust/regulatory/docs/CCR_Ch16_10_2018.pdf↩︎
The Association of Bay Area Government (ABAG). [accessed 2021 Nov 21]. https://abag.ca.gov/↩︎