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BUSTR Tier 2 Evaluation - Site-Specific Target Levels and Pathway Analysis

BUSTR Tier 2 evaluation for petroleum UST sites in Ohio: land use determination, site conceptual exposure model, SSTL development, and POE identification.

Published March 29, 2026 18 min read

Overview

A Tier 2 Evaluation is conducted when COC concentrations at a BUSTR petroleum UST site exceed the Tier 1 action levels and the owner/operator wants to develop site-specific target levels (SSTLs) rather than immediately remediating to Tier 1 values. The Tier 2 process allows default assumptions used in Tier 1 to be replaced with site-specific data, potentially resulting in higher (less restrictive) cleanup targets.

The Tier 2 Evaluation is conducted under OAC 1301:7-9-13 and the 2022 BUSTR TGM. The Tier 2 Evaluation report must be submitted within 18 months of the approval of the Tier 1 Investigation report.

When Tier 2 Is Required

A Tier 2 Evaluation is one of three options (along with an IRA or RAP) available after the Tier 1 Investigation report is approved and COC concentrations remain above applicable action levels. Tier 2 is typically chosen when:

  • The site has favorable conditions that differ from Tier 1 default assumptions (coarser soils, deeper groundwater, non-residential land use, non-drinking water groundwater)
  • The consultant believes site-specific modeling will demonstrate that COC concentrations are protective despite exceeding Tier 1 generic action levels
  • The cost of developing SSTLs is less than the cost of remediating to Tier 1 action levels

Delineation to Tier 1 Action Levels

Before the Tier 2 analysis begins, the distribution of COCs must be defined to the lowest applicable Tier 1 action level for each environmental medium. This may require additional SB/MWs beyond what was installed during Tier 1 Delineation, because Tier 1 delineation uses delineation levels (which are generally higher than action levels).

If COCs have migrated off-site above action levels, the owner/operator must attempt to gain off-site access. Contact affected property owners at least three times within 90 days. Submit denial documentation to BUSTR within 45 days.

Land Use Determination

Tier 1 assumes residential land use everywhere. In Tier 2, the land use may be reclassified as non-residential if either of the following is true:

The 75% Rule

The UST site is non-residential AND 75% of the area within 300 ft of the UST site’s property boundary is non-residential. The area of the UST site itself is not included in the calculation. Roadways within the 300 ft area must be divided in half - each half is counted toward the land use of the adjoining property.

Land Use Restriction

A land use restriction approved by BUSTR has been recorded in the county, or the owner/operator has entered into an environmental covenant with BUSTR restricting the land use.

What Counts as Residential vs. Non-Residential

Residential: Housing (single and multi-family), educational facilities, day care facilities, agricultural land, prisons, long-term care facilities. Assumes adults and children are full-time residents subject to all exposure pathways.

Non-residential: Commercial and industrial properties, warehouses, gas stations, office buildings, retail, hospitals, religious institutions, hotels, fire departments, parks, golf courses, parking facilities. Assumes adult workers spend a typical work week on the property.

Land Use Survey

A survey of all properties within 300 ft of the UST site is required to establish non-residential land use. Send the Land Use Survey form letter (TGM Appendix D) to all property owners. If less than 75% respond or results are inconclusive, BUSTR may require additional methods (phone calls, door-to-door visits).

Site Conceptual Exposure Model

A site conceptual exposure model (SCEM) must be developed during the Tier 2 Evaluation. The SCEM describes the points and pathways through which exposure to COCs may occur. It must evaluate:

Receptors:

  • Adults and children (residential scenarios)
  • Adults only (non-residential scenarios)
  • Excavation workers
  • Aquatic life and recreational receptors in surface water within 300 ft

Environmental media:

  • Soil, groundwater, surface water, indoor air, outdoor air

Fate and transport mechanisms:

  • Atmospheric dispersion, volatilization, enclosed space vapor accumulation, soil leaching, groundwater transport

Routes of exposure:

  • Ingestion, inhalation, direct contact

Pathway Completeness

An exposure pathway can be determined incomplete (and therefore eliminated from evaluation) when:

  • There is no point of exposure (POE) for a COC in the identified media
  • Site-specific data demonstrates no transport mechanism exists to move COCs from source to POE
  • Site-specific data demonstrates no routes of exposure exist for the identified receptor
  • POEs are eliminated by groundwater or land use restrictions enforceable through an environmental covenant with BUSTR

Engineering controls (slurry walls, capping, vapor barriers, point-of-use water treatment) can also eliminate pathways, but must be designed through a Remedial Action Plan and maintained through an environmental covenant.

Points of Exposure

The POE is the point at which a receptor may be exposed to COCs. The POE closest to the source area is used in the evaluation. POEs differ by pathway:

Groundwater Ingestion and Soil Leaching POE

Determined by the following hierarchy (use whichever is closest to the source area):

  1. Any potable well on the UST site
  2. The property line when a potable well exists within 300 ft
  3. The property line when the site is in a DWSPA
  4. The DWSPA boundary if a DWSPA is within 300 ft
  5. The property line, unless specific conditions are met (mandatory municipal tie-in ordinance, well prohibition ordinance, or 100% municipal water connection within 300 ft)
  6. If a roadway separates the source from a property where a well could be installed, the POE extends across the roadway to that property’s boundary
  7. Surface water body within 300 ft (POE is where groundwater discharges to the surface water)
  8. If no other POE applies: 300 ft from the source area, or an alternate POE approved by BUSTR

Direct Contact POE

  • Residential: 0-10 feet below ground surface
  • Non-residential: 0-2 feet below ground surface
  • Excavation worker: evaluated case-by-case

Indoor Air POE

  • Residential or other buildings located or anticipated to be located above soil or groundwater containing COCs
  • Subsurface structures (utility manholes, tunnels) directly above COC concentrations

Developing Site-Specific Target Levels

SSTLs are calculated using the BUSTR spreadsheets by replacing default values with site-specific data. The following parameters may be adjusted with supporting documentation:

  • Total porosity in vadose zone
  • Volumetric water content in vadose zone
  • Volumetric air content in vadose zone
  • Volumetric air and water content in capillary fringe soil
  • Fraction organic carbon
  • Dry bulk density
  • Saturated hydraulic conductivity (vertical)

Other values determined during investigation (depth to groundwater, soil type, building type, land use classification) may also be changed with supporting documentation. Exposure parameters can only be changed from residential to non-residential based on the land use determination.

All field methods and calculations used to obtain site-specific values must be clearly documented. Once site-specific data is obtained, it must be used for all future SSTL calculations.

Important limitations:

  • Direct contact and TPH action levels cannot be recalculated using site-specific data. If these pathways are exceeded, an IRA, RAP, or Tier 3 Evaluation is required.
  • Building type (residential or non-residential) must match the land use determination in the spreadsheets.

BUSTR-Screen Fate and Transport Modeling

BUSTR-Screen is an Excel-based groundwater fate and transport model that predicts COC concentrations at each POE. It combines a user interface with BIOSCREEN (a USEPA model based on Domenico fate and transport equations). BUSTR requires that BUSTR-Screen be used if groundwater fate and transport modeling is performed during the Tier 2 process.

BUSTR-Screen predicts natural attenuation of dissolved hydrocarbons in confined or unconfined aquifers with horizontal flow and constant seepage velocity. The model can be downloaded from the State Fire Marshal website.

SSTLs can also be developed using a combination of BUSTR spreadsheets and BUSTR-Screen (back-calculation approach). For example, BUSTR-Screen can predict the maximum acceptable groundwater concentration in the source area that would be protective of the POE, and that concentration can then be used as the action level in the soil leaching spreadsheet.

Free Product and Tier 2 Modeling

If free product is present at the site, the Tier 2 Evaluation report must still be submitted within 18 months of Tier 1 approval. However, BUSTR will not evaluate groundwater-related pathways (groundwater ingestion, groundwater to indoor air, soil leaching to drinking water) until free product has been recovered to the maximum extent practicable.

The report must identify which pathways are affected by free product. After free product recovery terminates, all remaining pathways must be re-evaluated and a Tier 2 Evaluation Addendum submitted.

Surface Water Quality Standards

If a surface water body is within 300 ft of the UST site, determine whether it is used as a public water supply by checking OAC 3745-1-08 through 3745-1-30 on the Ohio EPA Surface Water Standards Program website.

  • If the water body is used for public water supply and the intake is within 1,500 ft of the POE: use public water supply standards from OAC 3745-1-33
  • If not used for public water supply (or intake is beyond 1,500 ft): use human health fish consumption standards from OAC 3745-1-34

BUSTR considers the POE to be where groundwater discharges to the surface water body, prior to mixing. Additional mixing calculations are not allowed.

Reporting

Submit a completed Tier 2 Evaluation Form 2022 within 18 months from the approval of the Tier 1 Investigation report. The form provides an outline for the report.

For Analytical Groups 4 or 5, a multiple chemical adjustment (MCA) must be conducted when 10 or more non-carcinogenic or 10 or more carcinogenic COCs are present.

Tier 2 Decisions

After BUSTR reviews and approves the Tier 2 Evaluation:

  • All COCs below action levels or SSTLs for all pathways, no monitoring required: NFA status
  • All COCs below SSTLs but fate and transport modeling was used: A monitoring plan may be required to validate modeling results. Submit with the Tier 2 report.
  • One or more COCs above SSTLs: Must conduct one or a combination of:
    • Interim Response Action
    • Remedial Action Plan
    • Tier 3 Evaluation Plan
    • A calibration plan to disprove assumptions used in BUSTR-Screen (submit with Tier 2 report)

Source

BUSTR Technical Guidance Manual (TGM) 2022, Chapter 3: Corrective Action Requirements (Sections 3.11 through 3.12). OAC 1301:7-9-13. TGM Tables 3.4 through 3.9: Action Level Tables.