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BUSTR Tier 1 Source Investigation and Delineation

BUSTR Tier 1 source investigation and delineation for petroleum UST releases in Ohio: COC sampling, drinking water determination, and action level comparison.

Published March 29, 2026 20 min read

Overview

When a petroleum release from an underground storage tank (UST) system is confirmed in Ohio - typically when closure assessment results exceed action levels - the site enters the BUSTR corrective action process under OAC 1301:7-9-13. The first step is the Tier 1 Source Investigation and Delineation.

The Tier 1 investigation has two parts. The source investigation determines what COC concentrations exist in the source area. The delineation defines how far the contamination has spread. Both are reported together and must be submitted to BUSTR within one year of the confirmed release.

This guide covers the 2022 corrective action rule (OAC 1301:7-9-13, effective September 1, 2022). Sites with releases reported before that date may elect to use this rule or continue under the previous version.

What Triggers a Tier 1 Investigation

A Tier 1 Source Investigation is required when any of the following occurs:

  • Closure assessment results exceed action levels (confirmed release reported to BUSTR within 24 hours of receiving analytical results)
  • A Site Check is conducted as Option 1 (choosing to investigate rather than close the system)
  • Site Check Options 2 or 3 produce results above action levels
  • The owner/operator elects to transition a pre-2022 release into the current corrective action rule
  • BUSTR transfers a release into the current rule due to missed compliance deadlines

Source Investigation

Identifying Source Areas

All potential sources must be identified, including existing, closed-in-place, or removed USTs, piping systems, and dispensers. Areas of known or suspected surface spills or overfills must also be identified. If the source is unknown, all potential sources on the UST site must be investigated using site plans, personal interviews, fire department records, Sanborn maps, deeds, and other historical records.

A source area is the location of the highest concentrations of COCs in soil or groundwater. There may be multiple source areas at a single site. Areas containing free product must be evaluated as potential source areas.

Subsurface Investigation

Install a minimum of three soil borings/monitoring wells (SB/MWs) within or as close as possible to the source area to determine the highest concentration for each COC. Consider the location of potential sources, the likely distribution of COCs, and temporal variations in concentrations when determining placement.

If a portion of the source area is not accessible, at least one SB/MW must be installed immediately downgradient of the source area. More than three SB/MWs may be necessary to adequately characterize the source area.

During the subsurface investigation, evaluate the following site characteristics:

  • Direction and gradient of groundwater flow (if groundwater is encountered)
  • Faults, fissures, fractures, or geologic transport routes (based on published data from ODNR or USGS)
  • Soil type based on field observations and/or laboratory analysis (soil survey maps are not acceptable)
  • Depth to groundwater
  • Location of man-made structures such as sewer lines, water lines, electrical conduits, or drainage tiles

Chemicals of Concern

COCs are determined by the product stored in the UST system using the same five analytical groups as closure assessments (TGM Table 3.1):

  • Group 1 (light distillates): BTEX, naphthalene, 1,2,4-TMB, MTBE, 8 PAHs, EDB/EDC (conditional), TPH C6-C12
  • Group 2 (middle distillates): BTEX, naphthalene, 1,2,4-TMB, MTBE, 8 PAHs, TPH C10-C20
  • Group 3 (heavy distillates): Benzene, naphthalene, 8 PAHs, chlorinated hydrocarbons (full 8260), TPH C10-C20, TPH C20-C34
  • Group 4 (used oil): Benzene, naphthalene, MTBE, EDB, EDC, 8 PAHs, chlorinated hydrocarbons, all TPH ranges
  • Group 5 (unknown/other): Contact BUSTR for COC determination before sampling

EDB and EDC Requirements for Corrective Action

The EDB/EDC rules for corrective action differ from closure assessment:

  • USTs in service before January 1, 1986: EDB and EDC must be analyzed in all soil and groundwater samples
  • USTs installed between January 1, 1986 and January 1, 1996: EDB and EDC may be eliminated if closure sampling showed them below detection limits (with detection limits below action levels), AND the site is not in a DWSPA, not in a Sole Source Aquifer, and no drinking water sources exist within 1,500 ft. Written BUSTR approval is required before excluding them.
  • USTs installed after January 1, 1996: EDB and EDC are not required
  • Aviation gasoline, racing fuel, and used oil USTs: EDB and EDC are always required regardless of installation date

Groundwater Determination

The saturated zone encountered during investigation is assumed to be groundwater unless testing demonstrates otherwise. The saturated zone is classified as groundwater if it can yield at least 1.5 gallons within 8 hours of purging AND has an in situ hydraulic conductivity greater than 5.0 x 10-6 cm/sec. If either criterion fails, no further evaluation is needed - the zone is not groundwater.

Action Level Determination

For the Tier 1 Source Investigation, determine the appropriate action levels using the following assumptions:

  • Land use: Assume residential
  • Soil class: Classify based on field observations or geotechnical data. Bedrock is classified as Class 1.
  • Depth to groundwater: Use site-specific data. If unknown, assume less than 15 feet.
  • Groundwater: Assumed to be drinking water (unless a determination is made during Tier 1 Delineation)

Compare the highest detected concentration of each COC to the action levels for every applicable pathway. The most restrictive pathway drives the decision - not just one table, but the lowest value across all applicable tables. The applicable pathways depend on the groundwater scenario (drinking water, non-drinking water, or no groundwater).

Action level tables are published on our BUSTR corrective action standards page.

Tier 1 Delineation

After the source investigation, the Tier 1 Delineation defines the extent of contamination and makes the drinking water determination. This is often the most time-consuming and expensive part of the Tier 1 process.

Defining the Extent of Contamination

Install additional SB/MWs to define the horizontal and vertical extent of COCs to at or below the delineation levels (TGM Table 3.3) in all directions. Delineation continues outward from the source until all COC concentrations are below delineation levels.

Delineation levels are not action levels. They are screening values designed to define the investigation boundary. They cannot be revised or adjusted with site-specific data.

Key delineation levels for common COCs (soil / groundwater):

  • Benzene: 1.67 mg/kg / 0.417 mg/L
  • Toluene: 1,240 mg/kg / 217 mg/L
  • Naphthalene: 52.7 mg/kg / 1.68 mg/L
  • MTBE: 150 mg/kg / 134 mg/L
  • Benzo(a)pyrene: 1.2 mg/kg / 18.3 mg/L

BUSTR reserves the right to request delineation of additional COCs not listed in the delineation table, including TPH.

If the plume has migrated off-site, the owner/operator must use best efforts to obtain off-site access. Contact affected property owners at least three times within a 90-day period. Submit off-site access denial documentation to BUSTR within 45 days.

Drinking Water Determination

This is a critical decision point that affects which action levels apply. Groundwater is always considered drinking water if any of the following are true:

  • The UST site or surrounding area (within 1,500 ft) is in a Drinking Water Source Protection Area (DWSPA)
  • The UST site is in a Sole Source Aquifer
  • A surface water body is within 300 ft of the UST system
  • An existing drinking water source (private or public well) is within 1,500 ft of the UST system

If none of the above apply, groundwater may be classified as non-drinking water if any of the following are true:

  • Groundwater yields less than 3 gallons per minute
  • Background total dissolved solids are 3,000 mg/L or greater
  • The site is within an Urban Setting Designation (check Ohio EPA DERR VAP)
  • No potable wells within 300 ft (based on physical survey) AND a local ordinance requires mandatory municipal water tie-in for all properties within 1,500 ft
  • No potable wells within 300 ft AND a local ordinance prohibits installation of potable wells within 1,500 ft
  • No potable wells within 300 ft AND 100% of properties within 300 ft are connected to municipal water or municipal water is readily available

If groundwater cannot be classified as either drinking water or non-drinking water using these criteria, it must be treated as drinking water.

To evaluate drinking water potential, conduct a well log search on the ODNR Division of Water website (waterwells.ohiodnr.gov) for all wells within 1,500 ft of the UST system. The interactive search with the “Points & Radius” buffer tool set to 1,500 feet is the most effective method.

Review each well log to confirm it has been properly geolocated. Also search unlocated wells using the Advanced Search by county and township - these wells are in the database but have not been mapped. If an unlocated well cannot be confirmed to be outside the surrounding area, it is assumed to be within it.

Submit copies of all identified well logs and the ODNR well map to BUSTR, even if no wells are found.

Physical Survey

If trying to eliminate the drinking water pathway, a physical survey of all properties within 300 ft of the UST site property boundaries may be required. This includes sending the Water Well Survey form letter (TGM Appendix D) to all property owners within 300 ft. If less than 75% of surrounding properties respond, BUSTR may require additional survey methods (phone calls, door-to-door visits).

Reporting

Tier 1 Notification Form

If COC concentrations exceed action levels upon completion of the source investigation, submit a Tier 1 Notification Form 2022 to BUSTR within 90 days of the triggering event (date of analytical report, date of election into 2022 rule, etc.).

Tier 1 Evaluation Form

If COC concentrations are at or below action levels, submit a Tier 1 Evaluation Form 2022 within 90 days.

Tier 1 Investigation Report

The full Tier 1 Investigation report (combining source investigation and delineation) must be submitted within one year of the triggering event. This report summarizes all investigation activities, analytical data, the drinking water determination, and the comparison of COCs to applicable action levels for all pathways.

What Happens Next

After BUSTR reviews and approves the Tier 1 Investigation report:

  • All COCs below action levels for all pathways: BUSTR issues NFA status
  • One or more COCs above action levels: The owner/operator must conduct one or a combination of the following:

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

Source

BUSTR Technical Guidance Manual (TGM) 2022, Chapter 3: Corrective Action Requirements. OAC 1301:7-9-13. TGM Table 3.1: Chemicals of Concern. TGM Table 3.3: Delineation Levels.