Idaho risk evaluation

The Idaho Department of Environmental Quality (DEQ) says its remediation programs use a site-specific and risk-based approach to investigate and clean up contamination. For petroleum releases DEQ says the decision is guided by a rule, the Standards and Procedures for Application of Risk-Based Corrective Action at Petroleum Release Sites (IDAPA 58.01.24), and by its accompanying guidance, the Idaho Risk Evaluation Manual for Petroleum Releases. It is not a list of sites.

In a Phase I ESA

When an Idaho record says a site needed no cleanup, or was cleaned up, the next question is what that decision was measured against. DEQ's pages give the outline.

• The answer depends on the site. DEQ says the approach depends on the type and extent of contamination, the geology and hydrology at the site, the current and likely future uses of the site, and other relevant factors. A decision made for one use may not hold for another, so compare the use assumed in the file with the use planned.
• Petroleum releases follow the rule and the manual. DEQ says the rule and the manual outline standards and procedures to determine whether risk-based corrective action measures should be applied, and which ones, at properties where petroleum releases have occurred. DEQ says it uses EPA's Regional Screening Level and Vapor Intrusion Screening Level calculators to evaluate whether contaminated soil, groundwater or surface water at a location poses a human health risk.
• Other chemicals use EPA screening levels or an approved method. For nonpetroleum chemicals of concern such as metals and solvents, DEQ says the EPA Regional Screening Levels and the associated risk calculator, or another DEQ-approved risk assessment approach, can be used to determine risk-based criteria.
• Contamination can be left in place. DEQ says the use of environmental covenants recognizes that not all contamination needs to be cleaned up, provided the risks from the residual contamination are adequately addressed. When a file shows a risk-based decision, check for an environmental covenant.
• DEQ reviews the data it is given. DEQ says it evaluates data submitted by external parties to determine that the data are of sufficient quantity, type and quality before they are used to make decisions. It links its quality assurance plans and standard operating procedures for tank closures, petroleum site assessments and corrective action.

DEQ names two tools that assist in evaluating risk: a Mann-Kendall calculator for trend analysis, and EPA's ProUCL calculator for the 95% upper confidence limit of the mean. When a file relies on either, check that it shows the inputs.

Where the programs use it

DEQ says the need for remediation at a Leaking Underground Storage Tank site is guided by IDAPA 58.01.24. It lists the use of site-specific risk-based cleanup standards among the benefits of the Voluntary Cleanup Program. It says the current property owner is responsible for cleaning up leaks from a residential heating oil tank under IDAPA 58.01.02 and 58.01.24.

Where to read the rule and the manual

• DEQ's Risk Evaluation page links the rule, the manual and the calculators.
• DEQ's Sample Collection page links the documents on how DEQ evaluates site assessment and corrective action data.

Sources

Idaho Department of Environmental Quality pages, read October 1, 2026:

• Risk Evaluation, last modified January 7, 2026.
• Sampling, Investigation, and Cleanup, last modified April 14, 2026.
• Sample Collection, last modified January 7, 2026.
• Leaking Underground Storage Tanks in Idaho, last modified January 8, 2026.
• Environmental Covenant, last modified January 7, 2026.
• Voluntary Cleanup Program, last modified January 7, 2026.
• Regulated Underground Storage Tanks, last modified August 6, 2026.