When contamination occurs, it is important to determine whether it presents a human health risk, such as:
- Contaminated soils or groundwater releasing harmful vapors into the air inside living or working spaces
- Contaminated groundwater polluting a drinking water well
- Contaminated rivers and lakes impacting drinking water systems and affecting recreationists
The risk evaluation process is used to characterize the nature and extent of health risks to humans and ecological receptors from chemical contaminants and other factors that may be present in the environment.
Risk Evaluation for Petroleum Releases
DEQ uses EPA’s Regional Screening Level (RSL) and Vapor Intrusion Screening Level (VISL) calculators to evaluate whether contaminated soil, groundwater, or surface water at a particular location poses a human health risk. The Risk Evaluation Manual for Petroleum Releases helps to evaluate whether a petroleum release investigation or cleanup is needed and, if so, its scope and nature.
The manual provides a consistent method for addressing contamination and describes the risk evaluation process:
The following tools assist in evaluating risk:
- Mann-Kendall Calculator: A tool to conduct a Mann-Kendall analysis of trends for up to five chemicals and 10 sampling events.
- EPA ProUCL Calculator: A tool to calculate the 95% upper confidence limit (UCL) of the mean for chemical concentration data. The utility calculates the 95% UCL for normal and log-normal distributed sample data, as well as a bootstrap estimate.
Updates to the Risk Evaluation Manual for Petroleum Releases
DEQ is currently revising the Idaho Risk Evaluation Manual for Petroleum Releases. The public is invited to provide input on the draft manual with revisions through November 6, 2026:
Submit comments on the draft manual revisions to:
Kristi Lowder
UST/LUST Program Manager
(208) 373-0347
kristi.lowder@deq.idaho.gov
DEQ hosted a revision meeting on October 22, 2026, to gather public input. The revision meeting presentation slides are available as an additional resource to learn about the proposed manual revisions.
- Revision Meeting Presentation Slides – October 22, 2026
Nonpetroleum Contamination
The EPA Regional Screening Levels and associated risk calculator or another DEQ-approved risk assessment approach can be used to determine risk-based criteria for nonpetroleum chemicals of concern, such as metals and solvents.