Risk-based Approach to Cleanups

As part of the process for investigating and cleaning up contaminated sites, ADEQ conducts a risk assessment, which considers the risks associated with the presence and movement of chemicals in the environment. The purpose of this assessment is to evaluate the potential for adverse health effects from exposure to the chemicals. It helps ensure that an appropriate remedy and level of cleanup is implemented.

ADEQ risk assessment activities include:

  • Administering an inter-agency service agreement with the Arizona Department of Health Services (ADHS)
  • Processing risk assessment review requests for ADEQ's Superfund, Voluntary Remediation, Hazardous Waste, Underground Storage Tank and other programs
  • Evaluating chemicals that do not currently have regulatory cleanup standards
  • Educating the public on the principles and use of risk assessment processes

Levels of Cleanup

Options for cleaning up soil and groundwater are based on standards. A standard indicates a level at which a pollutant is considered harmful to human health and the environment.

Tier 1— standards are the most conservative and use pre-determined assumptions (i.e., federal soil remediation levels or aquifer water quality standards) in the risk assessment calculations.
Tier 2 —standards use the same equations to calculate risk as Tier 1 but substitute site-specific information for the predetermined values. The site-specific standard may be a higher concentration than the pre-determined standard, but it also may cost less to attain while also continuing to protect human health and the environment.
Tier 3 —standards offer the most flexibility in assessing risk by allowing use of any methodology or set of equations that are valid and appropriate to the site conditions. A Tier 3 standard calculated at one site may differ considerably from one calculated at another site. More information is required to complete Tier 3 calculations, and this additional information must account for potential exposures outside the property boundaries to verify that human health and the environment will continue to be protected.

Deciding Which Tier to Use

If contaminant concentrations at a site are less than the Tier 1 standards, the site is eligible for closure without further work. However, if contaminant concentrations exceed the Tier 1 standards, an owner may consider using site-specific values to calculate Tier 2 and/or Tier 3 standards. Complying with the tiered corrective action standards demonstrates that the site meets the regulatory requirement that remaining concentrations adequately protect human health and the environment. Using one tier may be more efficient and cost-effective than using another tier. The same process is used to decide between Tier 2 and Tier 3 standards.

Indicators that Tier 2 or Tier 3 standards may be useful may include:

  • Site conditions that are significantly different than those assumed for Tier 1
  • Small changes in the assumptions result in a large change to the calculated standard
  • Site chemical concentrations are close to or less than the estimated Tier 2 or Tier 3 standards
  • Conducting the proper calculations for and cleaning up the site to Tier 2 or Tier 3 concentrations costs less than cleaning up the site to Tier 1 concentrations