Empirical demonstrations under MTCA

Empirical demonstrations and cleanup

Empirical demonstrations rely on observations and practical experience to verify that a hypothesis is correct.

In 2001, the Legislature made changes to the state’s cleanup law, the Model Toxics Control Act (MTCA), that give the option to use site-specific soil and groundwater data to empirically demonstrate that soil concentrations measured at a site have not caused — and will not cause — the applicable groundwater cleanup level to be exceeded.

MTCA’s cleanup rule outlines the standards and other requirements that cleanups must meet, including cleanup levels that protect people’s health and the environment.

What are MTCA’s requirements for performing empirical demonstrations at a cleanup site?

The MTCA Cleanup Regulation (WAC 173-340-747(9)) states:

1. The measured groundwater concentrations must be less than or equal to the applicable groundwater cleanup levels; and
2. The measured soil concentrations will not cause an exceedance of the applicable groundwater cleanup levels at any time in the future. This requires applicants to demonstrate that:

• Enough time has elapsed for hazardous substances to migrate from soil to groundwater; and
• The characteristics of the site (such as depth to groundwater and infiltration) are representative of future site conditions. Demonstrations may include measuring or calculating the attenuating capacity of soil between the source of the hazardous substance and the groundwater table, using site-specific data.

What guidance can help me prepare an empirical demonstration?

• FAQs regarding empirical demonstrations and related issues (2016). This implementation memo can be used at any cleanup site that meets the requirements for empirical demonstrations, including Ecology-supervised sites and independent cleanup sites.
• Response to comments (2016) was prepared after the FAQs were publicly reviewed and provide more background.

How can I stay informed about empirical demonstrations?

Get on our email distribution list by contacting Mark Gordon at mark.gordon@ecy.wa.gov or 360-407-6357.

We also use this list for other technical updates such as model remedies and vapor intrusion. If you're already receiving those technical updates from Mark, you're already on the list, and will periodically receive empirical demonstration updates, too.