Empirical Demonstrations and MTCA

In 2001, one of the changes made to the Model Toxics Control Act (MTCA) dealt with empirical demonstrations for contaminated site cleanups.  The revised law added 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, an exceedance of the applicable groundwater cleanup levels. 


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

MTCA’s requirements in WAC 173-340-747(9) state that:

  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:

  1. Enough time has elapsed for hazardous substances to migrate from soil to groundwater, and
  2. The characteristics of the site (such as depth to groundwater and infiltration) are representative of future site conditions.

Demonstrations may include a measurement or calculation of the attenuating capacity of soil between the source of the hazardous substance and the groundwater table using site-specific data.


Is guidance available to help prepare empirical demonstrations?

Yes. In June 2016, the Toxics Cleanup Program (TCP) finalized guidance in an FAQ format. These “frequently asked questions” can help stakeholders and potentially liable persons (PLPs) provide sufficient information to Ecology when preparing an empirical demonstration request. This collaborative effort of TCP’s Northwest Regional Office and Headquarters is available on Ecology’s Publications site: Frequently Asked Questions (FAQ’s) Regarding Empirical Demonstrations and Related Issues (Implementation Memorandum No. 15), Publication No. 16-09-047 

Ecology also prepared a response to the comments received during the public review period. 


How can I stay informed about this topic?

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