(1) Before permanent closure or a change in service is completed, owners and operators must measure for the presence of a release where contamination is most likely to be present at the UST site. When measuring for the presence of a release, owners and operators must:

(a) Collect soil samples, as soon as possible after the tank, piping, or both have been removed, at the base of the tank excavation and piping trench at suspected worst-case locations, which locations may include:

(i) areas around the tank and piping that record the highest concentrations of hydrocarbon vapor recorded with vapor monitoring instruments;

(ii) areas around the tank and piping that look stained or discolored;

(iii) the lowest point of the tank;

(iv) where the tank meets the piping; and

(v) beneath the fill lines. For tank removal, at least two soil samples, one from either end of the tank or at suspected worst-case locations, shall be taken at least one to two feet below the base of the maximum excavation depth for each tank over 600 gallons being closed. One soil sample shall be collected beneath tanks with a capacity of 600 gallons or less. If contaminated soil is removed from the excavation site, at least one composite sample of the contaminated soil shall be collected for analysis. For piping removal, soil samples shall be collected every 20 feet at the base of the piping trench, and at suspected worst-case locations. Up to five piping trench samples may be composited.

(b) If ground water is encountered in the tank excavation, the presence of free product should be measured and a sample of the water collected for analysis.

(c) In selecting sample types, sample locations, and measurement methods, owners and operators must consider the method of closure, the nature of the stored substance, type of backfill, depth to ground water, and other factors appropriate for identifying the presence of a release. The department and the implementing agency should be consulted to assist in determining sample types, sample locations, and measurement methods. The Montana Quality Assurance Plan for Investigation of Underground Storage Tank Releases should be used as a guide for the collection, preservation and analysis of field samples.

(d) Field hydrocarbon vapor analyzers can be used as screening tools to determine the presence of a release and to assist in determining the extent of contaminated soil to be removed. These analyzers, however, should not be used to confirm the absence of soil or water contamination. Only laboratory analysis of samples will be accepted by the department to confirm the absence of soil or water contamination.

(2) If sampling indicates contaminated soils, contaminated ground water, or if free product as a liquid or vapor is discovered under (1) , or by any other manner, owners and operators must begin corrective action in accordance with subchapter 6. A release must be reported to the department and to the implementing agency by the owner or operator within 24 hours.