(1) Before permanent closure or a change in service is completed, the owner or operator shall 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, the owner or operator:
(a) shall collect soil samples, as soon as possible after the tank or piping has 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 each tank with a capacity of over 600 gallons that is being removed for closure, at least two soil samples, one at each end of the tank, or at suspected worst-case locations, must be taken. For a tank with a capacity of 600 gallons or less, one soil sample must be collected beneath the tank. Each sample must be take at least one-to-two feet below the base of the maximum excavation depth. If contaminated soil is removed from the excavation site, at least one composite sample of the contaminated soil must be collected for analysis. For piping removal, soil samples must 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, shall measure the presence of free product and collect a sample of the water for analysis;
(c) in selecting sample types, sample locations, and measurement methods, shall 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; and
(d) may use field hydrocarbon vapor analyzers 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, the owner or operator shall 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.