Owners and operators must monitor tanks for leaks using the methods, or combination of methods, of release detection specified in this section.

(1) Tanks requiring secondary containment. Tanks requiring secondary containment under WAC 173-360A-0310 (4)(a) must be monitored for leaks at least every thirty days using interstitial monitoring (WAC 173-360A-0655).

(2) Tanks not requiring secondary containment. Except as provided for previously deferred UST systems under subsection (3) of this section, tanks not requiring secondary containment under WAC 173-360A-0310 (4)(a) must be monitored for leaks at least every thirty days (except as otherwise provided) using one of the following methods, or combination of methods, of release detection:

(a) Combination of inventory control (WAC 173-360A-0620) and tank tightness testing (WAC 173-360A-0635). Tank tightness testing must be performed at least every five years. This combination of methods may be used only if the tank has been installed for less than ten years;

(b) Combination of weekly manual tank gauging (WAC 173-360A-0625) and tank tightness testing (WAC 173-360A-0635). Tank tightness testing must be performed at least every five years. This combination of methods may be used only if:

(i) The tank has been installed for less than ten years; and

(ii) The tank's capacity is less than or equal to two thousand gallons;

(c) Weekly manual tank gauging (WAC 173-360A-0625). This method may be used as the sole method of release detection only if:

(i) The tank's capacity is less than or equal to five hundred fifty gallons; or

(ii) The tank's capacity is five hundred fifty-one gallons to one thousand gallons and the tank's diameter is either sixty-four inches or forty-eight inches;

(d) Automatic tank gauging (WAC 173-360A-0630);

(e) Interstitial monitoring (WAC 173-360A-0655). This method may be used only if the tank is secondarily contained and meets the performance standards in WAC 173-360A-0310 (4)(b);

(f) Vapor monitoring (WAC 173-360A-0660);

(g) Groundwater monitoring (WAC 173-360A-0665);

(h) Statistical inventory reconciliation (WAC 173-360A-0670); or

(i) Other release detection methods (WAC 173-360A-0675).

(3) Additional methods for certain previously deferred UST systems. Field-constructed tanks not requiring secondary containment under WAC 173-360A-0310 (4)(a) with a capacity greater than fifty thousand gallons must be monitored for leaks in accordance with subsection (2) of this section or using one of the following methods, or combination of methods, of release detection:

(a) Tank tightness testing (WAC 173-360A-0635) performed at least annually, except the method must be able to detect a 0.5 gallon per hour leak rate;

(b) Combination of automatic tank gauging (WAC 173-360A-0630) performed at least every thirty days, except the method must be able to detect a one gallon per hour leak rate, and tank tightness testing (WAC 173-360A-0635) performed at least every three years, except the method must be able to detect a 0.2 gallon per hour leak rate;

(c) Combination of automatic tank gauging (WAC 173-360A-0630) performed at least every thirty days, except the method must be able to detect a two gallons per hour leak rate, and tank tightness testing (WAC 173-360A-0635) performed at least every two years, except the method must be able to detect a 0.2 gallon per hour leak rate;

(d) Vapor monitoring (WAC 173-360A-0660) performed at least every two years, except the method must use a tracer compound and be able to detect a 0.1 gallon per hour leak rate;

(e) Combination of inventory control (WAC 173-360A-0620) performed at least every thirty days, except the method must be able to detect a leak of at least 0.5 percent of flow-through and be performed in accordance with Department of Defense Instruction 4140.25, Air Transport Association Airport Fuel Facility Operations and Maintenance Guidance Manual, or equivalent procedures, and either:

(i) Tank tightness testing (WAC 173-360A-0635) performed at least every two years, except the method must be able to detect a 0.5 gallon per hour leak rate;

(ii) Vapor monitoring (WAC 173-360A-0660) performed at least every thirty days; or

(iii) Groundwater monitoring (WAC 173-360A-0665) performed at least every thirty days; or

(f) Other release detection methods (WAC 173-360A-0675), except owners and operators must demonstrate the method can detect a leak as effectively as any of the methods allowed in (a) through (e) of this subsection and the department must approve the use of the method. In comparing methods, the department will consider the size of release that can be detected and the frequency and reliability of detection.