Which is NOT an approved method of release detection for single-walled tanks?

Single-walled tanks are not considered safe for certain types of release detection methods that are primarily suited for double-walled tanks. Interstitial monitoring, which involves checking the space between two walls of a tank or piping system for signs of leaks, is specifically designed for double-walled systems. This method is effective in identifying leaks early because any fluid escaping from the inner tank would be detected in the annular space before reaching the surrounding soil or groundwater.

On the other hand, groundwater monitoring, automatic tank gauging, and vadose zone monitoring can be used as release detection methods for single-walled tanks. Groundwater monitoring checks the surrounding area for any signs of contamination, automatic tank gauging continuously measures the tank's fuel levels and can detect discrepancies indicating a leak, while vadose zone monitoring looks at the area between the tank and the groundwater table for signs of a leak.

Thus, interstitial monitoring is not applicable to single-walled tanks, making it the correct answer to the question regarding which method is NOT approved for release detection in those tanks.