A container or drum holding a liquid or solid can leak and spill. If the substance goes far enough, it may reach a drain and end up contaminating the environment. In order to prevent contamination, a secondary containment system for liquids, solids, and any hazardous materials needs to be in place. At the same time, the system must always be effective in containing a substance, and in order to determine this, testing must be performed regularly on secondary containment.

Secondary containment systems come in various forms, from portable containers or liners to double-walled tanks. A system may be in place under a dispenser or STP sump or surrounding a primary container. The goal of testing is determining if the system still functions as well as it did upon installation. In installation, as well, a test must be done on a secondary containment system to gauge its original effectiveness. Testing additionally indicates if a system continues to meet a manufacturer's guidelines, industry codes, or engineering standards.

Secondary containment testing comes in two forms: hydrostatic or vacuum testing. The former is considerably quicker than vacuum testing but not as accurate as the latter. If a hydrostatic test is performed, the sump, or container, needs to be filled with water, and the level of the water inside must be marked with paint. In 24 hours, the water level needs to be checked again and measured for a quarter- or half-inch drop.

Although hydrostatic testing indicates that the water level inside dropped, it does not involve calculating the rate in which a liquid escapes the system. Vacuum testing, on the other hand, does. For this test, the sump is also filled with water, but rather than go away for 24 hours, you wait five minutes for a dispenser or 30 minutes for an STP sump and then measure the 0.5 gallon per hour leak rate and level of change.

SB 989 has required secondary containment testing since 2001 and specifies that all systems must be checked upon installation, six months later, and every three years from that point. Before testing occurs, the owner or operator must inform a local agency 48 hours in advance and, after, has 30 days to submit a report. If in the instance a secondary containment system cannot be tested or does not pass testing, it needs to be placed with one that's functional.