The disposal of municipal (RCRA Subtitle D), coal combustion residuals (RCRA Subtitle D) and hazardous waste (RCRA Subtitle C) is regulated by the Resource Conservation and Recovery Act (RCRA). RCRA also applies to the protection of groundwater resources by regulating constituents of concern that may migrate into groundwater. A RCRA monitoring system must consist of at least four wells: one up-gradient from the unit and three down-gradient. Up-gradient wells are used to measure background water quality. Down-gradient wells are tested for the presence of any statistically significant amounts of constituents that migrate from the waste management area. Groundwater compliance is based on a comparison of the data from up-gradient and down-gradient wells.

Depending on the status of a RCRA site, one of several groundwater monitoring programs may be in place. 

• Background monitoring determines the naturally occurring levels of constituents in the groundwater. Monitoring at up-gradient wells must occur quarterly for a full year.
• Detection monitoring examines groundwater for elevated levels of indicator parameters that suggest that contamination may be occurring. During the detection monitoring phase, facility owners/operators must develop an assessment program outline in the event that an assessment monitoring program is required.
• If the results of detection monitoring suggest that groundwater contamination has occurred, assessment monitoring will be implemented to determine the nature, extent, and rate of the groundwater contamination. Assessment monitoring must continue on a quarterly basis until the facility undergoes final closure or monitoring requirements are established in conjunction with obtaining a permit and/or performing corrective action.
• A groundwater protection standard (GWPS) is established when a statistically significant release is detected at the waste management unit boundary under a detection monitoring program. The GWPS consists of a list of constituents and concentration limits for which groundwater monitoring must be conducted, the point of compliance, and the compliance period. The concentration limits of the GWPS are set at background levels, maximum contaminant levels (MCLs), or alternate concentration limits (ACLs).
• Information developed in an assessment monitoring program is used by regulators to evaluate the need for corrective action. A corrective action is required when constituents of concern exceed the GWPS. The owner/operator is required to remedy the situation by removing hazardous constituents or treating them in place. The corrective action groundwater monitoring program must demonstrate that the corrective action is working.