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Landfills are required to monitor the underlying groundwater for potential contamination during their active life and post-closure care period. A groundwater monitoring system consists of a series of wells placed upgradient and downgradient of the landfill. Groundwater samples from the upgradient wells indicate background concentrations of constituents in the groundwater, while the downgradient wells may show groundwater contamination caused by a landfill. Groundwater monitoring systems must be certified by a qualified groundwater scientist and must comply with the sampling and analytical procedures outlined in the regulations.

Potentially, there are three phases of groundwater monitoring: detection monitoring, assessment monitoring, and corrective action. During the detection monitoring phase, landfills typically have to monitor quarterly for a list of constituents throughout the facility's active life and post-closure care period. If at any time during the detection monitoring phase, one of the sampled constituents is detected at a statistically significant higher level than the established background level or a set maximum contaminant level, the landfill must notify the state regulatory agency.

At that time, an assessment groundwater monitoring program is established involving additional sampling. If it is determined that groundwater contamination has occurred beyond the facility boundary, corrective action measures are put in place to protect human health and the environment. The facility must continue these remedial measures until it can demonstrate that all required actions have been completed. At that time, the facility may return to the detection monitoring phase.