Long-Term Performance Monitoring at Natural Attenuation Site

Long-term monitoring is an integral part of the natural attenuation remediation process. It is needed to monitor the migration of a contaminated groundwater plume over time and to evaluate the extent of natural remediation that is occurring. It involves the placement of four types of monitoring wells: up-gradient, side-gradient, impacted source, and down-gradient wells. The monitoring of groundwater samples from these wells should provide information to evaluate if the objectives of the natural attenuation process are being met. The sampling analytical plan depends upon the type of contaminants involved. The frequency of sampling should be sufficient to detect in a timely manner any changes in plume contaminant degradation behavior. It should also be flexible enough to make changes, should there be a need. The monitoring program should be continued until the objectives of the natural attenuation process have been met, and possibly longer, if necessary, to ensure that health and environment threats are protected. If during the monitoring process the plume behavior is found to change for the worse (i.e., plume migration is faster or further than before), a contingency plan should be in place to protect the down-gradient receptors. The contingency plan usually requires an engineered remediation plan to reduce the negative impact of the plume on the down-gradient receptors.