Abstract

Environmental monitoring is a major component of any remedy that relies on natural attenuation processes. The objective of this document is to identify data needs and evaluation methods useful for designing monitoring networks and determining remedy effectiveness.

Effective monitoring of natural attenuation processes involves a three-dimensional approach to network design and clearly defined performance criteria based on site-specific remedial action objectives. Objectives for the monitoring program will be met through routine evaluations of institutional controls and measurements of contaminant, geochemical, and hydrologic parameters. These data are used to evaluate changes in three-dimensional plume boundaries, contaminant mass and concentration, and hydrological and geochemical changes that may indicate changes in remedy performance.

Data interpretation focuses on detection of spatial and temporal changes, and assessment of their impacts on the achievement of site-specific goals. Particular changes of interest include:

• Progress toward contaminant removal objectives and indications of additional contaminant releases
• Contaminant detections at the horizontal and vertical plume boundaries that may indicate plume expansion
• Geochemical changes (e.g., oxidation-reduction [redox] conditions) indicative of possible changes in contaminant transformation rates
• Changes in ground water flow rates or directions such that contaminants may move into previously unaffected areas
• Changes in land and resource uses that threaten the effectiveness of institutional controls

Decisions regarding remedy effectiveness and the adequacy of the monitoring program will generally result in continuation of the program, program modification, implementation of a contingency or alternative remedy, or termination of the performance monitoring program. Such decisions are appropriately based on specific, quantifiable performance criteria defined in the monitoring plan.

Continuation of the program without modification would be supported by contaminant concentrations behaving according to remedial expectations, while ground water flow and geochemical parameters remain within acceptable ranges.

Modification of the program, including increases or decreases in monitoring parameters, frequency, or locations, may be warranted to accommodate changing conditions or improved understanding of natural attenuation processes at the site. Situations that may trigger implementation of a contingency or alternative remedy include:

• Increasing contaminant concentrations or trends not predicted during remedy selection or indicative of new releases
• Contaminant migration beyond established plume or compliance boundaries
• Contaminants not decreasing at a rate sufficient to meet remediation objectives
• Changes in land or ground water use that have the potential to reduce the protectiveness of the remedy
• Contaminants observed at locations posing or having the potential to pose unacceptable risks to receptors

Contact

Steven D. Acree

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