Once the remedial action is implemented, monitoring is required to assess the protectiveness and effectiveness of the chosen interim remedial measures (IRM). The type and frequency of monitoring should be based on the CSM developed for the site which identifies the LNAPL type, source and distribution, site specific receptor issues, hydrogeologic influences on LNAPL behavior and other factors affecting LNAPL migration and recoverability. The operational monitoring should validate the assumptions developed for the site and document that the selected IRM is effective in preventing migration, reducing mass, when practicable, and is protective of known human and ecological receptors.
Type of Monitoring
Monitoring the LNAPL Body
Since a remedial investigation should have resulted in a detailed understanding of the source(s) and distribution of the LNAPL at the site, it is anticipated that an appropriate monitoring well network was installed during the remedial investigation phase. This monitoring well network should include wells located within the LNAPL body and an appropriate array of wells around the perimeter of the LNAPL body. The spacing of these wells should reflect and account for site specific subsurface characteristics as well as the characteristics of the LNAPL being monitored. A sufficient number of monitoring points shall be located within the LNAPL body to establish and document LNAPL distribution, to predict behavior and migration and to assess recoverability. Perimeter wells should be located just beyond the LNAPL body, but close enough to verify that LNAPL is not migrating.
Monitoring should be conducted on a regular basis at all LNAPL sites during the interim remedial measurement phase. Operational monitoring should include gauging events that collect depth to water (product 1: ), , depth to LNAPL and LNAPL thickness measurements (product 1:) using appropriate field instruments. Adjustments in monitoring may be needed based on performance monitoring data and changing site conditions.
Receptor Monitoring
Receptor monitoring, if applicable, should be conducted to assess that the IRM is protective of known risks. The frequency will be based on the professional judgment of the investigator and should consider the CSM developed for the site. Receptor monitoring should be conducted in accordance with other relevant Guidance Documents and Regulations.
Interim Remedial Measure Monitoring
The specifics of remedial system monitoring will vary with the type of IRM selected for a site. All IRMs should include a means to monitor the response of the LNAPL body to remedial efforts and should include the collection of sufficient system data to allow an assessment of IRM effectiveness.
An IRM monitoring protocol should include the following:
- Hydraulic gauging which includes depth to water/depth to LNAPL/LNAPL thickness
measurements in pertinent monitoring wells and recovery points both within and along the perimeter of the LNAPL body; - Determination regarding the amount of LNAPL recovered at each recovery point during the reporting period;
- System-specific monitoring
For IRMs which include groundwater extraction as a component of remedial system design,
system monitoring should include:
- Regular hydraulic gauging of both pumping and non-pumping wells;
- Verification of depths of pump intakes in all dewatering points;
- The pumping rate established at each dewatering point during the reporting period; and
- The degree of drawdown established in both active pumping wells and nearby monitoring points.
For IRMs which utilize total fluids extraction using a drop tube technology, system monitoring
should include:
- The depth to which the drop tube is placed in each extraction point;
- Measurements of total applied system vacuum during system operation;
- The casing vacuum as determined in each extraction point during system operation;
- Photo-ionization detector (PID) readings of the extracted vapor phase mass (if applicable) at each extraction point;
- Hydraulic gauging and vacuum gauging at nearby monitoring points; and
- The amount of groundwater and LNAPL recovered from each extraction point during the reporting period.
For IRMs which include the injection of surfactants to increase the mobility of LNAPL in order to enhance recovery rates in the liquid phase, system monitoring should include:
- Pre-injection groundwater sampling for both the compounds of concern and compounds/by-products associated with the injected material;
- Details regarding the volume, rate, duration, and depth of introduction of the injected material at each injection point;
- Details regarding the recovery phase of the IRM including depth of pump intake(s), pumping rate at each extraction point and duration of the recovery phase;
- Hydraulic gauging during the recovery phase at both the recovery points and nearby monitoring wells to determine pumping zone of influence;
- Post-injection groundwater sampling for both the compounds of concern and the compounds/by-products associated with the injected material