Designing and Implementing Perimeter Air Monitoring Programs

Perimeter or fenceline ambient air quality and meteorological monitoring programs have become increasingly valuable at hazardous waste, landfill, dredging, ports, and MGP remediation sites. Air quality monitoring programs protect public health and reduce owner liability related to excavation. When designing monitoring programs, it is important to define overall program objectives and systematically address key components to make the program technically defensible and cost-effective. Core components include identifying target parameters and action levels, monitoring instrumentation, data/ telemetry, archiving and quality control. With these component considerations, a viable and cost-effective air monitoring plan can be developed to submit to the regulatory authority for approval. Absent these considerations, significant liabilities for the site owner can result, as well as overly elaborate design/cost issues, and the potential for regulatory delays or rejection.

Perimeter Monitoring/Data Quality Objectives

At its core, a monitoring program protects human health and the environment and enhances worker health and safety. Beyond these paramount objectives monitoring programs should:

  • Evaluate the need and effectiveness of vapor and/or dust controls
  • Document air quality during site activities
  • Establish background levels of target compounds prior to site activities
  • Develop site-specific action levels that protect public health
  • Monitor and document air quality during site activities near sensitive receptors
  • Ensure data collection quality and defensibility
  • Provide risk management information and address public confidence
  • Reduce potential owner liabilities due to site activities

Parameters and Action Levels

Determining target parameters for monitoring and their action levels at various sites is a challenge. Most remedial site activities where sub-surface soil is disturbed or excavated indicate elevated levels of toxic compounds such as: sulfur dioxide, nitric oxides, PCBs, semi-volatile organic compounds, polycyclic aromatic hydrocarbons (PAHs) volatile organic compounds (VOCs), particulates and metals. Based on the presence and toxicity of these compounds, these should be the primary targets for monitoring.

Action Levels

Action levels are toxic air concentrations which trigger remediations and/or shut down of site activities to help reduce the possibility of adverse health impacts near a project site. Action levels for targeted parameters should be developed as risk-based, consistent with EPA guidelines (USEPA, 1989) to protect human health and the environment for short-term acute and long-term chronic exposures. The approach and methodology used to calculate the various action levels should be described in detail.

Determining parameter-specific action levels during the initial monitoring program design will help determine what monitoring instruments should be used. The first step in a risk-based action level calculation is to determine the specific chemicals of interest at a site, e.g., from the target parameter of VOCs to speciated compounds like benzene or toluene. Or in the case of metals, monitor for specific metals such as lead or mercury. Note that target parameters will likely change from site to site. It is critical to evaluate the exposure potential at the site to human and ecological receptors. Human receptors include residents, workers at nearby businesses, schools, hospitals, day care centers, etc. Ecological receptors include waterbodies (lakes and ponds), and other wildlife habitat. Wind direction and other meteorological data can be used to identify up to three receptors that are likely to receive the highest air concentrations.

The main exposure pathway for human receptors is inhalation of volatile chemicals or particulates during site activity. Depending on the receptor, the length and type of exposure will vary. For example, if a residential location was maximally affected, adult and child receptors should be evaluated. For a business location, only an adult receptor would be evaluated.

It is critical to evaluate the exposure 
potential at the site to human and 
ecological receptors.


The average length of exposure for residential versus worker receptors also varies. Residential receptors have longer exposure times than workers. The distance of the receptor from the site should be factored into developing action levels. Higher action levels are appropriate when the receptors are distant. Simple air models can be used to factor the receptor distance.

Toxicity assessment for action levels involve identifying toxicity values for the chemicals of interest. In human health risk assessment, chemicals should be evaluated for noncarcinogenic and carcinogenic properties. Toxicity values are available from USEPA databases and occasionally from certain states. In addition to identifying toxicity values, state or federal ambient air quality guidelines should also be identified. It may be necessary to base the action levels on these guidelines.

Once the exposure and toxicity information is collected, action levels should be calculated following USEPA risk assessment guidelines at receptor locations. Action levels should be derived using human health based criteria for different averaging times, subjected to normal variations in atmospheric dispersion. The results of air modeling will then be used to calculate action levels at the fenceline. The fenceline action levels can then be used to design the final perimeter monitoring program.


Instrumentation plays an important role in air monitoring programs. Select instrumentation or methods that fit the level of sophistication dictated by the site-specific risk assessment and resulting action levels. Different instruments and equipment are necessary for the most effective sampling of monitoring parameters.

Sampling for SO2, CO, NOx compounds can be conducted using either portable or more conventional continuous monitors. Volatile organic compounds (VOCs) can be measured using continuous or portable PID/FID type sensors, or by time integrated methods, such as Summa canisters. More sophisticated gas chromatographs can also be used if speciation of compounds is needed. Sampling for airborne PCBs and/or particulates, PAHs and Metals may be conducted for three reasons: concern over fugitive releases from site activities, health related concerns based on sensitive receptors, and the need for particulate measurements as a surrogate for other pollutants (i.e. PAHs/Metals). Sampling for PCBs is generally performed using an integrated PUF sample approach over a given period of time and a subsequent laboratory analysis. Particulates can be measured using continuous instrumentation or integrated approaches. PAHs and Metals can be approximated as surrogates to particulates or measured directly using continuous integrated sampling approaches.

Number and Placement of Monitoring Locations

Since a perimeter air monitoring system is intended to protect public health in the vicinity of any site, a monitoring program should be designed to warn of short-term exposure levels of target compounds. These warnings should be designed so that acceptable risks for acute and subchronic exposures are not exceeded. Evaluate a site for the following factors to determine the optimum number and location of the perimeter air monitoring locations:

  • Availability of electrical services
  • Security of site perimeter
  • Extent and length of perimeter boundaries
  • Proximity of site activities to local residents and other sensitive receptors
  • Toxicity of contaminated soils
  • Risk analysis for nearby sensitive receptors
  • Directional location of remedial activities with respect to sensitive receptors
  • Predominant wind directions, based on climatological analyses
  • Site activity plan (e.g., one or. multiple locations at one time)
  • Ability to mobilize monitors from one site to another
  • Time schedule for installation
  • Budget considerations.

Frequency of Sampling

Sampling frequency can be separated into three categories depending on the field study.

If continuous analyzers are used, data should be available and averaged on a short-term basis (5, 10 or 15 minutes), depending on the program requirements. This frequency provides detailed and continuous information to help control emissions and protect sensitive receptors. Continuous monitoring instrumentation requires rigorous calibration and maintenance protocols which often limit data availability. Due to the relative immobility of continuous monitoring sites, it is often necessary to install enclosures at a number of locations.

Portable instrumentation provides more flexibility for fenceline monitoring and can be brought down wind of site activities to ensure that measurements are taken where emissions are exiting the site. The mobility of portable instrumentation also allows for periodic site surveys of the fenceline. A disadvantage of using portable instruments is that it is usually only possible to measure at a single location at a time. If a site is large, a survey may be impractical due to the time required for a single trip around the fence line.

Integrated samplers collect a sample over a designated period and can be set up at multiple locations around the fenceline. Integrated samplers may be relocated easily without disruption of site activities. Analytical results of integrated samplers (usually provided by an off-site laboratory) will usually provide more speciated data than from continuous or portable instrumentation. Only one sample is collected per day and the data are generally not available from the analytical laboratories for up to 4 weeks.

Independent of the equipment used and the sampling frequency, perimeter sampling should be performed daily during all site activities. In addition, background perimeter monitoring should also be conducted prior to site activities to help establish background levels for the target parameters and potential local offsite sources of the various analytes.

Data Telemetry and Archiving

Perimeter monitoring programs monitor and document the air quality during site activities and at sensitive receptors. No matter what instruments or sampling frequency is used, all data should be archived and reported systematically. Real-time data telemetry or a manual data archiving system should be employed. Whether the system provides a continuous real-time telemetry or requires manual archiving, all data should ultimately be archived in a central computerized database. The sophistication of the data telemetry system should be based upon the data quality objectives and the need for real-time action level exceedance alarms. To determine the type of data telemetry, collection and archiving system required evaluate the installation schedule, target parameters, instrumentation and mobility requirements, site size, sampling frequency, costs, and the need for real-time exceedance alarms.

It is usually prudent to prepare operator check sheets that can be used to remind site operators to address the various routine maintenance tasks.

QC Protocols

The operational protocol for providing defensible data relates directly to calibration. Calibration procedures and data must be adequately documented so that records can be established and maintained. Records of instrument maintenance, documented in operator checklists, are necessary to demonstrate that proper quality control measures have been applied to the monitoring equipment.

Continuous instrumentation is normally calibrated daily. If the sensor calibration is conducted automatically through a signal from an on-site data logger, a record of the event is available as part of the data set for the day. It is often necessary to conduct calibrations of continuous instruments between automatic calibrations. Formalized calibration forms and standard operating procedures should be used. Calibration and routine maintenance checklists are also recommended.

Portable instruments are normally calibrated at the beginning of each workday and at least once during mid-day. This calibration frequency is recommended because many portable instruments are influenced by changes in ambient temperature and/or humidity. Normally, instrument manuals are sufficient guides for conducting a calibration but the results must be documented. It is normally not necessary to keep an instrument-specific check sheet for maintenance as long as maintenance and other quality control measures are recorded in the on-site logbook.

The calibration cycle for integrated samplers is usually either weekly, monthly or quarterly. Specific calibration forms and charts are kept as a record. Normally, quality control tasks are also recorded. Specific calibration procedures are usually generated by the operating team since few are normally available in the product literature. Field and trip blanks should be included as part of the sample collection protocol. These blanks should be analyzed with the test samples and are an important quality control measure for this type of equipment. Chain-of-custody shipments to laboratories are also a critical QC function.

Conceptual Design/ Regulatory Review

A key component to determining and gaining acceptance of any perimeter monitoring program's objectives and initial design is an upfront discussion with the regulatory authority (e.g., state, EPA, Port Authority, etc.) These initial discussions (i.e., submittal of a monitoring program conceptual design) can be invaluable in understanding the regulatory position concerning the monitoring objectives. These discussions should guide the perimeter air monitoring program's design and could save valuable time in the ultimate design and implementation.

Monitoring Plan Generation

Based on the guidelines presented in this paper, plus any initial conceptual design discussions with the regulatory body overseeing the site-specific project, a formal Perimeter Monitoring Plan should be generated. This formal Monitoring Plan should then be submitted to the regulatory body overseeing the program for approval prior to implementation.

The formal Monitoring Plan should include the following sections:

  • Monitoring objectives
  • Monitoring site locations
  • Monitoring protocols and frequency of sampling
  • Instrumentation
  • Action limits for the various parameters
  • Routine operational procedures
  • Data telemetry approach
  • Data reporting formats and frequency
  • Monitoring system calibration and
  • QC protocols.


Many issues must be addressed when designing and implementing a perimeter fenceline monitoring program for any site. One must clearly identify the program's specific objectives to truly design the most technically defensible and cost-effective program possible. Failure to strategically address these objectives could result in significant liabilities for the site owner or a perimeter air monitoring program being designed significantly more elaborate and expensive than necessary.

Leo J. Gendron (; 978-589-3518) is an Air Measurements Senior Program Manager at ENSR International with more than 30 years of experience. He is currently managing several ambient air monitoring programs, including two fenceline programs. Mr. Gendron has a B.S. in Meteorology and an MS in Applied Management.

Anthony M. Sacco P.E., QEP (; 978-589-3516) is a Senior Specialist at ENSR International with 40 years of experience in the design and operation of ambient air monitoring programs. He has prepared most of ENSR's standard operating procedures (SOPs) for air measurements and has provided expert witness testimony in several litigations.

Ishrat S. Chaudhuri Ph.D., DABT (; 978-589-3052) is a Senior Toxicologist in ENSR's Risk Assessment Department. Over the past 18 years, Dr. Chaudhuri has conducted risk assessments under CERCLA, RCRA, and state programs, as well as multipathway risk assessments for facilities involving air emissions.

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