Today's municipal solid waste landfills are required to monitor air, surface water and groundwater - an activity that has become a major component of a fa-cility's operating budget.
Regulations that pose considerable challenges for landfill operators include the Clean Air Act, Section 110 (CAA), the Clean Water Act, Section 402 (CWA) and Subtitle D of the Resources Conservation and Re-covery Act, Title 40 Part 258 (RCRA 40 CFR 258). The revised regulations are intended to help:
* Identify the site's environmental impact;
* Differentiate between a release from a site and from other sources;
* Determine the magnitude and extent of a release; and
* Assess the effectiveness of on-going cleanup activities.
Ultimately, the requirements will help reduce and manage environmental risks, but landfill operators run the risk of incorrectly implementing monitoring programs and triggering costly, unnecessary as-sessment and remediation programs.
Groundwater Monitoring RCRA prohibits the migration of hazardous constituents into groundwater and mandates groundwater monitoring at landfills in the United States. RCRA 40 CFR 258 specifies national criteria for detection, assessment and corrective action monitoring.
The first stage, detection monitoring, determines if there has been a release from the site. A small list of indicator parameters are monitored regularly to determine if there is any difference between back- ground and compliance concentrations.
A groundwater detection monitoring system collects water samples from monitoring wells to detect a potential re-lease from a landfill. The monitoring wells are strategically placed to compare unaffected, background water quality with the water quality determined from wells in the uppermost continuous aquifer, im-mediately downgradient from the landfill.
Most detection monitoring systems incorporate single or multiple background wells with multiple downgradient compliance wells to make this comparison. A common misconception is that, if results from downgradient wells are not comparable to background, the landfill must be the cause. Often, little consideration is given to the natural variations of groundwater chemistry that can be caused by the local hydrogeologic regime. A careful understanding of the site's hydrogeology and a monitoring system that accurately reflects those conditions is essential to as-sess a landfill's impact on the local groundwater quality.
The geologic environments of landfills vary. The background-downgradient detection monitoring comparison is justified when a relatively uniform, continuous groundwater aquifer extends beneath the landfill, but many geologic environments don't meet these conditions.
For example, in regions where earthquake faulting is common, fault zones often contain highly fractured material that is more easily eroded than surrounding terrain. Historically, these have been prime landfill locations, but just as faults juxtapose distinctly different rock formations, they may also segregate hydrologic regimes that can develop distinctly different groundwater chemistries. Because comparing these chemistries from wells across the fault may not be appropriate, the landfill may be falsely implicated as the reason for the difference.
How is a landfill's background de-termined? Many facilities presume that concentrations from upgradient wells must be used as background, which is only appropriate if there is little or no spatial variability in the underlying hydrogeology at the site. In many cases, facilities should use historical data from downgradient compliance wells as background.
Selecting indicator monitoring parameters that will provide an early, accurate indication of a re-lease from the facility is equally challenging. For example, where highly variable zones of metals exist, those metals should not be used as monitoring parameters.
Parameters should be chosen based on the facility's leachate and background groundwater samples. The best parameters usually are present in the leachate, and its concentration exceeds the background concentration at least 10 times.
Background selection and indicator monitoring parameters also affect the determination of the most appropriate statistical test to evaluate the water quality data. The statistics are used to determine if current concentration values exceed background concentrations. If you use historical data from downgradient compliance wells as background, then the statistics that were designed for upgradient to downgradient comparisons such as the ANOVA are not appropriate. Other statistics that look at changes over time at each well are more appropriate.
But statistics are not foolproof. Each statistical result must be in-terpreted within a site-specific hydrogeologic, sampling and analytic context. False positives (a statistical indication of a difference in concentrations when in fact there is no difference) are possible in every statistical test, and statistics cannot distinguish between a difference in the concentrations caused by a fa- cility's release and a lab or sampling error.
Use retest protocols and site-wide false positive computations to reduce the chance of errors. Consider using specialized groundwater statistical software to automate the regular review of data and to ensure that only the most appropriate statistical tests are used. A consultant with practical experience in groundwater quality statistics can help set up a monitoring program.
Assessment Monitoring If detection monitoring indicates a release at the site, implement an as-sessment monitoring program to de-termine its nature and extent.
Evaluation wells determine the vertical and horizontal extent of the release's migration; characterize the chemical properties of the plume, including lateral and vertical variations; and evaluate the direction and migration rate of the plume, as well as the physical and hydrogeologic factors that control groundwater and plume migration.
Identify the release boundaries to determine the vertical and horizontal extent of the migration. To characterize the plume migration, wells must be placed hydraulically downgradient from the impacted detection monitoring well into the unaffected groundwater. Consider the following when designing the well placement:
* Is the release limited to a single aquifer, or could multiple, deeper aquifers be impacted?
* Are groundwater flow rates and plume migration rates relatively uniform as would be expected in a sedimentary aquifer such as sandstone, or could flow rates and plume migration distance be highly variable, such as in bedrock terrain?
* Do existing wells adequately monitor the entire thickness of the aquifer, or are the wells selectively screened in the uppermost portion only? Many contaminants are denser than water and will sink and concentrate at the lower boundary.
* Considering future costs, will the evaluation monitoring wells be used as groundwater extraction wells? This may modify the well size and screen length.
Under assessment monitoring, the facility must accurately assess constituent concentrations against health-based or background standards. Insisting that all observations lie below a standard ignores the reality of false positives caused by lab or sampling errors. On the other hand, using the average ignores information about the distribution of the higher values that are critical to as-sessing any true risks associated with a particular constituent.
Use a groundwater statistics ex-pert to determine the most appropriate approach and to educate regulators on the pros and cons of alter- native approaches, or corrective ac-tion may be required even when there is no risk to human health and the environment.
Determining if the concentrations are increasing or decreasing is challenging. If strong decreasing trends are evident, expensive corrective remedies may not be needed. Trend tests such as linear regression are mathematically unsuited to groundwater quality data, especially if there are a few very high values in the da-ta set. Nonparametric trend tests such as Mann Kendall are more ap-propriate for assessing trends.
Corrective Action If a site goes into corrective action, monitoring determines if the remedy is reducing contaminant concentrations and shows when cleanup standards have been met.
Technically impossible cleanup levels are often established. EPA and other studies have shown that the efficacy of groundwater cleanup technologies usually declines as contaminant concentrations drop. In fact, an asymptotic limit often is reached, and the contaminant concentrations will not drop below that limit. Facilities should review action levels regularly and establish monitoring programs that will provide data to evaluate technical feasibility.
Determining when to terminate treatment is closely related to the feasibility of action levels. Treatment should not necessarily cease as soon as the concentration values fall be-low an action level. In fact, the de-cision process is more complex due to the inherent variability in groundwater monitoring and since treatment creates a non-steady state in the aquifer. Trend tests monitor the effectiveness of corrective action and predict a termination date, which usually doesn't occur until comparisons between observations and ac-tion levels indicate favorable results.
Demonstrating that cleanup ac-tion levels have been met while the corrective remedy is operating does not ensure that those levels will continue to be met. Sampling can begin only when the transient effects of the treatment have decayed (steady state).
Determining that steady state conditions have been met often re-quires a combination of groundwater monitoring, trend testing and nu-meric modeling. Three or more years of monitoring are usually required by EPA to show that clean-up levels have been met; then the site returns to detection monitoring.
Surface Water Monitoring The Clean Water Act (CWA) regulates discharges of pollutants to U.S. waters. Discharges from any point source such as landfills are prohibited, and a National Pollutant Dis-charge Elimination System (NPDES) permit must be issued. NPDES provides the framework for regulating municipal and industrial stormwater runoff.
Landfills that discharge industrial stormwater either directly or indirectly (through a municipal water supply) to surface waters must be covered by a NPDES permit and monitoring program. The program monitors the stormwater discharge quality relative to discharge prohibitions, effluent limitations and receiving water limitations.
Landfill surface water monitoring programs are designed for two general situations: stormwater monitoring and monitoring natural-surface water bodies.
Stormwater is monitored to evaluate the quality of surface water that collects in the landfill's drainage system and that is typically discharged off-site into mu-nicipal stormwater run-off systems or natural water systems. Precipitation and surface flow can directly contact waste if daily cover is not properly or completely placed or if runoff causes active erosion through the landfill cover. In either case, the quality of the surface water flow that contacts waste may be affected, and that flow may be carried off-site, violating the landfill's NPDES permit regulations.
Several monitoring stations are necessary to effectively monitor most landfills. Moni-toring points are typically located in areas of high potential risk and where landfill drainage systems discharge runoff off-site.
Assessing Landfill Impacts When surface contaminants accumulate during dry periods, stormwater runoff often exhibits a surge of contaminants in the initial stages of a storm. This is often true at the landfill maintenance and fueling stations, where oil and grease concentrate, so an initial sample must be collected within the first hour of runoff and another during peak flow.
To overcome the logistical difficulties of collecting several samples during a fixed time at several monitoring stations, use automatic sampling devices that can collect water samples at pre-programmed intervals. Sampling during peak flow is also recommended to monitor runoff at its maximum erosive capacity, when it may directly contact waste due to erosion of the landfill cover.
Stormwater monitoring should verify that landfill waste does not come in contact with precipitation and stormwater runoff and that dai-ly cover practices are adequate. The monitoring results may warrant changes in management practice. For example, high levels of total suspended solids may indicate that the cover materials, in combination with the drainage design, are insufficiently compacted or lack enough surface vegetation.
Monitoring natural surface water bodies adjacent to a landfill identifies potential impacts from surface water or groundwater flow discharging into the stream or lake.
To assess the impact on surface water, compare samples against background samples - but identifying background can be a significant challenge. Often a facility will use a monitoring station in an adjacent lake or stream to determine natural background water quality characteristics, but these characteristics fluctuate due to seasonal changes. If natural fluctuations are not identical in both the background and compliance monitoring points, it can be difficult to identify landfill impacts on the surface water bodies. Also, if the background monitoring point is close to the landfill, it may be affected by the facility.
The compliance monitoring stations must be located hydraulically downgradient and directly adjacent to the landfill drainage discharge point to ensure that discharges from adjacent facilities are not influencing the results. In shallow groundwater, place stations where migration from beneath the landfill may emanate in-to the surface water. Surface seeps or springs should also be sampled.
The seasonal nature of surface water quality usually requires at least quarterly monitoring because landfill impacts can be di-luted during periods of peak runoff. For example, if seeps or groundwater are being impacted, the probability of detection in surface water bodies is much greater during low surface water flow periods.
Statistics are often used to identify natural background characteristics, to compare background a-gainst compliance data, to detect early trends in concentrations and to identify variables that may confuse the analysis. Factors such as precipitation or natural stream flow may hide or magnify a landfill impact. For example, if months of dry weather are followed by a sudden storm, sediment from fields or roads could be washed off into a lake or stream. Samples taken im-mediately after the storm might magnify the facility's long-term impact.
Landfill air emissions are regulated and monitored ac-cording to RCRA 40 CFR 258. The major emissions are explosive gases, toxic or hazardous emissions and odorous or nuisance emissions. Under the regulation, State Implementation Plans developed un-der Section 110 of the CAA regulate most aspects of a facility's emissions and air monitoring program.
Landfill emissions, usually monitored quarterly, are typically nitrogen, carbon dioxide, methane and other organic constituents.
The rule limits concentration and monitoring frequencies for potentially explosive or toxic gases within the landfill's boundary and for off-site structures potentially affected by migration. These gases collect in basements and foundations where concentrations may reach explosive or toxic levels. Specifically, 40 CFR 258 requires:
* A minimum of quarterly methane monitoring in structures within the property boundary;
* A maximum methane concentration of 25 percent of the lower explosive limit (LEL) within facility structures; and
* A maximum methane concentration of the LEL at the property boun-dary.
It is important to establish an ap-propriate monitoring frequency, en-sure complete monitoring of potentially affected structures and plan for corrective action.
The monitoring program must be suitable to the site-specific conditions that control gas generation and migration. Landfills with no gas ex-traction system may require more frequent monitoring due to gas levels within the fill. Also, if native materials that contact the waste are very porous, gas migration potential might call for frequent monitoring.
Establishing comprehensive sampling locations within structures is critical. Sampling stations must re-flect areas of high gas migration and collection potential, as well as high explosive potential. Ignition sources or areas with open flames must also be included.
When any property is separated from waste by 25- to 50-foot buffers, landfill gas could pose an explosive, toxic problem that may require more detailed monitoring with sensors and alarms .
If gas migration is detected, 40 CFR 258 requires that corrective measures must be taken within 60 days. A facility must develop emergency plans, then design and construct long-term corrective actions. A contingency solution might in-clude positive ventilation of affected structures or installation of air barrier systems; long-term corrective ac-tion may require a landfill gas ex-traction system.
In certain jurisdictions, particularly urban air districts, ambient air quality testing is required in re-sponse to point-source gas emission requirements. Ambient air monitoring characterizes the concentration and chemical composition of the landfill's overall emissions. A series of sampling stations are placed a-round the landfill in dominant up-wind and downwind directions, and in areas where vapors collect, to fully monitor the facility.
Because ambient air monitoring results are directly influenced by meteorological conditions, wind speeds and direction must be accurately measured. Monitoring plans may have wind speed limitations which may require a number of at-tempts at sites in windy locations.
The Challenges Ahead Consultants and regulators will continue to face landfill monitoring challenges. Since most monitoring regulations establish broad objectives but provide little detail on how to meet those objectives, site-specific, practical monitoring programs will require creative regulatory and technical solutions.
Realistically, many facilities will be dismayed by their initial monitoring results. Many landfills, especially older, unlined sites, will have initial indications of groundwater problems, so review the initial results carefully before determining that a release has occurred. Findings can be misleading due to many hydrogeologic, monitoring network and statistical factors.
Monitoring professionals should learn about new technologies and how to use statistics to interpret the results. A poorly conceived and im-plemented monitoring program can be expensive. A single error in a statistical test of groundwater quality can force a facility to immediately re-sample its wells for more than 250 constituents.
In order for both sides to benefit from the landfill monitoring regulations, facilities must forcefully dem-onstrate compliance efforts. Mean-while, regulatory agencies must rec- ognize that, until the complexity of the monitoring and statistical tools are fully understood, errors are in-evitable. Giving facilities time to verify their monitoring results will be in the public interest.