A best management practice (BMP) is a practical and effective process, practice, or technique to achieve a desired outcome. BMPs also generally reflect practices that have proven themselves over time for a large number of facilities or organizations. They are offered as “good ideas” that may need to be adjusted to account for individual needs or site-specific circumstances. BMPs are not rules, regulations, or mandatory standards.
CIWMB contracted with SCS Engineers to study the functionality of monitoring wells and the results of that study, “Landfill Gas Monitoring Well Functionality at 20 California Landfills“, were presented to the Board on July 15, 2008. At that July meeting, the Board directed staff to initiate an informal rulemaking process to modify existing disposal site gas monitoring and control regulation Title 27, California Code of Regulations (CCR), section 20925 to provide additional requirements for landfill gas (LFG) monitoring probe design and construction and probe functionality assessment. They also directed staff to incorporate input from stakeholders as well as a Technical Advisory Group into BMPs developed from the findings of that study. Applying the BMPs should minimize the need for operators to replace probes currently being designed and installed to comply with recent changes to the landfill gas monitoring regulations that became effective on September 20, 2007.
These BMPs provide voluntary guidance to landfill operators, consultants, and local enforcement agencies regarding effective design and construction of LFG monitoring wells/probes.
- Probes should be constructed with longer screened segments (as opposed to shorter).
The longest screened section practical for the given site-specific situation should be used. For example, within the zone of preferential path (e.g., sand lens), a longer screen encompassing the entire zone is preferred. Longer screened sections reduce the possibility of blockages by bentonite, dirt, roots, and other organic material. In general, screened segments should not be shorter than 5 feet in length. However, in multidepth probes, screened sections should be installed so that they do not overlap at the same depths. In addition, in order to prevent cross-contamination of the ground water by fluids (perched water or LFG), probes should not be screened across confining layers separating perched water zones from the regional aquifer (see BMP No. 7). In very shallow wells, the screened length for probes should be as long as possible without compromising the well seal.
- Probes should be assembled using materials and in a manner that provides an adequate seal and does not interfere with sampling trace constituents.
Threaded and gasketed assemblies that prevent leaks at connection points or continuous pipe will help ensure that gas samples are collected from the screened portion of a probe as opposed to a leaking slip coupling or screwed together joint. Glued and/or solvent welded joints may also interfere with any trace gas sampling and should be avoided. It is understood that some portions of a probe (e.g., end-cap and wellhead) cannot be pre-constructed and, thus, may require a slip-type fitting. PVC is not required for probes, but it was the only material used for probes evaluated in the study.
- Minimize the number of probe pipe connections by using longer sections of pipe.
The use of longer pipe sections will limit the number of points in the probe with any potential to leak ensuring the isolation of the monitored zone and well integrity. For PVC, the use of commercially available 10-20 foot sections of pipe is encouraged. Other materials such as HDPE are available in continuous spools, but no such materials were encountered during the study.
- Probes should be constructed using a non-specialized valve on the probe head assembly.
The use of a labcock, ball, or similar valve that is easily opened and closed without a special connection or adapter will help ensure that valid pressure and gas readings can be obtained from probes by on-site personnel as well as regulatory agencies. If a specialized or proprietary valve is used, such as a Schrader valve or quick-connect valve, then an adaptor should be provided and stored on-site at the first well or other readily accessible location.
- LFG wells and probes should be properly labeled and identified.
There should be a visible labeling method for locating and identifying monitoring wells in the field and a durable method for the labeling of probes. The minimum information on the label should include well identification, and probe identification (e.g., shallow, intermediate, deep). The labeling should be consistent for all wells at a particular site. For wells with more than three probes, the labeling should clearly identify the shallowest to deepest probes. Probe screen interval is also preferred on the labels. Non-oxidizing labels are preferred as some metal labels may rust and become unreadable. Labels should be replaced as necessary by the operator to allow easy identification of the wells and probes.
- LFG probes should be constructed to allow access by a bore monitor.
Probes should have an unobstructed bore interior with an inside diameter of ½ to ¾ inches to allow visual access by a bore monitor (i.e., down-hole camera).
- The depth of the probe(s) in relation to the water table should be a design consideration.
In order to maximize the effectiveness and life of the monitoring probe, the depth to the water table plus seasonal fluctuations in the water table should be taken into account when determining the depth of the well and screen interval of the probe(s). The screen interval should be long enough so that at least a portion of the screen interval will always be above the water table. An exception to this is when the longer screened interval would overlap with another probe screen (see BMP No. 1). Wells need to be designed and constructed to prevent cross-contamination of ground water through the well/probes by fluids (perched water or LFG). Each LFG monitoring well must: be properly sealed in accordance with 27 CCR 20925(d) (3) which includes a surface seal; seal off any aquitard and/or perch water zone; in wells with multiple probes, seal off well between each probe; and seal off well just above the high water mark, if a probe will be screened within the fluctuation zone. Each probe should have only one screened interval. One design option, given no perched groundwater, would be to have a probe that is screened to just above the seasonal high ground water level and a separate dedicated probe screened in the fluctuation zone to the permanent, seasonal low ground water level. Alternatively, the probe in the fluctuation zone could be screened and properly sealed in a separate well (borehole). During high water years or seasons this lower probe may be inundated, but would detect any migrating LFG during dry times.
- Probes should be preferentially located as far away from surface vegetation as possible in order to avoid root intrusion into shallow probes.
In order to minimize the possibility of root intrusion in a probe, the probe location should be placed as far away from deep-rooted vegetation as possible without compromising the ability to monitor for LFG and the probe should be periodically inspected and cleared of vegetation. Probes that are located close to deep-rooted vegetation can experience some degree of root intrusion either in the screened interval or at the joints of the probe. Roots can crack probe casings, block visual monitoring, inhibit depth soundings, and provide a surface for debris build-up.
- A certified engineering geologist/ registered civil engineer must “field design” the screened interval for the probes and certify installation/completion of wells/probes in the as-builts required by the regulations.
The LFG regulations (Title 27, California Code of Regulations, Section 20923 and 20925) require that (1) the monitoring network is designed by a registered civil engineer or certified engineering geologist; (2) monitoring wells are drilled by a licensed drilling contractor or a drilling crew under the supervision of a design engineer or engineering geologist; (3) wells are logged during drilling by a geologist or geotechnical engineer; (4) the specified depths of monitoring probes within the wellbore are adjusted based on geologic data obtained during drilling and probes placed adjacent to soils which are most conducive to gas flow; and (5) as-builts for each monitoring well are to be maintained by the operator and submitted to the EA upon request.
The lengths and depths of screened intervals of probes constructed should be based on subsurface conditions (i.e., lithology, contacts, geologic structure, and ground water) and should consider zones that are the most likely pathways for LFG migration. Correlating the geology to the screened lengths and depths is essential for the effective monitoring for LFG and is considered part of the design of the monitoring network that must be certified by a registered civil engineer or certified engineering geologist. The as-built description should include the rationale for screen placement based on the geology and preferential pathways for migration including placement of mid-depth probe(s).
For more information contact: LEA Support Services, PermitTrainingAssistance@calrecycle.ca.gov