Landfill Compliance

Piezo-Penetrometer Cone
The diagram below shows three instruments. The first piezo-penetrometer cone is the side-sensing piezo-cone. This cone is used in the trash prism because it can better withstand the harsh enivronment than the tip-sensing cone, which is the second cone shown below. The tip-sensing cone is more sensitive than the side-sensing cone and is used in the vadose zone and native soil.

Tranducers with different pressure ranges can be installed in the cones for greater sensitivity. A porous element is placed around the transducer chamber to allow gas or liquid to enter. Inclinometers are installed inside the cones to insure more precise depth control.

The third cone shown below is a slide sampling cone. The slide sampler is closed and sealed with an o-ring and pushed to the desired sampling depth. Once the depth is achieved the push rods are pulled up which opens the screened section. Teflon tubing, attached to the sampler and is threaded through the push rods conveys the sample to the surface, where it is collected and analyzed.

Other devices can be attached to the CPT cone which will be discussed on another page.

Internal Conduit

It is possible to increase the production of existing collectors by mapping out the actual zone of influence of the collector. If a PPT is being performed within 50 feet of an existing collector the vacuum should be increased so that the vacuum will be very apparent if it is encountered in a zone in the PPT sounding. This information will assist in determining the zone of influence of the collector. The PPT has shown that vacuum influence flows outward from a collector like fingers, not like a balloon.

When the PPT encounters a high induced vacuum from a nearby collector and gas pressuures in the same sounding, which is common, it is not necessary to install another collector. As demonstrated in the attached diagram, an internal conduit can be installed using 3/4" diameter perforated PVC pipe connecting the vacuum zone to the gas pressure layer. The PVC pipe provides a pathway for the gas pressure to flow through to the vacuum zone and into the existing collector. The top of the PPT hole is grouted up. This eliminates the need to connect a collector to the header and the associated possibilty of air infiltration. This can increase the amount of gas being extracted by this collector by as much as 100%.

If this process is used around every existing collector in a landfill it could double the amount of gas for use in co-gen. operations, without the cost of adding another collector. It will also assist in keeping the landfill in compliance.

Multi-Stage Perimeter Probes

Current regulations require that landfills have perimeter probes installed, if there is evidence that methane gas is migrating from the landfill. A multi-stage probe is a probe with several chambers at various depths in the same bore hole. These types of probes are "allowed" but not required by the regulations to save costs. The chamber levels are sealed from each other with Bentonite clay. Many active landfills today have multi-stage perimeter probes installed at various spacings around the landfill.

Prior to performing a PPT investigation where an impacted perimeter probe is involved it is necessary to verify the readings from the probe. To do this, an Integrity Test was developed to insure that each chambler was sealed from the other chambers in the probe. It was during these tests that it was discovered that some of these chambers were cross-connected and were providing false low readings. It is believed that the Bentonite clay seals had dried out and created leaks.

Upon review of several landfill records of probe readings it is possible that many landfills have cross-connected probes. The current regulations do not require landfill operators to test the integrity of a probe after construction or over time. If records indicate that there is LFG in two or more chambers in a probe there is a possibility that they are cross-connected. Only an Integrity Test can verify this.

Unfortunately, a cross-connected probe will always give a false low reading never a false high reading. This may sound good at first but what is really happening is that higher concentrations of LFG are migrating off-site and to the groundwater. It is much more cost effective to intercept migrating LFG than it is to pump and treat groundwater. A leaking probe may never reach a concentration of 5% where an operator must take action.

Vacuum Influence on Probes
Many landfill operators and their consultants feel the best way to lower the LFG concentration in a perimeter probe is to install a LFG collector close to the impacted probe and put it under the vacuum influence of the collector. Some state regulations say this is not permitted but it is not enforced. This practice does return the probe to compliance but it is no longer monitoring the static conditions around the landfill. It is monitoring the condition of the nearby collector. Once outside the vacuum influence, LFG could still be migrating off-site. Only another probe in a static state can verify that the LFG is under control inside the trash prism. The PPT can pretest a location for a probe to verify that it is outside the vacuum influence of a collector.

Back Flow Prevention

PPT Profile investigations have identified another cause of impacted probes and groundwater. Based on PPT information it has been discovered that collectors that are installed on the perimeter of landfills to control LFG migration are not high producing gas collectors. However, when the flare or power plant is shut down, the header can become pressurized from high producing collectors in the center of the landfill and the LFG is injected down the low producing collectors on the perimeter of the landfill and allowed to migrate to probes or the groundwater. This is another reason why perimeter probes should not be in the zone of vacuum influence of collectors. One way to address this problem is to turn off the valves of perimeter collectors prior to shutting down the vacuum system. This is fine for planned shut downs but sometimes the vacuum systems go down in the middle of the night and no one is going to close valves on a dark landfill. There is also the time consuming problem of resetting the valves when the vacuum system is operating again. A better alternative is to install check valves on all perimeter collectors, which will prevent gas from entering the collectors and impacting probes or the groundwater.