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Bonsall Landfill

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The Bonsall Landfill is a closed landfill owned and maintained by the County of San Diego Department of Public Works. It was opened in 1968, and stopped accepting waste in 1985.
The landfill is highly regulated by a number of agencies including the Regional Water Quality Control Board (RWQCB), the Department of Environmental Health Local Enforcement Agency (LEA), and the Air Pollution Control District (APCD). To comply with the regulations and as owner of Bonsall Landfill, DPW is responsible for the landfill’s maintenance and monitoring, operations of environmental controls, and any required corrective actions. Over the years, DPW has made repairs to the landfill cover and drainage and installed groundwater monitoring wells and a landfill gas extraction system.


There are numerous challenges common to any landfill. As the waste decomposes it becomes more compact which can cause the landfill to settle unevenly. This settlement can allow water to pond on the surface of the landfill. Settlement can also create cracks in the landfill cover. Ponding or cracking on the landfill may allow surface water to come in contact with waste which could contaminate groundwater. The surface of the landfill also must be protected from erosion so that the cover is not washed away exposing waste.

As the waste decomposes it also produces landfill gas and liquids. The gas and liquids may contain Volatile Organic Compounds (VOCs) which are also commonly found in household waste. Without preventive measures and other environmental controls these VOCs can seep into the groundwater or escape into the atmosphere.

Bonsall Landfill SlopeThe Bonsall Landfill is an old, unlined landfill that is situated on fractured rock, which increases the challenge of ground water seepage. However, numerous monitoring wells have been installed around the site and are sampled semi-annually to monitor groundwater quality. Two wells show small amounts of  dichlorodifluoromethane (DCDFM).  The amounts found are within the range of levels established for safe drinking water.  DPW has also identified and sampled private wells in the vicinity of the site. No contamination from the landfill has been found in drinking water wells in the area.


What the County is doing

DPW is working closely with the various regulatory agencies to continuously maintain and monitor the Bonsall Landfill site and implement corrective actions. This work includes semiannual sampling and reporting of groundwater wells, regular maintenance and monthly inspection and reporting of the gas extraction system, as well as monthly inspections and as-needed repair of the protective ground cover, erosion controls, and drainage systems. The County recently completed road improvements to prevent ponding, as well as slope and drainage improvements.

With these efforts, detected amounts of VOCs have significantly reduced over time. DPW will continue its efforts and work closely with the RWQCB.

A letter to area property owners has been sent with specific information about the Bonsall Landfill.

Reports are filed with the Regional Water Quality Control Board. Their Cleanup and Abatement Order letter of October 1, 2010 is available here. 



Some Frequently Asked Questions: 

Is groundwater beneath my house safe to drink?

Generally the groundwater in the vicinity of the landfill is of poor quality and not suitable for drinking. Due to this, a majority (if not all) of residences near the landfill are serviced by municipal water sources. The most recent data from semi-annual groundwater monitoring performed at the Bonsall Landfill shows that there were no volatile organic compounds (VOCs) found at limits that exceed the Maximum Contamination Limit (MCL) for drinking water.  No drinking water wells in the vicinity of the site have shown to have impacts resulting from the landfill.


Can I irrigate with water impacted with low level volatile organic compounds (VOCs)?

Prior water quality studies in the vicinity of the landfill indicate groundwater has naturally-occurring elevated concentrations of some general chemistry constituents including chloride, sulfate, and total dissolved solids. Elevated concentrations of these constituents make the groundwater unsuitable for irrigating some types of crops. Recent sampling completed in July 2012 indicated detected a low concentration of a VOC. Due to the volatile nature of these types of contaminants and the low concentrations observed in groundwater, irrigation using sprinklers would likely alleviate most if not all of the contamination present

Since the contaminants are “volatile” should I be worried about it coming up through the ground?

The County monitors the perimeter landfill gas (LFG) migration probes surrounding the landfill monthly to document that the levels of methane at the landfill boundary are safe. Additionally, the County annually collects samples from the probes located closest to residences near the landfill to evaluate the presence of VOCs in soil vapor. Recent soil vapor sampling indicates that VOCs detected do not pose a risk to human health. Currently there is no significant risk to nearby residences due to landfill gas migration.

Are there any landfill gas (methane) problems at the site? Who oversees the flare system?

The landfill is routinely monitored for methane gas emissions, by the County Department of Public Works (DPW), the County Department of Environmental Health, Solid Waste Local Enforcement Agency (LEA).  Monthly methane monitoring conducted by the County indicates there are no fire or explosion hazards associated with landfill gas/methane. The Air Pollution Control District regulates the emissions from the flare system. The flare and the flare emissions are in compliance with all regulatory requirements.

Why weren't residents notified of the contamination sooner?

Testing and monitoring has been ongoing at the landfill since the early 1990s. Currently, offsite property owners who have their wells tested as part of the County’s monitoring program receive the results to the tests along with an explanation of the data. Interest in the concentrations of organic compounds in groundwater was initiated by the San Diego Regional Water Quality Control Board in September 2008. Since that time, the County has been working with the San Diego Regional Water Quality Control Board to perform studies to define the impacts to groundwater and take action to address them.

What is the County doing to ensure safety and improve groundwater quality?

The source of the groundwater contamination is likely attributed to landfill gas at the Bonsall Landfill. Removal of a large amount of waste from within the landfill is not currently a feasible option to address these issues. Therefore, the County has been proactively focusing on increasing control over the sources of impacts by upgrading the landfill gas control system and improving surface water drainage from the site. Landfill gas system upgrades were completed in March 2010, and the first phase of drainage improvements were completed in early October 2010. These measures are expected to have a positive impact on groundwater quality near the landfill. In addition, the County is currently in the planning stages for the second phase of drainage improvements which are scheduled to be completed by the fall of 2011.

How long do landfills emit gas/contaminants? Has the Bonsall landfill reached its maximum potential? When will it become dormant?

There are many variables that contribute to the gas production of a landfill, including the nature of the waste, the moisture content of the waste, the climate of the area, etc. In dry areas such as southern California, the waste within landfills decays more slowly than in wetter areas, and therefore landfills here generate lower volumes of gas over longer periods of time than landfills in wet climates. It appears that the Bonsall landfill, which stopped accepting trash over 25 years ago is likely to continue to decline in gas production, however it may continue to produce low quantities of gasses for decades.

What if I have other questions, or need more information?

You can contact Michele Stress at (858) 694-2691, or Candace Gibson, program coordinator, at (858) 495-5447.