Dieldrin - Timeline

See the summary timeline of events related to dieldrin and the mitigation efforts by GWA and other government services involved to protect our island's water system.

History and Background

Dieldrin is a pesticide; commonly used as a termiticide
Its use was limited to certain applications in the 70s and production was completely banned nationally in 1987
With respect to the Safe Drinking Water Act, there is no federal regulation for dieldrin and this may be due to a combination of factors to include:
- No widespread detection across U.S. systems
- At the time regulation was being considered, the pesticide was banned, and it was no longer in production
- There was an assumption that the occurrence of dieldrin would decrease over time since these insecticides were no longer being produced
Ultimately in 2003, the US EPA made a determination not to enact any primary drinking water regulation for dieldrin.

Presence on Guam

It was highly probable the pesticide was used on Guam, but GWA does not have information on where or how long it continued to be used. Its likely use would be application to soils under concrete slabs/foundations during building construction.
GWA does not have any information on whether the national ban was enforced on Guam, or if its use as a pesticide was allowed here after 1987
GWA does not have any information whether the disposal of the banned pesticide or soils to which it may have applied as a termiticide was regulated or monitored
GWA does have information that as far back as the mid-90s, there is data indicating its presence in groundwater

GWA’s Sampling and Reporting

GWA has regularly sampled for and reported on pesticides, including dieldrin, and other organic compounds in accordance with national Safe Drinking Water Act (SDWA) regulations to both U.S. EPA and Guam EPA
While Dieldrin was not regulated, other pesticides are and the analysis GWA utilized for pesticides under the SDWA included dieldrin. For this reason, GWA has accumulated this data on the chemical.
The results of all such testing and the presence of detected contaminants, regulated and unregulated have been reported to and is accessible to all customers
The manner in which this type of information is provided to customers is standard across all jurisdictions in the U.S. and is via Annual Water Quality Reports
These reports are sent to every customer and also made available on our website annually, as far back as 2007. Dieldrin has appeared on these reports with the range of concentrations found in all water samples collected throughout the year for the applicable report.

U.S. EPA Communications on Dieldrin

In 2012, shortly after federal inspectors reviewed GWA’s water system and laboratory reporting, U.S. EPA did send a letter to GWA, Guam EPA indicating the presence of dieldrin in groundwater and expressing that they could work with Guam EPA and GWA to assess any risks from the presence of this chemical in groundwater.
Our records show, that other than that initial letter, no information on risks associated with dieldrin, or further efforts on the part of U.S. EPA to enact regulations or require treatment for this unregulated chemical were shared with GWA.
Since 2016, GWA's discussions and meetings with various officials from U.S. EPA from 2016 and onward, dieldrin was never raised as an issue until 2022.
At that time, after another set of U.S. EPA inspections, GWA and Guam EPA were advised by U.S. EPA that dieldrin persisted to show presence in groundwater.

Guam EPA Regulatory Action

It is important to note that Guam EPA was not authorized to enact an action level for any contaminant until 2020 and only regained "primacy" or the status as the primary enforcement authority over specific rules within the Safe Drinking Water program for Guam in 2023.
In 2022, when Guam EPA was made aware of potential risks associated with the presence of dieldrin by U.S. EPA, the current leadership at Guam EPA expressed that it intended to look into whether regulations would be needed to address any risks.
In late 2023, GWA was advised by Guam EPA that it was beginning the first steps to establish regulations and described the expertise required, complexity of the process to evaluate risks and develop regulations
In 2024, GWA was provided the draft risk assessment document and proposed regulations. GWA provided comments to the proposed regulations.
In 2025, GWA was informed that the Guam EPA Board would be meeting to approve and enact the proposed regulations. On July 5, 2025 GWA was notified that the regulations would take effect on August 01, 2025.

GWA Action to Address the Presence of Dieldrin

Once GWA was informed in 2023 that there was a concern, we immediately engaged our engineering team to begin the planning and engineering required to construct permanent treatment systems
While GWA did not know what the regulations would be, or what regulatory levels would be established, discussions with Guam EPA indicated that they were concerned with three particular wells: Y-15, D-17 and M-4
As Guam EPA continued their work to assess the risks and develop proposed regulations, GWA issued a task order for the design work to include re-purposing existing treatment equipment that was no longer in active use, to two of the three wells identified (D-17 and M-4), and new treatment equipment to be procured and installed for the third site (Y-15).
Later in 2024, as Guam EPA detailed their proposed regulations and action levels, this information was used by the engineering team to complete design work for the treatment systems needed to effectively treat for dieldrin.
When the design work was completed in 2024, the construction project was put out for an IFB which was issued on February 27, 2025
Bids were due on April 21, 2025. The sole bid submitted was for $7,144,094.00, approximately $1,559,719 (or 28%) over the estimated cost of $5,584,375.00
GWA followed appropriate procurement procedures to align the cost of the project closer to the available funding, and obtained funding approval.
Final approval required for issuance of the construction contract was obtained in September 2025.

Extent of Dieldrin in Source Water

Historic data GWA has collected on this pesticide indicate that:

Of all GWA sources, including wells, springs, surface water and purchased water there are 124 sample locations
31 showed non-detectable levels, including spring, surface water and purchased water sources
Of the 93 showing detections, the vast majority of 84 locations showed detections of less than 0.1 PPB – this indicates widespread presence in very low concentrations, well below the Guam EPA Action Level.
7 wells has concentrations higher than 0.1 PPB but still below the Action Level, of which only 5 are in regular operation (primarily in northern Mangilao and Dededo)
Only 2 wells were above the Guam EPA Action level, only one of which remains operational
- Y-15 – for which an advisory was issued to comply with EPA’s new regulations on dieldrin
- D-17 – which is off-line

Public Discussions & Notifications about dieldrin prior to September 12, 2025:

Guam EPA Board of Directors Meeting – December 14, 2023 (Dieldrin in Guam’s Drinking Water)
Oversight Hearing of the Northern Guam Lens Aquifer and the Status of Guam’s Water Resources – January 4, 2024
GWA Work Session – January 16, 2024 (GWA GM Report, ID/IQ for Professional Project-Construction Management Services for DCA Task Order for Groundwater Wells Treatment Design CIP for Dieldrin)
CCU Board Meeting – February 27, 2024 (GWA Resolution 11-FY2024 Relative to Approving the Five-Year Financial Plan and Capital Improvement Plan to support the continued operations and maintenance of the Authority and to timely meet necessary and urgent impending regulatory compliance requirements for PFAS and Dieldrin)
Informational Briefing – Guam Legislature – April 3, 2024 (Updates on Dieldrin Regulations)
Dieldrin Draft Risk Assessment Document (DRAD) and first public comment/hearing notice Issued – June 10, 2024
DRAD second public comment/hearing notice issued – June 17, 2024
Dieldrin Proposed IAL Public Hearing held – June 24, 2024
GWA Work Session – July 16, 2024 (DCA Task Orders for Groundwater Wells Treatment Design CIP for Dieldrin)
Guam EPA Board of Directors Meeting – November 21, 2024 (Motion to Adopt IALs Passed)
Guam EPA Board of Directors Meeting – December 19, 2024 (motion for the board to direct Guam EPA to communicate with all PWS and to advise the board on implementation of the IAL for dieldrin
GWA Work Session – May 20, 2025 (GWA Resolution 42-FY2025 Relative to the Approval of Construction Contract for the Deep Well D-17, Y-15, and M-4 GAC System Dieldrin Treatment)
CCU Board Meeting – May 27, 2025 (GWA Resolution 42-FY2025 Relative to the Approval of Construction Contract for the Deep Well D-17, Y-15, and M-4 GAC System Dieldrin Treatment)

Key Takeaways

While there is widespread presence of dieldrin in source water samples, the vast majority of these sources indicate low background concentrations well below the Action Levels
GWA did not put dieldrin in the groundwater, does not have the authority to find the source of this chemical, but has been working to implement permanent solutions to remove from the affected wells
Only one operating well out of GWA’s 90+ operating wells is above the Guam EPA IAL
Y-15 serves primarily the Santa Rosa zone in Yigo, and the number of customers affected by the Do Not Drink without Treatment advisory is limited
However, Y-15 cannot be shut-down for any extended period of time without causing outages to a larger area, if water from outside the Santa Rosa zone is used to make up for the loss of production
GWA has been making operational adjustments to reduce the affected area, and is consulting with Guam EPA to revise the boundary of the affected area
GWA has allocated millions of dollars to construct permanent treatment systems for the three wells identified by Guam EPA
GWA has expedited construction of an interim treatment system that should be operational by mid-October (2025).
The interim treatment system has been approved by Guam EPA and once testing confirms that treatment is effective, Guam EPA may eliminate and lift the September 2025 "Do Not Drink without Treatment
In order to address the inconvenience caused by this situation, GWA is also pursuing the use of Point-of-Entry or Point-of-Use systems at customer locations

GWA GM Miguel Bordallo updates mayors on dieldrin situation.

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