A joint legislative oversight hearing convened this week to examine detections of the banned pesticide dieldrin in Guam’s water system and to press regulators for timelines, testing data and mitigation measures.

Guam Water Works Authority (GWA), the Guam Environmental Protection Agency (Guam EPA) and the Department of Public Health and Social Services (DPHSS) told senators that dieldrin—a persistent organochlorine pesticide banned in the United States in 1987—has been detected intermittently in groundwater testing and that three production wells have been identified as sites of concern: Y‑15, D‑17 and M‑4. Guam EPA established an interim action level (IAL) for dieldrin in late 2024 and set an implementation date of Aug. 1, 2025.

Why it matters: Dieldrin is classified by U.S. EPA as a probable human carcinogen (Group B2) on the basis of animal data; Guam public health officials said human data are limited and that a definitive human health association would require a focused epidemiologic study. Residents have asked how long the contamination has been present, who is exposed and what will be done to keep tap water safe.

What agencies said about the contamination and immediate response

- Guam Water Works Authority said it was notified by U.S. EPA of dieldrin detections in 2012 but that no federal drinking‑water standard existed to trigger corrective requirements. GWA submitted designs and put an interim GAC treatment package out to bid; an interim GAC installation at production well Y‑15 was completed in October and initial post‑GAC samples returned non‑detect results. Thomas Cruz, assistant general manager for operations and acting GWA general manager, said the GAC treatment produced “no dieldrin detected in water after the GAC treatment.”

- Guam EPA described the IAL process, explaining the health‑risk basis used to set the range of levels considered and the regulatory timetable. Administrator Michelle Lastimosa said Guam EPA worked with U.S. EPA Region 9 and consulted toxicologists while preparing the IAL and that public comment and board review extended the rulemaking timeline; the board approved a final draft in November 2024.

- The Department of Public Health and Social Services presented a preliminary ecological review of cancer diagnoses on Guam from 2007 through early 2022, focusing on female breast cancer because some prior studies have suggested an association. Territorial epidemiologist Patrick Soto said ecological (group‑level) data cannot establish causation and recommended a case‑control epidemiologic study or targeted biomonitoring to evaluate individual exposures.

Testing, monitoring and public notices

Agencies said GWA continues to sample wells and entry points to the distribution system. For the wells driven by the IAL schedule, Guam EPA directed GWA sampling frequency increases: Y‑15 is being sampled weekly and the most‑affected wells will be sampled at least monthly while mitigation is underway. Guam EPA mailed compliance notices to about 1,112 ratepayers in the affected Santa Rosa/Jigo area under its tier‑1 advisory rules.

Guam EPA said it has coordinated closely with U.S. EPA technical staff, requested technical assistance and sought funding for sampling and remediation. U.S. EPA staff from the National Enforcement Investigation Center and CERCLA/Superfund personnel have conducted inspections and are discussing technical assistance; Guam EPA said a Superfund (CERCLA) pathway could be pursued if soil sampling confirms a localized source that meets the program’s criteria.

Interim and permanent mitigation steps

GWA said it relocated smaller, existing GAC vessels to Y‑15 as an interim fix; the interim vessels were operating in parallel and provided non‑detect finished‑water results in initial tests. Engineering lead Brett Raley said the interim vessels are expected to provide 12–18 months of service before media change or breakthrough, and that the permanent design will install larger vessels in a lead‑lag configuration. The permanent construction contract covering treatment for Y‑15, D‑17 and M‑4 was discussed in the hearing; GWA estimated contract value in testimony at roughly $7.4 million for the three wells (design and construction).

Guam EPA and GWA said U.S. EPA committed to funding short‑term household mitigation (point‑of‑entry/point‑of‑use filters) and longer‑term treatment systems. Guam EPA cited an estimated per‑household cost of roughly $2,700 for whole‑house POE units not including installation; GWA reported roughly 80 customers had registered for assistance and eight reimbursement requests had been received at the time of testimony.

Health data and further study recommendations

DPHSS presented preliminary cancer registry and mortality data and emphasized limits of ecological analysis. Patrick Soto said that, because dieldrin bioaccumulates in fat, a more informative approach would be biomonitoring and a case‑control study matching diagnosed individuals with controls to measure personal exposure histories. DPHSS estimated an initial community biomonitoring effort to cost on the order of $1.7 million (ballpark, based on the number of potentially affected residents and assumed per‑test costs) and said the agency would seek legislative support for any study funding.

Limits of current knowledge and enforcement

Agencies repeatedly told senators that dieldrin is an “emerging/unregulated” contaminant at the federal level (no current U.S. EPA MCL for dieldrin). Guam EPA used its locally authorized interim action level authority (adopted by the Guam EPA board) to require monitoring and mitigation. Officials said that detections reported in 2012 were disclosed in GWA annual water quality reports but that the absence of an enforceable federal standard delayed specific regulatory triggers until Guam developed its IAL and enforcement authority.

Next steps and open issues

Senators pressed for faster source‑identification sampling (soil sampling of demolition sites, construction debris and other possible historic sources), and Guam EPA said it has theories—demolition and disposal of pesticide‑treated construction materials in northwestern Guam among them—and is exploring Superfund/CERCLA technical assistance with U.S. EPA. Agencies agreed to continue frequent sampling, to post sampling results publicly, and to return to the legislature with updates and further plans. GWA committed to completing permanent GAC installations (contract timeline discussed by engineering was summer 2026) and to continuing interim customer relief (fill stations, bottled water, POE/POU support) until permanent fixes are in place.

What is unresolved: whether a specific localized source can be identified (Guam EPA said identification may be difficult and may require targeted soil sampling), the scale and cost of a public biomonitoring study, and the timetable for lifting public advisories (Guam EPA said the IAL procedure generally uses four quarters to form an annual running average for compliance but that it was consulting with U.S. EPA and stakeholders about how to lift an advisory if weekly/monthly data show repeated non‑detects before a full year of data are available).