Guam oversight hearing details detections of banned pesticide dieldrin in northern wells; agencies outline fixes and limits to what’s known

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…