What is Granular Activated Carbon (GAC) Treatment?

GWA's Surface and Springs Unit recently performed the annual replacement of Granular Activated Carbon (GAC) filters at the Tumon Maui Well. This same type of filtration treatment will be installed at other well sites to filter dieldrin and other contaminants found in the groundwater.

How Granular Activated Carbon Treatment Works to Remove Dieldrin from Well Water

When we turn on the tap, we expect safe, clean drinking water. But sometimes, legacy contaminants from past agricultural or industrial use can linger in groundwater. One such contaminant is dieldrin, a pesticide banned decades ago due to its toxicity and persistence in the environment. Unfortunately, because dieldrin breaks down very slowly, it can still be found in some in the environment and our groundwater today.

A highly effective solution to address this issue is granular activated carbon (GAC) treatment. This technology is used at the well level to protect communities by removing dieldrin and other organic contaminants before they ever reach your glass.

What is Granular Activated Carbon (GAC)?

Granular activated carbon is a filtration medium made from organic materials, like coal, coconut shells, or wood, that are processed to create a highly porous structure. Think of it as a sponge on a microscopic scale. One pound of GAC can have a surface area of over 100 acres. This massive surface area allows it to adsorb (not absorb) contaminants, meaning pollutants stick to the carbon surface rather than passing through.

How GAC Removes Dieldrin

Dieldrin belongs to a class of chemicals known as organochlorine pesticides. These compounds are:

• Hydrophobic: they don’t dissolve well in water

• Persistent: they don’t degrade easily

• Toxic: can be harmful even in small concentrations

Because of these properties, dieldrin is particularly well-suited for removal by activated carbon.

Here’s how the process works:

1. Groundwater Enters the GAC System

 Water pumped from the well is directed through tanks filled with GAC.

2. Adsorption Begins

As water flows through the carbon bed, dieldrin molecules are attracted to and held by the carbon’s porous surfaces. The chemical bonds and surface forces between the contaminant and the carbon keep the dieldrin in place.

3. Clean Water Exits

 By the time the water exits the treatment system, proper system sizing ensures all the dieldrin is removed.

4. Monitoring and Replacement

Over time, GAC becomes saturated and loses effectiveness. Routine monitoring ensures that the carbon is replaced or regenerated before contaminants can slip through.

Why Use GAC at the Well Level?

Installing GAC treatment directly at the wellhead has several advantages:

• Protects the entire water supply before distribution

• Removes a broad spectrum of contaminants (not just dieldrin, but also other pesticides, solvents, and even taste- and odor-causing compounds)

• Provides flexible and scalable treatment for small community systems

• Offers proven and reliable technology widely used in drinking water treatment

Safeguarding Communities

Dieldrin contamination is a legacy issue, but one that modern treatment can manage effectively. By using granular activated carbon at the well level, communities can ensure that their water is safe and free from harmful pesticides.

This technology doesn’t just remove contaminants, it restores peace of mind that every drop of water is clean, safe, and healthy.