What is Endothall and Why Does It Matter?

Endothall is an herbicide used to control aquatic weeds and algae in lakes, ponds, and irrigation canals. Water utilities sometimes apply it directly to source water reservoirs to manage plant growth. From there, it can pass through treatment systems and show up in finished tap water. It also enters water supplies through agricultural runoff, particularly in areas with heavy irrigation activity.

Across the U.S., endothall has been detected in 21 water systems. The average level measured is 3.061 parts per billion (ppb), with the highest recorded concentration reaching 9.6 ppb. While those numbers may sound small, even low-level exposure to pesticide residues in drinking water raises legitimate questions — especially for children and pregnant women.

At higher doses, endothall is toxic to the kidneys, liver, and gastrointestinal system. Animal studies show that prolonged exposure can damage the stomach lining and disrupt normal organ function (EPA). At the levels found in tap water, the immediate health risks are considered low for most adults. That said, the EPA acknowledges that long-term, cumulative exposure to pesticides like endothall warrants caution. Families with young children, people with compromised immune systems, or anyone drinking affected water daily should pay close attention to their local water quality reports.

The EPA has set a maximum contaminant level (MCL) for endothall at 100 ppb — the legal limit utilities must stay below. The good news is that none of the 21 systems where endothall was detected exceeded that limit. However, the EPA's legal limit reflects what's technically feasible to achieve at scale, not necessarily what's safe for long-term consumption. The Environmental Working Group (EWG) often points out this distinction, noting that regulatory limits sometimes lag behind the latest health science. No systems in this dataset tested above the EPA threshold, but detection itself is worth knowing about.

Geography tells an interesting story here. Florida leads all states with 9 systems detecting endothall — more than four times any other state in this dataset. That's not surprising. Florida's warm climate creates ideal conditions for aggressive aquatic weed growth, and water managers there rely heavily on herbicides to keep reservoirs and canals clear. Georgia and Minnesota each reported 2 systems with detections, followed by Wisconsin and Arizona with 1 each. Minnesota and Wisconsin's detections likely reflect lake management practices in those heavily lake-dependent states. Arizona's detection points to irrigation-heavy agriculture in an arid region where water is carefully managed and reused. In each of these places, the connection between water source management and what ends up in your tap is direct.

The most effective way to remove endothall from drinking water is reverse osmosis (RO) filtration. A quality RO system can remove up to 99% of pesticide residues, including endothall, by forcing water through a semi-permeable membrane that blocks contaminants at the molecular level. Activated carbon filters also help reduce herbicide levels, though they are generally less thorough than RO for this class of chemicals. If you live in Florida, Georgia, Minnesota, Wisconsin, or Arizona — especially near agricultural areas or managed water bodies — it's worth checking your annual Consumer Confidence Report (CCR). Your utility is required to send this to you every year, and it lists every contaminant detected in your local water. Echo Water's reverse osmosis systems are designed specifically to address the kinds of chemical contaminants that slip through standard municipal treatment. Knowing what's in your water is the first step. Filtering it out is the second.

Regulatory Standards for Endothall

Cities With the Highest Endothall Levels

States Most Affected by Endothall

Frequently Asked Questions

Related Contaminant Guides