Understanding Diquat

Background

Friends of Great Hill Pond works to manage the invasive aquatic plants in Great Hill Pond, including curly-leaf pondweed. Some of that work uses a herbicide called diquat, applied by a licensed contractor, Pond and Lake Connection, under a permit issued by the State of Connecticut. Diquat is sold for aquatic use under several brand names, including Reward.

This page summarizes what diquat is, how it is used in this setting, and what published research and regulatory reviews report about its behavior in water, its effects on aquatic life, and its human health profile. It is written as a plain-language overview of the evidence. Each claim links to a primary source, and the full list of sources appears at the end.

What Diquat Is and How It Works

Diquat is a fast-acting contact herbicide that has been used to manage aquatic weeds for decades [5][7]. "Contact" means it works on the parts of the plant it actually touches, rather than being carried throughout the plant. It is non-selective, so it affects a broad range of plants it contacts [5][7].

The plant absorbs it through its leaves, and it needs sunlight to work. In the presence of light, diquat triggers the release of reactive oxygen compounds that rapidly break down plant cells [4][5][7]. The effect is quick and visible: on a sunny day, treated plants can look water-soaked within hours, with browning and blackening of the tissue within a day or two [7].

In a pond setting, diquat does not act through plant roots in the sediment. It binds so tightly and so quickly to bottom sediment that it is inactivated before it can reach roots [4].

How It Is Applied

For aquatic treatment, diquat is applied by a licensed applicator, under a state permit, to the water over the target area at a low concentration. Pond and Lake Connection applies the product from an airboat, at labeled rates that are adjusted for water depth, plant species, density, biomass, and other conditions present at the time of treatment. The product label sets the maximum amount that can be used, and typical treatment concentrations reported in the research are well below the levels that regulators have reviewed for safety [5][10].

Diquat is labeled to control pondweeds in the genus Potamogeton, the group that includes curly-leaf pondweed (Potamogeton crispus), along with other common submerged weeds such as watermilfoil and elodea. The label controls every Potamogeton species except one, which is not present in Great Hill Pond [10].

The current product label directs that no more than half of a waterbody be treated at a time, with a wait of at least seven days before an adjacent area is treated, and it directs applicators to begin along the shore and work outward so fish can move into untreated water [1][10]. This staging addresses a specific risk: when a large mass of weeds dies and decomposes at once, the decomposition can lower dissolved oxygen in the water and harm fish. Treating in stages limits the amount of decaying material at any one time [1][4].

What Happens to It in the Water

Diquat leaves the water column quickly. Studies find that roughly 80 to 95 percent of the diquat applied binds to sediment within about two days [4]. Concentrations in the water itself drop to essentially undetectable levels within about one to two weeks [4]. In field trials, diquat was undetectable in open water more than about 200 feet from a treated plot, and gone within a week [5].

Diquat also does not build up in the food chain. It is non-volatile, so it does not evaporate off the water into the air, and it does not accumulate in the tissues of aquatic animals to any meaningful degree [4][6].

Once diquat is bound to sediment, however, it can remain there for a long time, sometimes many months or longer [1][5][6]. In this bound state it is inactive, locked onto clay particles rather than free in the water or available to organisms [1][6]. Diquat is therefore persistent in sediment, but as a bound, inactive residue rather than as a compound circulating in the water column.

Swimming and Recreational Exposure

EPA has concluded there are no dietary or residential risks of concern from diquat's registered uses, including exposure from swimming in treated water [1][2]. EPA also found that risks to drinking water from both land and aquatic uses are not of concern, largely because diquat stays with sediment rather than moving into water supplies [2].

The product label does not include a swimming or fishing restriction for aquatic use. The holding periods it does set are short and specific to certain uses, such as drinking water, watering livestock, and irrigation, on the order of one to a few days depending on the application rate [6][10].

Effects on Fish and Other Wildlife

Across 13 published experiments, diquat caused no direct fish deaths at concentrations at or below 1.0 part per million, which is below the label maximum [4]. In studies at concentrations somewhat above the recommended rate, bluegill growth, reproduction, and spawning were not affected [6].

Aquatic invertebrates are the more sensitive group, and the product label states directly that diquat is toxic to aquatic invertebrates [4][10]. Two factors provide context. First, the same sediment binding that clears diquat from the water also reduces exposure to these animals in a natural pond compared with a clean laboratory test system [6]. Second, observed effects in the field have tended to be local and temporary, with populations recovering, rather than pond-wide or permanent [4][6].

A significant practical consideration is not the chemical's direct toxicity but the decline in dissolved oxygen that can occur when treated plants decompose [1][4].

Human Health

Regulatory reviews characterize diquat's human health profile as follows:

• The greatest human exposure is to applicators handling the concentrate, not to the public. EPA's review identified an inhalation risk for handlers and, in response, required respirators and eye protection for applicators [1]. For this reason, application is carried out by licensed professionals using specified protective equipment rather than by the general public.
• EPA has set a Maximum Contaminant Level for diquat in drinking water of 0.02 milligrams per liter, a standard that states including Massachusetts have adopted [4].
• The reference dose, meaning the daily amount considered safe over a lifetime, is roughly 0.002 to 0.005 milligrams per kilogram of body weight per day, depending on the agency and study [4][8].
• The most sensitive long-term health effect identified is cataract formation, and that appears only after two or more years of continuous daily exposure at doses far higher than anyone would encounter from a treated pond [4][8].
• EPA classifies diquat as not likely to be carcinogenic to humans [4][8].
• Studies found no reproductive or developmental effects when diquat is taken by mouth [4][8].
• A small number of human deaths have been documented from diquat, and in every case the person had swallowed a substantial quantity, at least 15 milliliters, of the concentrated, undiluted product [4]. This is a substantially different exposure than swimming in water treated at label rates.

Common Questions

Did the European Union ban diquat?

The European Union chose not to renew diquat's approval in 2018, and products were withdrawn over the following months [9]. The basis for that decision was concern about exposure to people applying it and to bystanders in agricultural settings, and about risk to birds, rather than residues in food or water [9]. The European Union kept its food residue limits for diquat in place, indicating the concern related to application exposure rather than the safety of treated produce or water. That agricultural spraying scenario, in which diquat is used to dry down field crops, differs from subsurface application to a pond by a licensed applicator.

Is diquat the same as paraquat?

No. Diquat and paraquat are chemically related, which is a common source of confusion, but they are different compounds. Paraquat is substantially more acutely toxic to humans and is the compound associated with the most serious health concerns. Diquat is a distinct and considerably less toxic chemical.

Who faces the greatest exposure?

The most significant human exposure is to applicators handling the concentrate, not to the public. EPA's review identified inhalation risk for handlers and, in response, required respirators and eye protection for applicators [1]. For this reason, application is carried out by licensed professionals using specified protective equipment rather than by the general public.

Summary

Applied as described here, by a licensed contractor, under a state permit, at low concentration, and in stages, diquat is a well-studied compound with a favorable safety record for people and recreation, real but localized and short-lived effects on some aquatic invertebrates, and one long-term characteristic, inert binding in sediment, that does not translate into ongoing exposure in the water column.

Published by Friends of Great Hill Pond. greathillpond.org