Home and Landscape

Urban Pesticides, Fertilizers, and Water Quality


Pesticides and fertilizers applied around homes, residential or commercial landscapes, school sites, and other areas can enter our creeks, rivers, and oceans, degrading water quality. While pesticides and fertilizers can be very useful in controlling pests and helping plants grow, their overuse and misuse can harm the environment and human health.

What Are Pesticides and Fertilizers?

Pesticides are materials intended to control, prevent, kill, reduce, or repel pests. They can be made from natural ingredients or based on synthetic chemicals. There are many types of pesticides, and each is meant to be effective against specific pests. All pesticides are toxic at some level, but each varies in their toxicity to humans and other animals.

The term “-cide” comes from the Latin word “to kill.” Common pesticide types include insecticides, herbicides, fungicides, and rodenticides. Pesticides also include repellents, some pheromones, growth regulators, and disinfectants.

The chemical in a pesticide that directly affects the pest is called the active ingredient. The pesticide product label will list the percentage of how much of the active ingredient is contained in the product, which varies widely and is specific to how much of the ingredient is necessary to control the target pest. Active ingredients are different than trade names. BugOut is an example of a trade name, while imidacloprid is an example active ingredient. Multiple trade names may contain the same active ingredient.

Fertilizers are materials added to the soil to increase its fertility. They can be natural or synthetic and are made up of the three nutrients essential for plant growth and survival: nitrogen (N), phosphorus (P), and potassium (K). When you look at a bag of fertilizer, it will list the ratio, or grade, of N-P-K. A common ratio is 10-10-10, where the fertilizer is made up of 10 percent nitrogen, 10 percent phosphorus and 10 percent potassium. Different ratios of N-P-K exist because some plants require more of one nutrient than another, and some soils may have plenty of one nutrient, while not enough of another. Some fertilizers, like weed and feed products, contain herbicides in them as well. It is important to use the proper amount and correct N-P-K ratio to avoid overfertilizing and runoff into streams.

What Is Toxicity?

Toxicity is the ability of a chemical to injure a person, animal, plant, or other organism. All chemicals are toxic to some organisms to some degree, but pesticides are specifically designed to be toxic. Some pesticides are more toxic than others so present a higher risk of injury after exposure. The short-term (acute) toxicity of a pesticide is indicated by its signal word.

Pesticide toxicity can vary depending on the organism exposed. For example, a pesticide that presents low toxicity to humans and other mammals can be highly toxic to fish, bees, and birds.

Fertilizers can also be toxic. Overfertilization of plants can result in leaf burn, reduced production, and plant death. When these nutrients enter our water, they change the water quality. Fertilizer pollution can make water unsafe for recreation and drinking and can result in dead zones—areas of water where aquatic life cannot survive.

What Is Water Quality?

Water quality is a measure of the suitability of water for a particular use such as drinking, irrigation, fishing and recreation, or aquatic habitat. You can’t determine the quality of water just by looking at it.  Water quality is based on physical, chemical, and biological characteristics. Different water quality standards exist for each of the uses above.

Why is water quality important?

Pesticides and fertilizers can pollute our drinking water, potentially posing serious health risks, especially for young children. If you get your water from a well, pesticides and fertilizers can filter through the soil and make their way into the groundwater that you eventually drink. Even if your water is processed at a wastewater treatment facility, not all pesticides and fertilizer nutrients are removed during treatment.

When these products, especially pesticides, enter waterways, they can be deadly to aquatic organisms. Tiny creatures known as aquatic invertebrates are most susceptible to these chemicals. Many larger organisms in the aquatic ecosystem rely on these smaller aquatic invertebrates to survive. When aquatic invertebrates are killed by pesticide and fertilizer runoff, it affects the entire food chain. In the case of pesticide use, these aquatic organisms are often referred to as nontarget organisms. Other non-target organisms may include birds, reptiles, and bees.

How Do Pesticides and Fertilizers Get into Waterways?

Pesticides and fertilizers can enter storm drains, indoor drains, and groundwater through improper disposal and runoff. These drainage points often flow directly into rivers, streams, lakes, and oceans.

Storm drains

Storm drains are commonly found in curbside gutters along streets and are intended to move excess rain, snow melt, and irrigation water into nearby waterways. Storm drains are meant to carry clean water, but contaminants like pesticides and fertilizers often end up in these drains. Irrigation, wind, or rain can carry pesticides and fertilizers applied around homes and other buildings from hard surfaces like sidewalks and driveways to storm drains. They may also enter storm drains through spills and illegal dumping (improper disposal).

Indoor drains

Indoor sewer drains collect wastewater from toilets, sinks, washing machines, and showers in homes, restaurants, businesses, and other buildings. Pesticides and fertilizers can enter these drains by improper disposal or laundering clothing contaminated with chemical residues. Sewer drains carry the wastewater to treatment plants where the water is filtered and disinfected before being released into waterways. However, these wastewater treatment plants are unable to remove all pesticides, so these chemicals, when present, are released with the treated water.


Applying pesticides to the soil as granules or drenches, or spilling or dumping garden chemicals onto the soil can allow them to enter the groundwater below. Some pesticides can percolate with water through soil and leach downward into underground aquifers. This contaminates groundwater that may be used for drinking water by residents with wells and can eventually enter surface waters.

How to Protect Waterways

Pesticides and fertilizers are valuable resources for indoor and outdoor pest and landscape management. However, their overuse and misuse in the home and garden can harm water quality. Below are some simple tips for protecting water quality and minimizing environmental harm.

Use nonchemical solutions

Before applying any pesticide or fertilizer, first decide if it is necessary. Often problems are incorrectly diagnosed, leading to wasteful pesticide and fertilizer use. Use an integrated pest management (IPM) approach to accurately identify, treat, and prevent pest problems. Are you sure it is an insect pest causing damage and not something else? Does your plant need to be fertilized, or does it just need more consistent watering? To learn more about how to reduce pesticide and fertilizer usage with IPM, visit Applying IPM in your home and landscape.

Read the label!

Before using a pesticide or fertilizer you must read, understand, and follow the product label for safe use. The label will provide you with valuable information that you need to safely apply the product, such as how much you should use, when you can safely apply, and what species are sensitive to the product.

Correct disposal

When disposing of leftover or unused pesticides or fertilizers, take the product in its original container to your local household hazardous waste disposal facility. Go to the earth911 website or call 1-800-CLEANUP (1-800-253-2687) to find a disposal site in your area. Never dump pesticides or fertilizers down drains, into waterways, or into trash cans—this is illegal and can contaminate waterways. To read more about safe use and disposal, read the Quick Tips: Garden Chemicals: Safe Use and Disposal.

Check the weather

To ensure pesticides and fertilizers don’t end up in waterways, and to protect yourself when applying, make sure you apply under the right weather conditions and check the forecast for the next 24 hours. You can safely apply when the weather is calm, meaning wind speeds less than 10mph, and no rain or snow forecast.

Know which pesticides are toxic to aquatic organisms

Pyrethroids, organophosphates, and fipronil are all highly toxic to aquatic organisms and are commonly found polluting waterways. Limit the use of these pesticides or use them in ways that minimize their movement into water.

Pyrethroids are a group of insecticides that are persistent in the environment and capable of attaching to soil particles. They can be found in many household and garden pesticides and include the active ingredients bifenthrin, cypermethrin, allethrin, fluvalinate, permethrin, cyhalothrin, and cyfluthrin.

Organophosphates are a group of insecticides that are highly water soluble. They include the active ingredients diazinon, acephate, malathion, bensulfide, and chlorpyrifos. Many products in this group were removed from the California non-agricultural market decades ago, but they continue to be found in our waterways. Acephate and malathion are still available on the consumer market.

If you discover old products containing diazinon and chlorpyrifos on a shelf in your garage, they are illegal and unsafe to use so take them to your local household hazardous waste disposal site.

Fipronil is a common active ingredient in many household and garden pest control products. In addition to aquatic organisms, fipronil and its byproducts are highly toxic to animals and bees. Fipronil is often found in spot-on flea and tick pet treatment products which can enter waterways during pet bathing by being washed down tub or sink drains or washed off into storm drains.

To learn more about common active ingredients and their potential hazards, visit our active ingredient database.

Use less toxic insecticides

While all types of pesticides can harm water quality, insecticides are most likely to harm aquatic organisms. Less toxic insecticides include insecticidal soaps and oils, and products obtained from plant materials. These products tend to break down quicker in the environment. To learn more about these products, see Quick Tips: Less Toxic Insecticides.

Storm drain showing text “No dumping! Flows to river.”
Storm drain showing text “No dumping! Flows to river.” Credit: Lauren Fordyce, UC IPM
Person applying granular pesticide to a landscape bed.
Person applying granular pesticide to a landscape bed. Credit: Cheryl A. Reynolds, UC IPM, Davis
Person spraying a lawn with a pesticide.
Person spraying a lawn with a pesticide. Credit: Cheryl A. Reynolds, UC IPM, Davis
Aquatic invertebrates such as the water flea, <i>Ceriodaphnia dubia</i>, can be harmed by pesticides.
Aquatic invertebrates such as the water flea, Ceriodaphnia dubia, can be harmed by pesticides. Credit: Jack Kelly Clark, UC IPM
A parks worker inspecting a water quality sample in a creek.
A parks worker inspecting a water quality sample in a creek. Credit: A. Katrina Hunter


Text Updated: 09/2023