What Is Alachlor?

Alachlor is a synthetic pre-emergent herbicide historically used to control annual grasses and broadleaf weeds in agricultural crops, especially corn, soybeans, peanuts, and other row crops. It belongs to the chloroacetanilide class of herbicides, a group designed to inhibit early plant growth by interfering with seedling development. Alachlor was widely known under trade names such as Lasso and became important in intensive row-crop agriculture because it could be applied to soil before weeds emerged.

In drinking water, alachlor is treated as a high-concern agricultural pollutant because it is a biologically active pesticide, has been associated with cancer findings in laboratory animals, and can move from treated fields into water under certain soil and rainfall conditions. Its use has declined substantially in many countries, and it has been banned, restricted, or replaced by related herbicides in several jurisdictions. However, legacy contamination, historic application areas, contaminated shallow aquifers, and continuing use where permitted mean alachlor can still be relevant for water testing.

Unlike nutrients such as nitrate, alachlor is usually present at trace concentrations measured in micrograms per liter or lower. These low levels can still matter because regulatory limits, where they exist, are often very low. Alachlor is not detectable by taste, odor, or appearance, so clear water from a rural well can still contain the herbicide or its degradation products.

Scientific Identity

Alachlor is an organic pesticide with the molecular formula C14H20ClNO2 and CAS number 15972-60-8. Its systematic chemical name is commonly given as 2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide. The compound contains a chlorinated acetamide functional group attached to a substituted aromatic ring, a structure characteristic of chloroacetanilide herbicides. Related herbicides include metolachlor and acetochlor, which have similar agricultural uses and environmental behavior.

From a water-quality perspective, alachlor is a synthetic organic chemical rather than a microbial or radiological contaminant. It has moderate hydrophobic character but is sufficiently mobile in some soils to reach drainage systems, streams, and shallow groundwater. Its persistence depends strongly on soil organic matter, microbial activity, temperature, pH, sunlight exposure, and rainfall timing after application.

Alachlor can degrade into transformation products, including alachlor ethane sulfonic acid and alachlor oxanilic acid in some environmental settings. These metabolites may be more mobile in groundwater than the parent herbicide. A comprehensive pesticide assessment may therefore require more than testing for alachlor alone, particularly in areas with historic use or vulnerable aquifers.

How Alachlor Enters Drinking Water

Alachlor enters water primarily through agricultural use on treated fields. After soil application, rainfall or irrigation can dissolve and transport the herbicide across the surface as runoff or downward through the soil profile as leachate. The highest transport risk usually occurs when heavy rain follows shortly after application, before the chemical has degraded or bound to soil particles.

Surface water contamination is most common in agricultural watersheds with row crops, erodible soils, tile drainage, drainage ditches, and limited vegetated buffer zones. Alachlor can be carried into creeks, rivers, ponds, and reservoirs used as drinking water sources. In tile-drained fields, water can bypass some natural soil filtration and move dissolved pesticides rapidly into streams.

Groundwater contamination is most likely in shallow, unconfined aquifers beneath coarse-textured or sandy soils, fractured bedrock, karst terrain, or fields with low organic matter. Private wells are especially vulnerable when they are shallow, poorly sealed, located downslope of cropped land, or near pesticide mixing, loading, storage, or equipment-rinse areas. Old farmstead wells and unused wells can also act as direct conduits if they are not properly sealed.

Spills and handling practices can create localized high-risk areas. Concentrated pesticide releases near mixing pads, chemical storage sheds, or washdown zones may produce contamination levels much higher than normal field runoff. These point-source releases are important because they can contaminate a single well even when regional monitoring does not show widespread alachlor problems.

Occurrence and Exposure

Alachlor occurrence is strongly tied to agricultural geography and crop history. It is most likely to be found in regions with intensive corn, soybean, peanut, or other row-crop production, particularly where historic alachlor use was heavy before restrictions or replacement by newer herbicides. Even where current use has ended, residues and degradation products can persist in groundwater depending on aquifer conditions and travel time.

Exposure through drinking water occurs when contaminated groundwater is pumped from a private or public well, or when runoff-affected surface water is used as a municipal source. Public water systems typically conduct regulated monitoring where alachlor is included under national or local drinking water rules. Private well owners, however, are usually responsible for their own testing and may not know to request a pesticide panel unless they live near agricultural land.

Seasonal patterns are important. In surface water, alachlor detections are most likely after spring or early-season herbicide applications followed by storms. Reservoirs and rivers may show short pulses that can be missed by infrequent sampling. Groundwater patterns are slower and may reflect cumulative historic use rather than a single application season. A well may show low but persistent concentrations long after alachlor use has declined locally.

People may also encounter alachlor through occupational routes, such as pesticide handling or farm work, but this profile focuses on drinking water exposure. For households using rural wells, the main concern is chronic ingestion over months or years, especially when the well is shallow or located in a farmed watershed.

Health Effects and Risk

Alachlor is considered a high-priority drinking water contaminant because toxicological studies have linked it to adverse effects at sufficient doses. Animal studies have reported tumors in organs such as the nasal turbinate, stomach, and thyroid, and regulatory agencies have historically treated alachlor as a potential or probable human carcinogen based on laboratory evidence. The exact cancer classification wording may vary by agency and has changed over time, but the carcinogenic concern is central to why drinking water limits are low.

Non-cancer effects reported in toxicological evaluations include impacts on the liver, kidney, spleen, eyes, and blood-related parameters at higher exposures. Developmental and reproductive endpoints have also been considered in risk assessments. Drinking water exposures are usually far lower than occupational exposures, but long-term ingestion can be important when water contains concentrations near or above a regulatory limit.

Risk depends on concentration, duration of exposure, body weight, water intake, and the presence of other agricultural contaminants. Infants, pregnant people, and individuals with compromised health may have less margin of safety, although alachlor-specific human drinking water data are limited. Co-occurrence with nitrate, atrazine, metolachlor, acetochlor, 2,4-D, or other pesticides may indicate broader agricultural contamination and should prompt a more complete water-quality evaluation.

Short-term emergency effects from alachlor in drinking water are uncommon unless there has been a spill or gross contamination event. The more typical concern is chronic low-level exposure from a well or source water that repeatedly receives agricultural inputs. Because alachlor cannot be detected by taste or smell, laboratory testing is the only reliable way to evaluate risk.

Testing and Monitoring

Testing for alachlor requires laboratory pesticide analysis; home test strips are not appropriate for confirming trace-level contamination. Certified laboratories commonly use gas chromatography with mass spectrometry, liquid chromatography with tandem mass spectrometry, or EPA-style methods for synthetic organic chemicals. The laboratory should report detection limits low enough to compare results with applicable drinking water standards or health-based guidance.

Private well owners in agricultural areas should request a pesticide panel that specifically includes alachlor and, where available, alachlor degradation products. A general “basic potability” test often includes bacteria, nitrate, pH, hardness, and metals but does not include herbicides. If the property is near corn, soybean, or peanut production, pesticide mixing areas, drainage ditches, or a history of herbicide use, a targeted agricultural contaminant panel is more appropriate.

Sampling should be performed carefully to avoid contamination. Use the laboratory-provided bottle, do not rinse preservative from the container, and follow instructions for cold storage and holding time. For wells, sampling after a period of normal use gives a representative result. If seasonal surface-water influence is suspected, sampling after spring runoff or major storms can reveal contamination that may not appear during dry periods.

A single non-detect result does not always prove absence. Alachlor concentrations can fluctuate, especially in surface-water systems. For private wells in vulnerable settings, repeat testing or periodic monitoring may be warranted, particularly if nitrate is elevated, nearby land use changes, or a spill has occurred.

Treatment Methods

Alachlor treatment should begin with source control because treatment devices can fail if concentrations change, cartridges are exhausted, or the contamination source worsens. The most reliable long-term approach is to prevent the herbicide from reaching the water supply through careful pesticide management, well protection, runoff reduction, and watershed controls. For household treatment, reverse osmosis and properly selected activated carbon are the main options.

Source control is the preferred first defense. For private wells, this may include maintaining a pesticide-free zone around the wellhead, ensuring the casing and sanitary seal are intact, diverting runoff away from the well, and sealing unused wells. At the farm and watershed scale, integrated weed management, proper chemical storage, calibrated application equipment, avoiding application before heavy rain, and buffer strips along waterways reduce transport to water sources. Source control may fail when contamination is already present in groundwater, when the aquifer receives regional agricultural recharge, or when upstream land use is outside the household’s control.

Reverse osmosis is usually most appropriate as point-of-use treatment at the kitchen sink because the main exposure route is ingestion. A certified RO system with carbon prefiltration can reduce many pesticides and also address other agricultural contaminants such as nitrate, depending on system design. RO is less commonly installed as point-of-entry treatment because whole-house RO is expensive, wastes water, may require corrosion control, and is rarely necessary for showering or laundry exposure to alachlor. RO can fail if membranes are not replaced, if prefilters clog, if bypass plumbing is present, or if the system is not tested after installation.

Activated carbon can be useful either as a point-of-use carbon block filter or as a larger granular activated carbon system. However, carbon has a finite adsorption capacity. Because alachlor has no taste or odor at relevant concentrations, users cannot detect breakthrough without laboratory testing. For serious contamination, carbon systems should be designed with sufficient contact time and ideally include lead-lag vessels or monitoring ports.

Regulations and Guidelines

Alachlor is regulated or addressed differently across jurisdictions. In the United States, the Environmental Protection Agency has established a federal drinking water maximum contaminant level for alachlor in public water systems of 0.002 mg/L, equal to 2 micrograms per liter. The EPA’s health-based goal has historically been set very low because of cancer concern. Public water systems subject to these rules must monitor and comply according to regulatory schedules.

Private wells are not generally regulated under the federal Safe Drinking Water Act, so owners must arrange their own testing and treatment. A private well can exceed a public-water standard without any automatic notification unless it is tested. State, provincial, or local health agencies may provide additional guidance, especially in agricultural regions with known pesticide detections.

International limits vary. The World Health Organization and national authorities may publish health-based guideline values, while the European Union applies a very stringent general parametric value for individual pesticides in drinking water rather than a compound-specific risk value for every pesticide. Some countries have prohibited or not approved alachlor use, while others may regulate residues in drinking water, food, or pesticide products differently. Because legal limits and pesticide approvals change, users should confirm current requirements with the relevant national or local drinking water authority.

Regulatory compliance should not be the only decision point for private wells. If alachlor is detected, especially with nitrate or other herbicides, it indicates a pathway from agricultural activity to the water supply. Even detections below a legal limit may justify well inspection, repeat testing, and source-control measures.

Related Contaminants

Frequently Asked Questions

Is alachlor still used in agriculture?

Alachlor use has declined sharply and it is banned, restricted, or no longer approved in several jurisdictions. In some regions it has been replaced by related herbicides such as metolachlor or acetochlor. However, historic use can still affect groundwater, and local regulatory status should be checked because pesticide approvals vary by country.

Can I smell or taste alachlor in well water?

No. Alachlor is usually present, if at all, at trace concentrations that do not produce a recognizable taste, odor, or color. Laboratory pesticide testing is required to know whether it is present.

Should I test for alachlor if my well is near corn or soybean fields?

Yes, testing is reasonable if the well is shallow, near treated fields, downgradient from cropland, close to pesticide storage or mixing areas, or located in sandy, karst, or fractured-bedrock terrain. Ask the laboratory for a pesticide panel that includes alachlor and related herbicides.

Does boiling water remove alachlor?

No. Boiling is not an effective treatment for alachlor. It can reduce microbial risk but does not reliably remove synthetic organic pesticides and may concentrate contaminants as water evaporates.

Which is better for alachlor: activated carbon or reverse osmosis?

Both can help when properly designed. Reverse osmosis is often preferred for point-of-use drinking and cooking water because it can reduce a broad range of dissolved contaminants. Activated carbon can also adsorb alachlor, but cartridge exhaustion and breakthrough must be managed with replacement schedules and, for higher-risk systems, follow-up testing.