Pesticides

What's left in the groundwater years after we stopped using it — and what we're still using that will be left there years from now

EPA MCL

Atrazine 3 μg/L; simazine 4 μg/L; alachlor 2 μg/L; DBCP 0.2 μg/L; 1,2,3-TCP no federal MCL (CA: 5 ng/L); glyphosate 700 μg/L; many emerging pesticides unregulated

Health concern

Cancer (atrazine — possibly NHL/breast/prostate; DBCP — male infertility, possibly cancer); endocrine disruption; developmental effects

Testing method

Pesticide-specific LC-MS or GC-MS panels; $150-300 for broad screen; not on standard well panels

Pesticides in groundwater are the long tail of every agricultural decision America has made for the past sixty years. Some of the worst contaminants in this category — DBCP, chlordane, lindane — have been banned for decades but still show up in private wells in former agricultural areas because they didn't biodegrade. Some of the most prevalent — atrazine, glyphosate — are still being applied at scale and continue to load into shallow groundwater every year. The category is messy because the regulatory framework, the agricultural practices, and the science of any individual compound have all moved on different timetables.

For private well owners, the practical question is: which pesticides are likely in my water given my geography, and which actually matter at the levels I might find? The answer depends heavily on where you are. In Iowa or Illinois corn country, atrazine is the dominant concern. In California's Central Valley, the persistent banned compounds DBCP and 1,2,3-TCP are the active enforcement issues. In Florida, the citrus and sugarcane industries have a different roster.

The major US pesticide groundwater concerns

Atrazine — the most-used herbicide in the US after glyphosate; applied to about 70% of US corn acreage. Banned in the EU since 2004. The EPA MCL is 3 μg/L; widespread detection in Midwest groundwater at concentrations occasionally exceeding the MCL, frequently approaching it. Atrazine is an endocrine disruptor; possible carcinogenicity is debated; the strongest concerns are reproductive and developmental.

Simazine, alachlor, metolachlor — older corn-soybean herbicides with similar shape to atrazine; still detected in groundwater across the corn belt.

Glyphosate (Roundup) — the most-used herbicide in the world. EPA MCL 700 μg/L; rarely detected in groundwater above this very high standard, but increasingly detected at lower concentrations as analytical methods improve. The carcinogenicity question (IARC Group 2A; multi-billion-dollar Bayer settlements) remains contested. Detection at typical groundwater concentrations is well below documented health effects but worth knowing.

Neonicotinoid insecticides (imidacloprid, clothianidin, thiamethoxam) — the dominant systemic insecticides for the past 20 years; bee-population collapse linked to them; emerging detections in groundwater. No federal MCL.

Persistent legacy compounds (banned but still detected):

• DBCP (dibromochloropropane) — banned 1977 after the Lathrop, CA male-infertility epidemic. EPA MCL 0.2 μg/L (one of the lowest of any pesticide). Still detected in Central Valley CA groundwater and a few other regions; many wells have had to install treatment.
• 1,2,3-TCP (1,2,3-trichloropropane) — Shell and Dow soil-fumigant impurity used 1940s-1980s. No federal MCL but California adopted a 5 ng/L (parts per trillion) state MCL in 2017 — one of the strictest drinking-water standards in the world. Hundreds of California wells exceed; multiple lawsuits.
• Chlordane — termiticide banned 1988; still detected near older homes treated for termites in the 1960s-1980s.
• EDB (ethylene dibromide) — soil fumigant banned 1984; persistent in some agricultural-area groundwater.
• Atrazine breakdown products — desethylatrazine (DEA), desisopropylatrazine (DIA) — sometimes detected at higher concentrations than parent atrazine; not always included in standard atrazine tests.

Where they show up

The pattern is straightforwardly geographic:

• Corn belt (IA, IL, IN, NE, KS, MO, OH) — atrazine, simazine, alachlor, metolachlor in shallow agricultural-area wells. The Cambrian-Ordovician deeper aquifer is generally protected; shallow drift wells are not.
• California Central Valley — DBCP, 1,2,3-TCP, plus current-use compounds (chlorpyrifos, others). The Central Valley aquifer page covers the broader story.
• Mississippi Alluvial — atrazine plus rice-specific compounds. See Mississippi Alluvial.
• High Plains — atrazine, plus cotton herbicides in Texas. See Ogallala.
• Florida — citrus and sugarcane region pesticides; Florida DEP has documented multiple legacy compounds in the surficial aquifer.
• Pacific Northwest — orchard pesticides (azinphos-methyl, others) in Yakima Valley and other irrigated regions. See Columbia Plateau.
• Northeast — golf course and home-use chemicals (much smaller per-acre but locally relevant).

Health effects

Pesticide health effects vary enormously by compound. The categories of concern:

• Endocrine disruption (atrazine, others) — effects on hormonal systems, reproductive development, fetal development. Strongest evidence for atrazine.
• Cancer — varies by compound. DBCP causes testicular cancer at high occupational exposures; atrazine has mixed evidence for breast cancer and non-Hodgkin lymphoma; glyphosate's IARC Group 2A classification is contested.
• Neurological effects — organophosphates (chlorpyrifos, malathion) and chlordane cause neurodevelopmental effects in children at chronic exposures.
• Reproductive effects — DBCP famously caused male infertility (the Lathrop case); other compounds have lesser-documented similar effects.
• Developmental effects — many pesticides cross the placenta and affect fetal development.

The dose-response for most pesticides at typical groundwater concentrations is uncertain. Most chronic pesticide exposures from drinking water are well below acute-toxicity thresholds and within the regulatory framework, but the long-tail concerns (cancer, endocrine, developmental) operate at the chronic low-dose level where the science is less settled.

Testing

• Method: LC-MS or GC-MS pesticide-specific panels. Different labs offer different scope panels; ask for one that matches your local agricultural concerns.
• Cost: $150-300 for a broad pesticide screen (~50-150 compounds). Higher for specialty panels or single-compound sensitive analyses.
• When: at well purchase if you're in a heavily agricultural area; every 3-5 years thereafter. After any nearby crop-application event (less practical but informative).
• Standard panels do not include pesticides — you have to ask specifically.

Treatment

• Granular activated carbon (GAC) — the standard for pesticide removal. Effective for the broad pesticide panel including atrazine, simazine, alachlor, DBCP, 1,2,3-TCP. Whole-house POE installations cost $1,000-3,000; carbon replacement annually depending on load.
• Reverse osmosis — works for most pesticides; point-of-use focus. Best paired with GAC for whole-house coverage.
• Air stripping — works for volatile pesticides; rare for residential.

Standard water softeners and basic carbon pitcher filters do not reliably remove pesticides at meaningful rates.

Aquifers where this is a concern