Weld County, Colorado: drinking water report. Weld County in northeastern Colorado has a population of about 330,000, including Greeley, Evans, and Windsor.
Weld County in northeastern Colorado has a population of about 330,000, including Greeley, Evans, and Windsor. The county is Colorado's agricultural heartland – the top-producing agricultural county in the state – and also one of the most active oil and gas producing counties in the country. Water comes from the Cache la Poudre River, the Colorado-Big Thompson Project, and groundwater wells, creating a three-way intersection of agriculture, energy extraction, and residential water demand.
The convergence of concentrated animal feeding operations (CAFOs), irrigated farmland, and over 20,000 active oil and gas wells creates a complex contamination landscape. According to the Colorado Oil and Gas Conservation Commission's 2024 data, Weld County has experienced 47 documented groundwater contamination incidents from oil and gas operations since 2010, including cases involving benzene, methane, and produced water.
Nitrate from agricultural operations is elevated in shallow aquifer zones across the county. A 2023 CDPHE assessment found that 15% of private wells tested in Weld County exceeded the nitrate MCL of 10 mg/L. The EPA's UCMR5 data shows PFAS detections in the Greeley system at low levels.
Weld County's combination of agriculture and oil and gas activity means private well owners face risks from both directions. Testing for nitrate, methane, and petroleum compounds provides a baseline that covers the most likely contaminants.
Check your water for current data on public systems. For nitrate, reverse osmosis is effective. For petroleum compounds, activated carbon filters help. Our water filter guide covers multi-contaminant scenarios. Pull your detailed report for trends, and visit our Colorado page for statewide patterns.
Drinking water is one exposure route. These federal datasets also cover Weld County, CO.
The EPA places Weld County, CO in Zone 1 - the EPA's highest-risk category, where the predicted average indoor radon level is above 4 pCi/L. Zone maps predict an area's radon potential from geology and existing indoor readings; they say nothing about a specific house. Testing is the only way to know your own level.
32% of homes in Weld County, CO were built before 1980, and 9% before 1950. Residential lead paint was banned in 1978, so housing age is the strongest available proxy for lead-paint exposure. This describes the county's housing stock, not any one home.
Modelled annual fine-particle pollution (PM2.5) in Weld County, CO averages 7.1 µg/m³, within the EPA's annual standard of 9 µg/m³. These are CDC modelled county estimates rather than readings from a specific monitor.
How radon zones work · Air quality by county · Lead exposure guide
Across 67 public water systems serving Weld County, CO with EPA lead testing on record, 4 (6%) reported a 90th-percentile lead result above the EPA's 15 ppb action level, and the median is 4 ppb. 18 water systems serving Weld County, CO carry a health-based EPA Safe Drinking Water Act violation.
| Most-cited violation type | Systems affected |
|---|---|
| Stage 1 Disinfectants and Disinfection Byproducts Rule | 6 |
| Surface Water Treatment Rule | 5 |
| Lead and Copper Rule | 3 |
| Nitrate | 2 |
| Interim Enhanced Surface Water Treatment Rule | 2 |
Source: EPA Lead and Copper Rule 90th-percentile tap sampling and EPA SDWIS Safe Drinking Water Act enforcement records, aggregated from the public water systems serving this county. Figures are counts of water systems, not a test of any individual home or tap.