By the Closer Look Inspectors team · Facts checked October 8, 2026 against MDH, MPCA and EPA sources · About our inspectors
DRO is a lab measure of heavier petroleum hydrocarbons such as diesel and heating oil. Minnesota Pollution Control Agency (MPCA) says petroleum has leaked at about 22,000 Minnesota sites, many from gas-station tanks and home heating-oil tanks. DRO results show whether water holds a petroleum mixture; specific chemicals are then tested and compared to MDH values.
All the water tests we offer
Bacteria
Shows whether surface water or waste is reaching the well.
- Surface contamination reaching the wellNone should be present (MDH)
- Fecal contamination from septic, manure or wildlifeNone should be present (MDH)
Health chemicals
No taste, color or smell. These are the long-term health checks.
- Fertilizer, septic and manure; highest risk to infants10 mg/L (EPA)
- Natural arsenic from Minnesota rock and soil10 µg/L (EPA)
- Lead service lines, solder and brass fixturesNo safe level (MDH)
- ManganeseNatural manganese; a concern for infants100 µg/L for infants (MDH guidance)
- FluorideNatural fluoride; dental health4.0 mg/L (EPA)
- UraniumNatural radionuclide in groundwater30 µg/L (EPA)
Petroleum & solvents
Fuel and chemical spills from old tanks or nearby sites.
Household quality
Staining, scale and taste, and choosing the right treatment.
- IronRusty staining and metallic taste0.3 mg/L, aesthetic (MDH)
- HardnessScale buildup; sizing a water softenerNo health standard
- pHCorrosive or scaling water6.5 to 8.5, aesthetic (EPA)
- SulfateTaste and odor; diarrhea risk for infants250 mg/L, aesthetic (EPA)
Limits from MDH, EPA and MPCA. Your lab report compares each result to these values.
Health standard: No health limit for DRO itself. MPCA: DRO/GRO results are not used to assess human health risk; specific chemicals are compared with MDH human-health water guidance values.
Where DRO comes from in Minnesota
DRO stands for diesel range organics, a laboratory measure used when petroleum may have reached groundwater. In Minnesota, the MPCA Petroleum Remediation Program publishes guidance (document c-prp4-25) on evaluating drinking water samples where petroleum has affected water. The guidance c-prp4-25 explains that DRO and gasoline range organics (GRO) are run as part of the investigative process, so a DRO test is most relevant to a home with a known or suspected fuel release nearby, an old heating oil or diesel tank, or a contaminated-site history.
What a DRO result does and does not tell you
DRO does not have a health limit of its own. MPCA guidance states that DRO and GRO results are used to determine whether a water plume is a mixture of petroleum products, and that they help inform which further chemical tests are needed, such as VOCs, semi-volatile organic compounds, and polycyclic aromatic hydrocarbons (PAHs). MPCA also states that DRO and GRO results are not used to assess human health risks. Health risk is evaluated by comparing the individual chemical results with MDH human-health-based water guidance values.
When to test and what to ask the lab
Consider a DRO test, together with VOC testing, when a well or property is near a petroleum release, a leaking or abandoned fuel tank, or a site under MPCA review. An accredited laboratory analyzes the sample. Ask which tests are included, since MPCA's approach pairs DRO and GRO with VOC, semi-volatile, and PAH analysis, and ask for the lab's reporting limits. MPCA guidance says reporting limits should be as low as feasible so they are not above the individual MDH guidance values. Home buyers can request any tank or release history for the property.
If your result is high: what to do next
A DRO detection signals that petroleum-related compounds may be present, but it does not by itself say whether the water is safe or unsafe. The next step is the more specific testing MPCA describes: VOCs, semi-volatile organic compounds, and PAHs, compared with MDH health-based guidance values. Contact MPCA's Petroleum Remediation Program or MDH to discuss results, particularly if a release is suspected. MDH advises removing the source of contamination or using a safer water supply where possible, and treatment decisions should rest on the specific chemicals identified.
Facts checked October 2026 against: pca.state.mn.us, Minnesota Department of Health.
DRO water testing in pictures
Minnesota private well testing timeline
| Test | Year 1 | Year 2 | Year 3 | Year 4 | Year 5 | Year 6 |
|---|---|---|---|---|---|---|
| Coliform bacteria | ||||||
| Nitrate | ||||||
| Arsenic | ||||||
| Lead |
Test again any time after flooding, after work on the well, or when the water's taste, color or smell changes, and always when you buy a home with a well.
How far a well must be from common hazards in Minnesota
- Building (closest projection)3 ft
- Lake, river, stream or pond35 ft
- Septic tank50 ft
- Drainfield (soil dispersal area)50 ft (100 ft sensitive well)
- Animal feedlot or building (over 1 animal unit)50 ft
- Cesspool or seepage pit75 ft (150 ft sensitive well)
- Unroofed feedlot, 300+ animal units100 ft
- Unprotected petroleum tank, 1,100+ gallons150 ft
Septic, feedlot or fuel tank near the well? A coliform and nitrate test shows whether it is reaching the water. Get a free instant quote
How long a private well lasts in Minnesota
Wells built before 1974, when Minnesota's well code began, are listed by MDH as higher risk for bacteria, along with leaking dug wells, wells in old frost pits and wells with corroded casings.
Wells built before 1974 often have no record on file. Test the water before you close. Get a free instant quote
Quick answers
What does DRO mean in a water test?
DRO means diesel range organics. In Minnesota petroleum cleanup work, MPCA runs DRO and gasoline range organics (GRO) as part of the investigation to determine whether a water plume is a mixture of petroleum products. It is a screening and investigative result, not a measure of health risk.
Is there a safe limit for DRO in drinking water?
DRO has no health limit of its own. MPCA guidance says DRO and GRO results are not used to assess human health risks. Instead, specific chemicals such as VOCs and PAHs are compared with MDH human-health-based water guidance values. Ask your lab and MPCA how to interpret a result.
Why would I need a DRO test on a house I'm buying?
A DRO test may be worth considering if the property has an old fuel or heating oil tank, or is near a petroleum release or contaminated site. It helps show whether petroleum may be present and which further tests are needed. Ask the seller for tank and release history as well.
What tests go along with DRO?
MPCA guidance describes DRO and GRO as informing follow-up testing for volatile organic compounds, semi-volatile organic compounds, and polycyclic aromatic hydrocarbons. Those individual chemical results are what get compared with MDH health-based guidance values. Ask an accredited laboratory which package fits your situation.
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