By the Closer Look Inspectors team · Facts checked October 2026 against Iowa DNR, Iowa HHS, State Hygienic Laboratory and EPA sources · About our inspectors
A DRO result reports petroleum compounds in the diesel range, carbon chains 10 to 28 atoms long, that can reach water through spills or leaks. Iowa's underground tank rules use a separate measure for diesel and other low volatile fuels, total extractable hydrocarbons (TEH), with a groundwater level of 1,200 micrograms per liter (µg/L) for diesel.
All the water tests we offer
Bacteria
Shows whether surface water or waste is reaching the well.
- Surface contamination reaching the well
- Fecal contamination from septic, manure or wildlife
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 rock and soil10 µg/L (EPA)
- Lead service lines, solder and brass fixtures
- ManganeseNatural manganese; a concern for infants
- 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 taste
- 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 shown are EPA drinking water standards. Compare your lab results with these values.
Health standard: Iowa DNR (567 IAC 135.14): groundwater action level of 1,200 µg/L for total extractable hydrocarbons (TEH) as diesel, measured by Iowa Method OA-2; ATSDR's fact sheet reports no federal regulations or advisories specific to total petroleum hydrocarbons.
Where diesel-range petroleum comes from in Iowa
EPA Method 8015D applies to petroleum hydrocarbons such as gasoline range organics and diesel range organics (DRO). It defines DRO as alkanes from C10 to C28. Iowa's rules group all grades of diesel fuel, fuel oil, kerosene, oil and mineral spirits as low volatile petroleum hydrocarbons. EPA's tank report with data through March 31, 2026 lists 6,463 cumulative confirmed petroleum releases from underground tanks in Iowa, with 241 still in the cleanup backlog. A 2004 Iowa health review describes Climbing Hill, where gasoline and diesel fuel leaking from two underground tanks contaminated several private wells. The contamination was first found in 1990.
Health effects and who is most at risk
DRO is a group measure, not one chemical. The federal Agency for Toxic Substances and Disease Registry (ATSDR) uses the term total petroleum hydrocarbons (TPH) for a family of several hundred compounds that come from crude oil. It says it is not practical to measure each one separately, and that measuring the total amount is useful. Effects depend on the dose, the duration, how you are exposed, personal traits and habits, and whether other chemicals are present. Some TPH compounds can affect the central nervous system. Animal studies show effects on the lungs, liver and kidney, and on reproduction and the developing fetus. ATSDR lists drinking water contaminated with TPH, and living near a spill or leak of petroleum products, among the ways people are exposed. Benzene is one TPH compound that the International Agency for Research on Cancer has found carcinogenic to humans. Iowa analyzes benzene with the high volatile compounds, apart from TEH.
Signs and when to test
Iowa's underground tank rules require tank owners to report a suspected release in certain cases. One is finding a released regulated substance, as a liquid not dissolved in water or as vapors, in soils, basements, sewer and utility lines, or nearby surface water. EPA lists an odor of gasoline or fuel oil near a gas station or buried fuel tanks as a reason to test a private well. Iowa's rules add TEH analysis to benzene, ethylbenzene, toluene and xylenes (BTEX) when a release may be anything other than gasoline or the source is unknown. At tank closure they do the same when the storage history is non-gasoline petroleum or is unknown. Iowa applies the levels it sets for drinking water wells to any drinking water well within 1,000 feet of a release source. A home purchase is a sensible time to get a baseline result.
If your result is high: what to do next
A DRO number does not name the chemicals in the mix. Iowa's 1,200 µg/L level is written for TEH by Method OA-2, while DRO is defined in EPA Method 8015D. Compare any DRO result with that level with care. Iowa's TEH values are based on the risks from naphthalene, benzo(a)pyrene, benz(a)anthracene and chrysene. Iowa's underground tank rules require BTEX analysis at every release they cover. They also let DNR require a tank owner to find out whether the tank is the source. That applies when regulated substances turn up in nearby drinking waters and another party brings it to DNR's attention. In Climbing Hill, residents with affected wells were given bottled water, then point-of-use carbon filters that must be maintained properly and changed regularly, and later new community wells. That review focused on benzene, toluene, ethylbenzene and xylene, the main chemicals of concern in gasoline and diesel.
Facts checked October 2026 against: legis.iowa.gov, wwwn.cdc.gov, epa.gov, epa.gov, atsdr.cdc.gov, legis.iowa.gov, epa.gov.
Iowa private well testing timeline
| Test | Year 1 | Year 2 | Year 3 | Year 4 | Year 5 | Year 6 |
|---|---|---|---|---|---|---|
| Coliform bacteria | ||||||
| Nitrate | ||||||
| Arsenic |
When to test your water
- Every yearColiform bacteria and nitrate, the basic safety checks for a private well.
- At least onceArsenic and manganese. Iowa DNR's testing list repeats arsenic every 3 years.
- Before a baby drinks the waterNitrate. Water above 10 mg/L as nitrogen should not be used for infant formula or juice.
- When you buy a homeA full baseline while you can still ask questions or negotiate.
- New well or well workWhen a well is new and after service on the well or pump.
- After a flood or power outageIf flood water was near or reached the well, or an outage dropped the water pressure.
- When taste, color or smell changesA change in the water is a reason to test.
- Near a fuel tankVOCs, plus DRO for diesel and fuel oil. Iowa DNR names gasoline among the VOCs some wells are susceptible to, and Iowa's well rules list 100 feet between a well and a liquid fuel storage tank other than propane.
- City water in an older homeLead. Iowa DNR says homes built before 1986 are more likely to have lead pipes, fixtures and solder.
Based on: Iowa DNR private wells, Iowa DNR testing frequency, Iowa DNR why test, Iowa DNR floods, State Hygienic Laboratory, 567 IAC 49.
How to get it done
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How far a well must be from common hazards in Iowa
- Property line4 ft
- Building sewer line10 ft
- LP gas tank15 ft
- Roadside ditch15 ft
- Stream, pond, lake or wetland25 ft
- Septic tank50 ft
- Septic absorption field100 ft
- Fuel oil or gasoline tank100 ft
- Fertilizer or chemical storage150 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
Buying a home with a well in Iowa: what happens at the sale
- Groundwater Hazard StatementIowa Code 558.69 requires a groundwater hazard statement filed with the declaration of value when a listed condition (including any known well and its status under 455B.190, or a private sewage disposal system) exists on the property, signed by a seller or agent, with a copy given to the buyer.
- Wells on the propertyIf known wells are on the property, the statement must give the approximate location of each and its status under Iowa Code 455B.190 (abandoned wells properly plugged); the owner is responsible for the accuracy of the information.
- Septic time-of-transfer inspectionIowa Code 455B.172(11) requires a private sewage disposal system serving a building to be inspected before any transfer of ownership, by a DNR-certified inspector, and the county recorder will not record the deed without the report or a binding acknowledgment.
- Failing systemsA system that is failing to treat wastewater must be renovated to current standards by the seller or, by agreement and within a reasonable time set by the county board of health or DNR, by the buyer (455B.172(11)(c)).
The paperwork tells you a well exists, not what is in it. Add water testing to your inspection. Get a free instant quote
Quick answers
What does DRO mean in a water test?
DRO means diesel range organics, the petroleum compounds in the diesel range. EPA Method 8015D defines that range as alkanes (carbon chains) from C10 to C28 and covers gasoline range organics (GRO) alongside it. The result is a group measure, not a single chemical, so it reports a total for the range rather than individual compounds.
Is there a safe limit for DRO in drinking water?
ATSDR's fact sheet, last reviewed in 2014, reports no federal regulations or advisories specific to total petroleum hydrocarbons. Iowa DNR does set groundwater levels for total extractable hydrocarbons: 1,200 µg/L as diesel and 400 µg/L as waste oil. They apply to drinking water wells within 1,000 feet of a release. They are written for TEH by Method OA-2, while DRO is defined in EPA Method 8015D, so compare with care.
What is the difference between DRO and TEH in Iowa?
DRO is defined in EPA Method 8015D as alkanes from C10 to C28. Iowa's rules call for total extractable hydrocarbons (TEH) analysis when a release may involve non-gasoline petroleum or the source is unknown. Under 567 IAC 135.16, Method OA-2 covers diesel fuel and fuel oil, and OA-1 covers gasoline and BTEX. Tank cleanup lab reports must say whether a sample matches the lab's waste oil, diesel or gasoline standard.
How far should a well be from a fuel tank in Iowa?
Iowa's private well rule, 567 IAC 49.8, sets a minimum of 100 feet between a private well and a liquid hydrocarbon storage tank, other than liquid propane gas (LPG). LPG tanks are 15 feet. Iowa's tank rules describe motor fuels and distillate fuel oils as petroleum hydrocarbon blends. Under the release rules, the levels set for drinking water wells apply to any drinking water well within 1,000 feet of a release source.
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