Petroleum Hydrocarbon Remediation

What is in a petroleum release

“Petroleum hydrocarbons” covers everything from gasoline to crude oil, and the product that was spilled decides how the site behaves. Gasoline is light, volatile and rich in BTEX (benzene, toluene, ethylbenzene, xylenes), the most soluble and most regulated fraction; it moves fastest and biodegrades fastest. Diesel, heating oil and jet fuel are heavier, less soluble and slower to degrade. Crude, lubricating oils and coal tar carry polycyclic aromatic hydrocarbons that sorb to soil and persist for decades. Laboratories report the mix as total petroleum hydrocarbons in gasoline, diesel and oil ranges (TPH-GRO, DRO, ORO) alongside the individual compounds. At most sites the mass sits in three places at once: free product (LNAPL) floating on the water table, hydrocarbons sorbed to the soil in the smear zone, and the dissolved plume moving downgradient. Each needs a different tool.

Product released	What drives the cleanup	Treatability in place
Gasoline	Benzene (5 µg/L MCL), vapor intrusion, MTBE where used	High: aerobic biodegradation and chemical oxidation both work
Diesel, heating oil, jet fuel	TPH-DRO criteria, naphthalene, sheens on groundwater	Moderate: biodegradation is slower; oxidation of the source zone speeds it up
Crude oil, used oil, coal tar	PAHs, TPH-ORO	Lower: heavy fractions sorb and resist; oxidation and soil mixing for sources, bio for the rest
Free product of any kind	Mobile LNAPL, product in wells	Recover it first; in situ chemistry is for what recovery leaves behind

The numbers that set the target

Benzene drives most petroleum cleanups: it is a known human carcinogen with a federal drinking water standard of 5 µg/L, two to three orders of magnitude tighter than toluene (1 mg/L), ethylbenzene (0.7 mg/L) and xylenes (10 mg/L). There is no federal standard for TPH itself; states set their own criteria by fraction, and at many sites the binding number is a benzene or naphthalene vapor-intrusion screening level rather than a groundwater standard. MTBE, where it was blended into gasoline, carries an EPA drinking water advisory of 20 to 40 µg/L for taste and odor and is harder to biodegrade than BTEX. Confirm the standard that applies before sizing any remedy, because it decides whether the job is polishing a plume or removing source mass.

An oxygen problem or an oxidant problem

Petroleum sites come to us as one of two problems. In a dissolved plume the bacteria that eat fuel are already present and starving: a hydrocarbon plume uses up the aquifer’s dissolved oxygen in days, recharge cannot keep pace, and degradation stalls. That is an oxygen problem, and the fix is to supply oxygen where the bacteria are. In a source zone there is too much mass to wait on biology, and the fix is to oxidize it chemically. The common program is an oxidant pass on the source followed by sustained oxygen on the plume. One requirement runs through both: in situ reactions happen in water, so the treated soil has to be saturated, either by injecting the amendment with water or by adding water during soil mixing.

Enhanced aerobic bioremediation: the oxygen math

Complete aerobic degradation of a hydrocarbon takes roughly three to three and a half pounds of oxygen per pound of fuel (benzene: C₆H₆ + 7.5 O₂ → 6 CO₂ + 3 H₂O, 3.1 lb of O₂ per lb; alkanes slightly more). Groundwater holds at most 8 to 10 mg/L of dissolved oxygen, so the aquifer cannot deliver that on its own, and the two chemical oxygen sources Hepure supplies cover the two timescales.

Calcium peroxide releases oxygen slowly as it reacts with water (CaO₂ + H₂O → Ca(OH)₂ + ½ O₂), which works out to 0.22 lb of oxygen per lb of pure CaO₂, less in proportion to the assay of a commercial grade. On paper that is about 15 lb of pure calcium peroxide per lb of hydrocarbon to be degraded; in practice the dose is set from the dissolved and sorbed mass in the treated volume plus the soil’s own oxygen demand, and the release lasts months, which is what makes it the plume amendment. Hydrogen peroxide delivers oxygen immediately (2 H₂O₂ → 2 H₂O + O₂, 0.47 lb of oxygen per lb of H₂O₂) and is the pulse used to kick-start a plume or to oxidize a source at higher strength. Because calcium peroxide’s release life is finite, many programs add a hydrocarbon-degrading bacterial culture from the bioremediation amendments line so the population is established while the oxygen is there, and check nitrogen and phosphorus, which a fuel plume also exhausts.

Where aerobic conditions cannot be maintained, BTEX and some other hydrocarbons degrade anaerobically, more slowly, with emulsified vegetable oil or sodium lactate as the electron donor. Ethylbenzene tends to be the compound that lingers in anaerobic plumes.

Chemical oxidation: the source-zone tools

Activated sodium persulfate is the usual petroleum oxidant. Persulfate on its own is modest; activated with alkali, iron, peroxide or heat it produces sulfate radicals that open the aromatic ring and carry BTEX, MTBE, TPH in the gasoline and diesel ranges, and PAHs through to carbon dioxide. It persists for days to weeks, long enough to spread through a treatment zone. The stoichiometry is heavy: complete oxidation of benzene takes 15 moles of persulfate per mole, about 46 lb of sodium persulfate per lb of benzene, and the figure for alkane fuels is similar, before the soil’s natural oxidant demand is added. Alkaline activation with sodium hydroxide to pH above 10.5 is the most common field choice because the activator and the pH buffer are the same drum; iron activation uses ferrous sulfate. Hepure supplies the persulfate in 55-lb bags and 2,204-lb bulk sacks and both activators.

Catalyzed hydrogen peroxide (Fenton’s chemistry: peroxide with ferrous iron) is the fastest oxidant, generating hydroxyl radicals that destroy BTEX and TPH within hours. On paper about 7 lb of H₂O₂ is needed per lb of hydrocarbon; in practice several times that, because much of the peroxide decomposes to oxygen and water before it reaches the fuel. The reaction releases heat and gas, which helps desorb hydrocarbons from the smear zone but has to be planned for in the injection design and the health and safety plan. Field programs specify 3% to 35% solutions.

Permanganate, the workhorse for chlorinated solvents, is weak on benzene and saturated hydrocarbons and is rarely the right choice at a fuel site, though it handles PAHs at manufactured gas plant sites. Two cautions apply to any oxidant: free product should be recovered before injection, because oxidant poured onto LNAPL is wasted, and a soil oxidant demand test on site soil should precede the dose, because clays and natural organic matter consume oxidant before the contaminant does. The options are compared on our chemical oxidants page.

Delivery: soil mixing

Soil mixing suits hydrocarbon source zones shallow enough to reach, roughly 20 feet below grade with excavator-mounted tools, where enough water can be added to saturate the soil. A bucket mix is partial; a rotary tool gives the contact chemical oxidation needs. Biological amendments are more forgiving of imperfect mixing, because they have months to diffuse through the saturated matrix; oxidants are not, because the reaction is over in days and only treats what it touches. Calcium peroxide and persulfate are both dry solids that blend well; peroxide and the liquid activators are added with the water.

Delivery: injection

Below the reach of mixing, and for dissolved plumes anywhere, amendments go in by injection: direct-push rods or wells advanced into the treated interval, with the chemistry and water pushed under pressure to saturate the soil around each point. Injection spacing follows the radius of influence measured in a pilot, and in low-permeability soils that radius shrinks, which is why clay sites often end up with closer points and smaller volumes rather than a bigger dose. How delivery fits the wider toolkit is covered on the groundwater remediation and soil remediation pages.

Matching the chemistry to the site

Situation	Usual choice	Why
Dissolved BTEX plume, no free product	Calcium peroxide, plus a bacterial culture	Months of oxygen where the bacteria already are
Source zone with sorbed mass, no free product	Activated persulfate, or catalyzed peroxide	Destroys mass in days to weeks; biology finishes the rest
Smear zone with residual LNAPL after recovery	Catalyzed peroxide	Heat and gas help desorb what recovery cannot reach
Shallow source, open site	Soil mixing with persulfate or calcium peroxide	Full contact at the lowest cost per ton
Diesel or heavy oil, slow biodegradation	Persulfate on the source, then calcium peroxide	Oxidation breaks the heavy fractions biology stalls on
Anaerobic aquifer where oxygen cannot be held	Emulsified vegetable oil or sodium lactate	Anaerobic BTEX degradation, slower but sustained
Manufactured gas plant or creosote site (PAHs)	Permanganate or persulfate by soil blending	Oxidation handles the fused rings; see the PAH page

What Hepure provides

We supply the chemistry and help you size it: calcium peroxide in 55-lb bags and 2,000-lb super sacks; hydrogen peroxide at the strength your design specifies; sodium persulfate in 55-lb bags and 2,204-lb bulk sacks; sodium hydroxide as a 25% solution and ferrous sulfate in 50-lb bags as activators; emulsified vegetable oil and sodium lactate for anaerobic programs; and the bacterial supplement. Send Pat (West, x1) or Bob (East, x2) the fuel type, the concentrations, the treated volume and depth, the soil type and whether free product is present, and you will get a product recommendation and a quantity estimate. We do not design the remedy, recover product or run the injection; that stays with your engineer and your contractor.

Common questions

Calcium peroxide or hydrogen peroxide for a fuel plume?

Both, often. Hydrogen peroxide for a fast pulse and for oxidizing a source at higher strength; calcium peroxide for months of sustained oxygen on the dissolved plume. On paper calcium peroxide yields 0.22 lb of oxygen per lb and hydrogen peroxide 0.47 lb per lb; the difference is release time, not oxygen content.

How much persulfate does it take to treat benzene?

About 46 lb of sodium persulfate per lb of benzene for complete oxidation (15 moles per mole), before the soil’s natural oxidant demand, which usually adds more than the contaminant itself. A soil oxidant demand test sets the real number.

Can permanganate treat gasoline?

Not well. Benzene resists it. Activated persulfate or catalyzed peroxide are the petroleum oxidants; permanganate belongs on chlorinated solvent and PAH sites.

Can I inject an oxidant into free product?

You can, but it is wasted. Recover the LNAPL first, then oxidize the residual in the smear zone and treat the dissolved plume biologically.

How deep can soil mixing go?

Roughly 20 feet with excavator-mounted tools. Deeper than that, inject.

How long does a petroleum remedy take?

Chemical oxidation of a source zone is a matter of weeks; aerobic bioremediation of a plume is months to a few years, set by how much mass is there and how well oxygen reaches it. Monitoring dissolved oxygen, not just contaminant concentrations, tells you whether the plume remedy is working.

References

• U.S. EPA. National Primary Drinking Water Regulations (benzene, toluene, ethylbenzene, xylenes); drinking water advisory for MTBE.
• U.S. EPA Office of Underground Storage Tanks. How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites (chemical oxidation, enhanced aerobic bioremediation chapters), 2017.
• Interstate Technology and Regulatory Council (ITRC). Technical and Regulatory Guidance for In Situ Chemical Oxidation of Contaminated Soil and Groundwater, 2nd ed., 2005.
• Siegrist, R. L., Crimi, M., and Simpkin, T. J. (eds.). In Situ Chemical Oxidation for Groundwater Remediation. Springer, 2011.