What are petroleum products?
Petroleum products are the refined products of crude oil, including gasoline, kerosene, diesel fuels, and oils. These products, including crude oil, are all composed of compounds called petroleum hydrocarbons. They have a broad range of characteristics depending on the amount of carbon in their molecular structure. Many of them can cause problems when present in the soil.
Why are petroleum products a problem?
Human health
Petroleum products have the greatest risk for human health when they are in drinking water. Because many petroleum hydrocarbons are highly mobile, if they are in soil they can be readily transported to water resources. Several petroleum hydrocarbons like benzene, toluene, ethylbenzene, and xylene can cause cancer. There are regulatory standards for safe levels in drinking water.
Humans can also be exposed to petroleum hydrocarbons by inhaling the fumes from contaminated soil. Alternatively, humans can ingest contaminated soil, which can be a primary pathway in children. Despite some individual petroleum hydrocarbons being carcinogenic at low levels, many are readily broken down into less harmful products in your body. Nonetheless, continued exposure, even at low levels, creates a risk for health problems for children and adults alike. Read about petroleum and other contaminants here.
Soil health
Soils contaminated by petroleum hydrocarbons can affect soil health. And it can do so at much lower concentrations compared to the effects on human health. They can harm soil microorganisms, reducing their number and activity. Soil microbes help make nutrients available to plants (read more here). Reducing microbes' numbers or activity also affects plants. Some petroleum hydrocarbons can be taken up by plants and pose a risk to grazing livestock, wildlife, and plant-eating insects. At high levels of contamination, seeds cannot germinate. In some cases, even mature plants cannot grow anymore. Some petroleum products can be very thick and sticky. These types of petroleum products can clog the soil so that water and air is difficult to move to plant roots, resulting in drought-like symptoms.
Examples of petroleum contamination
Small-scale contamination
This contamination can occur directly through leaks, car accidents, and spills. This type of contamination can be compounded in areas like parking lots and roads. In these high traffic areas, petroleum hydrocarbons can accumulate and be transported to surrounding soil during rain events. If you have ever seen a shiny puddle in a rainstorm, that shine comes from petroleum products. It can get into the stormwater system or the soil.
Large-scale contamination
The largest examples of petroleum hydrocarbon exposures are the result of industrial releases, including spills during transportation (such as train derailments, shipping accidents, or pipeline leaks), extraction (such as the Deepwater Horizon event), or storage (such as leaking storage tanks).
How to identify if petroleum products are a problem
For high concentrations of petroleum hydrocarbons, you can often smell or see (especially with oils) the contamination. If the concentrations are high enough to see or smell, they will likely have a negative effect on soil and plant health. In lower concentrations, contamination can still cause discoloration in the soil compared to surrounding areas and poor (or no) vegetative growth.
How to remediate a problem with petroleum products
Small-scale contamination
Many petroleum hydrocarbons are naturally-occurring compounds, as they are harvested from the earth. Of course, many are also further refined for use in our modern engines. At low levels, soil microorganisms already present in the soil can consume them. This breaks them down into less harmful substances. Thus, in small-scale contaminations, the best course of action may be to simply leave it alone. To increase the speed of natural degradation you can disturb the contaminated soil by digging up, tilling, or piling it. This exposes more of the petroleum product to air. The presence of more oxygen encourages soil microbes to become more active and feed on the petroleum product. Keeping the contaminated area warm and adding fertilizer (especially nitrogen) can speed up its breakdown.
Large-scale contamination
These concepts also apply at large-scale contaminations, taking the form of landfarming, biopiling, or natural breakdown. Additionally, petroleum hydrocarbon contaminated soils may be treated with chemical amendments to speed up their breakdown. Heating the contaminated soil until the petroleum hydrocarbons combust is another area of research.
Scientists have researched specific plants that can help clean up contaminated sites. Using plants to clean soil is called phytoremediation. Many common prairie plants, like coneflower, survive large amounts of hydrocarbon contamination. In other locations, trees that have deep roots can help dry out soil, which can help in remediation. Read more here about using plants to help remediate contaminated soils.