Oil pipelines have been in the news a lot this past year, between the proposed Keystone XL pipeline and various publicized oil spills. Potential human health effects of these spills are one of the concerns frequently raised, so I’m going to take a fairly high-level look at the potential risks here. Environmental effects are a separate topic that I’ll hopefully get to in the future.
First off, to have a human health risk, you have to have a few conditions met. The first is obviously that you have to have a potentially harmful chemical. Since any chemical, whether natural or synthetic, is potentially harmful at some dose, that one is kind of a given. The second condition is that chemical must get to where humans can be exposed. The third is that humans have to be exposed to enough of the chemical to have a potential for adverse health effects.
When oil is inside a pipeline there isn’t really any potential for exposure. So what happens when oil is released during a pipeline rupture?
When oil is released from a pipeline, it spreads out in the surrounding soil. How far it goes depends on the size of the rupture and how much time passes before it is noticed, the type of oil in the pipeline, and the geology. Most underground spills don’t go a huge distance, but can travel over tens of metres in some cases. Surface spills can travel further since they aren’t as confined, and spills into a water body can potentially travel fairly long distances.
When oil is present in soil, there are basically three things that can happen to it over the short term. The first is being physically bound to the soil; most of the heavier hydrocarbons end up in this state. The second is to enter a vapour form and become airborne; this really only happens to significant extents for fairly light hydrocarbons that might be found in light crude oil, for example. The third is to be dissolved into water between the soil pores, which could eventually move towards the water table – again mostly for light hydrocarbons. Oil spilled on the surface or in water is more likely to end up in vapour or dissolved phases than an underground spill.
So what is the potential for human exposure? The hydrocarbons bound to the soil are pretty much going to stay put, possibly degrading slowly over time (or until remediated); really the only potential for exposure is if a person comes into direct contact with the contaminated soils near the pipeline, and even then risks aren’t generally that high except in the case of something like children playing in the contaminated soil. There is also potential exposure if food crops are grown directly in the contaminated soil, though most petroleum hydrocarbons don’t bioaccumulate to a huge extent since they’re metabolized. Overall there are some localized risks, but once the contamination is identified they can generally be managed fairly readily.
Hydrocarbon compounds in the vapour phase are obviously more mobile, but at the same time they tend to break down fairly quickly in air. The biggest concerns are compounds like benzene which can be present at significant concentrations in light oils, but between the degradation in air, dilution due to air movement, and limited quantities of the volatile compounds, risks are fairly localized and short-term, unless there is a building such as a house on top of or within a few metres of the contamination (which wouldn’t be allowed in most places). Heavier oil mixtures or bitumen don’t have a lot of the light-end hydrocarbons that can enter air.
The portion that is dissolved in soil pore water is probably the portion with the biggest human health risk if it can enter an aquifer used for drinking water. Again this is really only an issue for light-end hydrocarbons; heavier hydrocarbons don’t really move very far and aren’t very soluble. This is really only an issue if there is a shallow aquifer used for drinking water that isn’t geologically isolated from the oil contamination. One of the concerns raised about the Keystone XL pipeline is the potential for impacts to the Ogallala Aquifer, which according to the Wikipedia entry ranges from 30 m to 122 m below the soil surface. I’ve seen soil and groundwater investigations from a lot of oil spills, and even spills of very light oil mixtures don’t travel more than a few metres downward. In the case of the Keystone pipeline the product would be bitumen (which is very insoluble and immobile), the shallowest depth is 30 m and surface recharge to the aquifer is known to be very slow, so there isn’t really any possibility of a pipeline break resulting in bitumen contaminating the aquifer. There are some more volatile compounds used to dilute bitumen for transport, but these compounds are more volatile and have a fairly short environmental life; vertical migration velocities are not high enough for these compounds to travel downwards 30 m before they degrade unless there’s some sort of conduit for them; in my experience it’s fairly rare for them to be found more than about 5 or 6 m below ground surface from a pipeline spill.
The biggest risks to human health occur when a pipeline release occurs into a surface water body, particularly if that surface water body is used as a source of drinking water or for recreational use, since in that case the oil is potentially released directly into the water. This is where efforts should be focused in my opinion – reducing unnecessary surface water crossings, and ensuring that when pipelines do cross surface water bodies that protective measures are taken and proper monitoring is in place.