The Keystone XL Pipeline (TransCanada) is a proposed addition to the Keystone Pipeline that would primarily carry dilute bitumen (dilbit) from the Athabasca oil sands in Alberta, Canada to refineries near the Gulf of Mexico in Texas. The pipeline addition would consist of 36" diameter pipes buried 4' underground, extend over 1,700 miles and carry up to 830,000 barrels per day (bpd).[1] Potential environmental impacts from developing the Keystone XL section include effects to wetland areas, shallow groundwater, nearby surface water, and sensitive plant and animal species. Of the potential environmental impacts, contamination of the Ogallala Aquifer is a major concern of many agencies as well as the public.[2][3][4] The Ogallala Aquifer supplies ~30% of the United States total irrigation water and ~82% of the drinking water for the >2.3 million people (1990 Census) who live within the aquifer boundary.[5] The region accounts for 19 percent of wheat, 19 percent of cotton, 15 percent of corn, 3 percent of sorghum, and 18 percent of cattle production in the U.S.[5] To properly assess the risk of contamination to the Ogallala Aquifer a step-wise process following hazard identification, exposure assessment, and uncertainty is outlined.
As of November 10, 2011, the US Department of State has decided to delay the pipeline decision after mounting concerns about the sensitive Sand Hills (Nebraska) area, especially regarding the Ogallala Aquifer.[6][7] A final decision is expected in early 2013.
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Oil spilled from the pipeline is not likely to reach the aquifer as dilbit is highly viscous at ambient temperatures. The major human health concern refers to the dissolution of benzene (collectively BTEX) from crude oil to drinking water in the aquifer . Typically after a spill hydrocarbons will volatilize, but being 4’ underground limits that process along with biodegradation and photodegradation for the most part. The likely scenario of aquifer contamination would consist of a leak in the pipeline, followed by a precipitation event, dissolution and equilibration of benzene in water, and percolation down to the aquifer. Benzene can cause many health problems and is carcinogenic with EPA Maximum Contaminant Level (MCL) set at 5 μg/L for potable water.[8] Although it is not well studied, single benzene exposure events have been linked to acute carcinogenesis.[9] Additionally, the exposure of livestock, mainly cattle, to benzene has been shown to cause many health issues such as neurotoxicity, fetal damage and fatal poisoning.[10]
Previous work[11] has shown that a ‘worst-case exposure scenario’ can be limited to a specific set of conditions. Based on the advanced detection methods and pipeline shut-off SOP developed by TransCanada, the risk of a substantive or large release over a short period of time contaminating groundwater with benzene is unlikely.[12] Detection, shutoff, and remediation procedures would limit the dissolution and transport of benzene. Therefore the exposure of benzene would be limited to leaks that are below the limit of detection and go unnoticed for extended periods of time.[11] Leak detection is monitored through a SCADA system that assesses pressure and volume flow every 5 seconds. A pinhole leak that releases small quantities that cannot be detected by the SCADA system (<1.5% flow) could accumulate into a substantive spill.[12] Detection of pinhole leaks would come from a visual or olfactory inspection, aerial surveying, or mass-balance inconsistencies.[12] It is assumed that pinhole leaks are discovered within the 14 day inspection interval, however snow cover and location (e.g. remote, deep) could delay detection. Benzene typically makes up 0.1 – 1.0 % of oil and will have varying degrees of volatility and dissolution based on environmental factors.
Although the Pipeline and Hazardous Materials Safety Administration (PHMSA) has standard baseline incident frequencies to estimate the number of spills, TransCanada altered these assumptions based on improved pipeline design, operation, and safety.[12] Whether these adjustments are justified is debatable as these assumptions resulted in a nearly 10-fold decrease in spill estimates.[11] Given that the pipeline crosses 247 miles of the Ogallala Aquifer,[13] or 14.5% of the entire pipeline length, and the 50-year life of the entire pipeline is expected to have between 11 – 91 spills,[11] approximately 1.6 – 13.2 spills can be expected to occur over the aquifer. An estimate of 13.2 spills over the aquifer, each lasting 14 days, results in 184 days of potential exposure over the 50 year lifetime of the pipeline. In the reduced scope ‘worst case exposure scenario,’ the volume of a pinhole leak at 1.5% of max flow-rate for 14 days has been estimated at 189,000 barrels or 7.9 million gallons of oil.[11] According to PHMSA’s incident database,[14] only 0.5% of all spills in the last 10 years were >10,000 barrels. This suggests that an accumulated leak is unlikely to release 189,000 barrels.
Benzene is considered a light aromatic hydrocarbon with high solubility and high volatility. It is unclear how temperature and depth would impact the volatility of benzene, so assumptions have been made that benzene in oil (1% weight by volume) would not volatilize before equilibrating with water.[11] Using the octanol-water partition coefficient and a 100-year precipitation event for the area, a worst-case estimate of 75 mg/L of benzene is anticipated to flow toward the aquifer.[11] The actual movement of the plume through groundwater systems is not well described, although one estimate is that up to 4.9 billion gallons of water in the Ogalla Aquifer could become contaminated with benzene at concentrations above the MCL.[11] The Final Environmental Impact State from the State Department does not include a quantitative analysis because it assumed that most benzene will volatilize.[12]
Of the two reports that have examined the potential for benzene to contaminate the Ogallala Aquifer, one assumes the quantity released is insignificant and the persistence minimal,[12] while the other assumes a high amount released with high persistence,[11] There are speculations concerning the corrosiveness of dilbit - especially when pumped at 158oF,[15] the detection and shutdown time, and the assumptions for spill frequency-volume.[11] However, TransCanada has offered to comply with 57 special conditions set forth by the PHMSA and claim to have superior pipelines compared to those documented in the past.[16]