Spills on Land: Contamination of Groundwater

Some of the most difficult and troublesome spills to cleanup are spills on land. The lower the viscosity of the spilled material, the greater challenge it will be to clean it up, and the greater cost of the cleanup. For example, Bunker C or Number 6 Oil is viscous at around 200° F (93.3 °C). Below that point, it is semi-solid and solid. A spill of hot oil contacting the ground will solidify quickly and will penetrate the ground only a few centimeters. Cleanup is accomplished with a shovel and only a very small fraction (<0.1%) of the oil is mobile or soluble to be conveyed to the groundwater.

By comparison, gasoline, kerosene, and many refined oil fractions and lots of chemicals have much lower viscosities. They can and will enter the soil and make their way down to the groundwater. It has been estimated that for 100 units (either gallons or liters) of a refined petroleum product, such as No 2 Diesel Oil spilled on to a sandy soil or sandy loam, approximately 75% will remain above the groundwater, smeared into the interstices of the soil particles. About 24% of the spill will be found floating on the surface of the groundwater in a lens-shaped structure. The remaining 1% of the material will be dissolved in the water column, migrating with the groundwater.

The cost of the cleanup is in inverse proportion to the amount of material: If the cleanup of the material in the interstices costs 1 unit of cost, the cleanup cost of the floating material will cost approximately 10 times the interstitial cleanup cost, and the dissolved material may cost 100 times the cost of the interstitial cleanup cost. This, of course, depends upon the viscosity of the material spilled, temperature, soil grain sizes, and other factors.

Remediation of the groundwater is generally out of scope of this article because of the complexities, and detail required for satisfactory explanation of the topic and techniques is too long for this article, and various treatises on the topic have been published in book form. However, the cleanup of the groundwater cannot be accomplished until the surficial and other soils above the groundwater have been cleaned up. The general techniques for groundwater cleanup include (i) pumping and treatment of groundwater by extraction and treatment of the pumped groundwater to remove the contaminants; (ii) vapor extraction - injection and/or withdrawal of the air above the groundwater to evaporate the contaminant materials in the soil interstices which may, depending upon the nature of the contaminant, also remove materials from the water column; (iii) groundwater vapor stripping by injection of air into the groundwater beneath the contaminant plume combined with vapor extraction of air above the groundwater; and (iv) chemical and organic enhancement of the groundwater to encourage natural attenuation. All of these remedial techniques are expensive and require the installation of a number of exploration and production wells, and long-term monitoring.

In recent years, and because of the cost of remediation, many states are engaging in natural attenuation for the lighter petroleum fractions. This approach lets Mother Nature perform degradation of the contaminants through action of the natural bacteria in the soils.

Another technique suitable for shallower soils (<10 m in depth) is specific types of phytoremediation. For shallow contamination, trees and certain plants will remove the contaminants and expire them through their leaves or treat them through their rhizomes. The process of natural attenuation is extremely slow, and for recalcitrant compounds, it can prove unsatisfactory.¹² The recalcitrant compounds described often are the phenol and asphaltene compounds and other high molecular weight and complex organic materials. The simpler compounds such as benzene, toluene, etc., are volatile and can often be Phyto remediated and bioremediated by natural means.

The best way of preventing groundwater and water contamination is not to allow it to occur in the first instance.

Further Reading:
1. David, L., Russell, P.E. (2011). Remediation Manual for Contaminated Sites. CRC Press, available in e-pub format from Amazon and others.
2. Russell, D.L. (1992). Remediation of Petroleum Contaminated Sites. Technomic Publishing.
3. Russell, D.L. (2012). Remediation Manual for Contaminated Sites. CRC Press.
4. Fingas, M. (2013). The basics of Oil Spill Cleanup, 3e. CRC Press