Recently, I released an article called: Mom, I Think I Spilled Some Oil. First, I want to assure my own mom that I didn’t actually spill anything. This clarification is rather important given how atrocious my room looked for most of my childhood. In this article, I want to talk a little bit about remediation strategies. How do we get oil (or other contaminants) out of the subsurface once they finds their way in?
While I’m only presenting three options, there is a whole array of available strategies, with the choice fundamentally coming down to the nature of the spilled substance, the location and severity of the release, and the costs at play. A keyword in any spill scenario is ‘plume‘. The plume is the ‘blob’ or mass of contaminants that spreads through the subsurface, largely following the path of groundwater flow. While groundwater doesn’t generally flow in large open channels like rivers on the surface, it does follow a preferential direction, which can be predicted with computer simulations based on some knowledge of the subsurface. (What I’m working on for my Master’s thesis involves just that!)
1. Pump and treat
Pump and treat is a widely accepted remediation strategy due to its cost-effectiveness and simplicity. A pump and treat operation consists of several wells that extract contaminated water from the subsurface, and a treatment plant either on or off-site that returns the water to an acceptable quality standard. Once treated, the water can be released into a surface water body or injected back underground.
2. Permeable reactive barrier
A permeable reactive barrier is essentially a curtain that is placed across the flow path of the contaminated groundwater. The curtain reacts with the contaminant, either immobilizing it or converting it into a non-toxic form. The ‘active ingredient’ commonly used in these barriers is zerovalent iron, which reacts with just about anything it happens to find. Since installation of such a barrier requires excavation and construction, the direction and speed of groundwater flow must first be known with certainty.
Bioremediation involves stimulating natural biological processes that break down (decompose) certain compounds. No matter where you look (provided you have a microscope), microbes are constantly breaking down organic matter into energy for themselves. A good example of this process is composting. What might look like an oil spill to us is simply dinner to a microbe. The limiting factors in the decomposition process are oxygen and nutrients. Thus, a bioremediation operation might include the injection of oxygen and ‘fertilizer’ into the subsurface to stimulate microbial activity.
Though these processes might sound simple enough, it’s important to remember that we’re dealing with a world that we can’t see. It’s like blindfolding someone and sending them into an unfamiliar house to repaint the walls. No matter what remediation technology is used, sites must first be painstakingly analyzed. There are risks involved in any cleanup operation. Misjudge groundwater flow or chemical reactions and the remediation strategy might prove ineffective. Drill a well in the wrong place and a confined pool of contaminants might have a new conduit to spread. If you’ve ever wondered what hydrogeologists and environmental engineers do – this is it.
Be warned: you may have just sustained a lethal dose of mostly harmless science.
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Cover photo courtesy of CameliaTWU, Flickr Creative Commons