Sometimes, we spill oil. Actually, in Alberta we spill quite a bit. On average, there have been two releases a day for the past 37 years, including crude oil, contaminated water and process chemicals, although this number has fallen steadily in the last decade. While we might see some of the larger spills on the news, such as October’s release of process water from a coal mining operation into the Athabasca River, most releases are small and pose little threat to the environment if managed quickly and with due care.
Part of the role of hydrogeology (the branch of geology dealing with water at or below the surface) is to model the behavior of these releases in the subsurface. To illustrate this, let’s take you through a hypothetical spill scenario. Our spill starts, as these things often do, with an industrial accident. A train carrying unrefined hydrocarbons derails, and two tanker cars of ‘product’ are spilled onto the gravel shoulder of the track. By the time the cleanup operation is underway, all of the product has drained into the subsurface.
The first question to ask is: “What exactly did we spill?” To understand how the contaminant will behave, including where it will flow and how fast, we need to know some of its properties, including its miscibility with water and its density. Let’s try this again in English. Miscibility simply refers to the ability of two liquids to mix. If we spilled brine (very salty water), it would mix into groundwater. On the other hand, if we spilled oil, it would either float on the water table (the groundwater surface) or sink to the bottom without mixing, depending on its density (weight). The technical term for such liquids is NAPLs (non-aqueous phase liquids), with LNAPLs lighter and DNAPLs denser than water.
A lot of tech talk, I know, but it is important especially where oil spills are concerned. Some hydrocarbons, such as gasoline and diesel are LNAPLs, while heavy oil and tar are DNAPLs. To complicate things further, certain constituents of oil mixtures will dissolve in water, while others will not. Moral of the story? It’s incredibly important to know what was spilled, both from a geological standpoint and an environmental one, since the nature of the product also defines the environmental threats it poses.
For the purposes of this exercise, let’s imagine that we have a DNAPL on our hands. The contaminant’s first subsurface stop would be the unsaturated zone. In this zone, the pore space (empty space) between rock grains is filled with air. The contaminant would flow downwards, much like syrup through a bucket of sand. Since we’re dealing with a large volume of ‘syrup’, most of it would eventually find its way to the water table. On its way, however, some of the contaminant will coat the grains it encounters, leaving behind an oily ‘smear’ (residual) along its path.
The water table marks the top of the saturated zone, where the pore space is occupied by water. The DNAPL will hardly notice the difference, continuing to migrate down through the aquifer until it reaches the confining layer (aquitard) that marks the base of the reservoir. The rock grains in this layer will be packed close enough together to prevent infiltration of groundwater or DNAPL. Provided there are no cracks in this layer, this is where the DNAPL will (for the most part) remain. Since geological layers are rarely perfectly horizontal, the DNAPL might continue to flow downslope before settling into a local dip or valley.
Stay tuned for Part 2: Mom, How Do I Clean It Up?
Be warned: you may have just sustained a lethal dose of mostly harmless science.
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