2.7. Equilibrium between Sediment and Pore Water

So, today we will continue from what we were discussing yesterday. The application of mass transfer in the environment, one of the yesterday’s and the last 2 classes we were looking at interfaces where there is a fluid. So, now we look at a case where there is a solid. Specifically, what we are interested in is this system where there is a sediment. One is a solid phase, the other one is a fluid phase. These are what is called as dense NAPL or dense non-aqueous phase liquids. D-NAPL are those chemicals which are dense and then there are L-NAPL which are light. D-NAPL will sink and they will land on the sediment.

In the context of environmental science, D-NAPL and L-NAPL are crucial for understanding how different substances behave in water and sediments. D-NAPL sinks because it has a higher density than water, meaning it will settle on the sediment surface. L-NAPL, being lighter than water, floats. This distinction is vital when assessing the environmental impact of chemical spills.

You can think of D-NAPL like a marble falling to the bottom of a glass of water. It sinks due to its weight. In contrast, L-NAPL is like a balloon filled with air that floats on the surface of the water.

When it enters here, one of the things that does happen to the sinkers, is that the dissolution starts taking place straightaway. Water is flowing, but there is a lot of surface tension when it comes to the small pores hence the water will not allow the material to just sink in nicely. It takes a lot of resistance effort to do that, and therefore, many of these chemicals do not. They find it difficult to get into, depending on the surface tension between these 3 systems.

After a D-NAPL sinks, it begins to dissolve in the water above it. However, it cannot easily percolate into the sediment because of the small pore sizes and high surface tension, which restricts how easily liquid can flow through small openings. Thus, the substance can remain largely on the sediment surface, with dissolution being a dominant process.

Imagine trying to pour syrup into a fine mesh sieve. The syrup might sit on top instead of flowing through due to the resistance and size of the mesh. This analogy illustrates how D-NAPL behaves on sediment surfaces.

Over a period of time, you start with this big spill on the surface and over a period of time, this spill can spread. This is similar to a plume because it marks the boundary of the chemical concentration. As the chemical dissolves, it diffuses slowly and spreads over time.

As D-NAPL dissolves, it creates a concentration gradient whereby the dissolved chemical spreads out into the surrounding water. This creates a 'plume' of contamination, much like smoke spreading through the air, marking the extent of contamination in the water and sediment over time.

Think of a drop of food coloring in water. Initially, it stays as a small dot but gradually spreads out creating a larger, colored area. Similarly, the dissolved chemical spreads as time goes on.

This is the reason why we call it as historically contaminated sediment and these things have a contaminated site. When we invoke the word history, it means that very long back, for instance, 2 decades or 3 decades ago, someone is responsible for this contamination.

The process of contamination can be lengthy, often taking years or even decades to manifest. When a contamination incident occurs, it might not be immediately evident. As time passes, the sediment can retain pollutants that were released long ago, leading to significant environmental and legal implications for remediation efforts.

Consider an old factory site that has left lasting impacts on the surrounding environment. Just like neglecting to clean up an old spill might lead to broader issues years later, past pollution can still affect communities long after it happened.

When we invoke the word history, it means that very long back, we are saying 2 decades, 3 decades and all that. So, this is the flux at a surface. We are interested in the flux n that is coming into the water with an interface with the sediment, defined as n = k × [ρ| - ρ∞].

In modeling the transfer of materials between the sediment and the water, it is important to establish a mathematical relationship describing how substances move across the interface. This relationship involves factors like concentration differences, denoted by n. Flux can fluctuate due to variables such as the degree of diffusion and the chemical concentrations present in the sediment versus the surrounding water.

If you think of it as a teabag in a cup of hot water, the rate at which tea color diffuses into the water depends on how strong the tea is in the bag versus the clarity of the water. Similarly, the flux depends on the concentration difference between sediment and water and how quickly each responds to changes.

--