2.4. Fate and Transport of Contaminants

So, 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. So, it is sediment, water or soil, air systems.

In this section, we focus on the interaction between solid sediments and fluids (like water) in the environment. Here, we consider sediments, which act as a solid phase, alongside fluids such as water. Both phases interact in many ways, affecting the way contaminants behave and move within the environment.

Think of a sponge soaked in water. The sponge represents the sediment, while the water is the fluid phase. When you add a drop of food coloring on the sponge's surface, the coloring slowly spreads through the sponge, similar to how contaminants can diffuse within sediment.

These are 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.

Contaminants can be classified into two types based on their density compared to water: dense NAPLs and light NAPLs. Dense NAPLs (D-NAPLs) sink in water, while light NAPLs (L-NAPLs) float. This classification is critical because it influences how these contaminants move and behave when spilled. D-NAPLs can settle on the sediment, making them harder to remove, while L-NAPLs may float on the water surface.

Imagine oil (an L-NAPL) floating on a puddle after it rains. The oil forms a layer above the water. In contrast, think of honey (a D-NAPL) being added to a glass of water; the honey sinks to the bottom, making it difficult for anyone to see it immediately.

When it enters here, one of the things that does happen to the sinkers, is that the dissolution starts taking place straightaway.

When D-NAPLs (dense non-aqueous phase liquids) land on the sediment, they undergo a process called dissolution, where they begin to mix with the surrounding water. As water flows, it creates a gradient that helps carry the dissolved contaminants downward into the sediment, but this process can be slow and complicated by several factors such as surface tension and pore resistance.

Think of sugar dissolving in a glass of water. When you drop sugar into water, it begins to dissolve immediately. However, if you have a lot of sugar at the bottom (like D-NAPL in sediment) and only a small amount of water above (the flowing water), it can take time for all the sugar to dissolve and spread evenly.

If percolation is possible, it will do percolation in porous medium. It is very hard especially in the presence of water in a pore provides lot of resistance for displacement.

Percolation refers to the movement of liquids through porous materials, such as sediment. In the case of D-NAPLs, the presence of water in the sediment pores can create resistance against their movement. Therefore, while some of these contaminants may be able to percolate down through the sediment, many times they simply sit on the surface, primarily undergoing dissolution instead.

Imagine you have a layer of sand and you slowly pour water over it. If the water cannot easily flow through the sand because it is already saturated, it will tend to stay on the top. Similarly, contaminants can get 'stuck' at the surface rather than moving down into the sediment.

Over a period of time, this spill can spread...We are calling this a plume because it marks the boundary of the chemical concentration.

As time passes, the dissolved contaminants spread out from the original spill site, creating a 'plume' of contamination in the water. This plume represents varying concentrations of the dissolved chemicals, similar to how a melting ice cream creates a streak on the surface beneath it as it spreads out.

Consider how a drop of food coloring in a clear glass of water gradually spreads out. At first, the coloring is concentrated in one area, but if you wait, it forms a larger, diluted color spread throughout the glass, analogous to how contaminants diffuse over time in water.

... these things have a contaminated site. When we invoke the word history, it means that very long back, we are saying 2 decades, 3 decades...

Contaminated sediments can be a lingering environmental issue because the effects of contamination can last for many years. Even if a spill happened decades ago, the residual chemicals can continue to impact water quality and aquatic life today, hence identifying who is responsible for these sites becomes challenging.

Imagine a factory that was dumping waste into a river decades ago. Even after the factory closes, the pollutants may remain in the sediment at the riverbed, affecting fish and water quality for years to come, much like an old wound that hasn't fully healed.

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