2.2. Sediment-Water System

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. So, both of them are somewhat similar, but we will start with sediment water, it is the simplest system in terms of what happens.

In the sediment-water system, we have a solid phase (sediment) and a fluid phase (water). These two phases interact with each other, and their relationship is critical for understanding environmental quality, particularly when it comes to contaminants. This system allows us to explore how substances migrate through water and sediment, being simpler than more complex systems involving air.

Imagine a sponge soaked in water. The water represents the fluid phase, while the sponge represents the solid phase. Just like pollutants can interact with water, they can also interact with the solid particles in the sediment, affecting how they move and spread.

So, in yesterday's class we looked at one example where there was a spillage of material. A chemical which sink down and lands on the surface of the sediment. So, these are what is called as dense NAPL or dense non-aqueous phase liquids. So, D-NAPL are those chemicals which are dense and then there are L-NAPL which are light. So, again as we discussed earlier, in D-NAPL the density is greater than density of water, for L-NAPL, the density is less than density of water.

Non-Aqueous Phase Liquids (NAPLs) are chemicals that do not mix with water. They are classified into two types: Dense Non-Aqueous Phase Liquids (D-NAPLs) which sink due to their higher density than water, and Light Non-Aqueous Phase Liquids (L-NAPLs) which float since they have a lower density. Understanding these distinctions is crucial when assessing the behavior of chemical spills in sediment-water systems.

Think of oil and water in a glass. The oil represents L-NAPL, which floats on top, while a heavy syrup could represent D-NAPL, which sinks to the bottom. This visual helps us understand how these substances behave in aquatic environments during spills.

So, when that happens, then there is dissolution occurring based on the mass transfer rate that we looked at yesterday using K A equation, and it spreads downwards, mainly again by dissolution, not by percolation.

When a chemical spill occurs, two processes can happen: dissolution and percolation. Dissolution refers to the chemical mixing into the water, while percolation involves the chemical moving through the sediment's pores. In most cases, the chemicals primarily dissolve in water rather than percolating through the sediment due to the resistance of the water in small pores.

Imagine sprinkling sugar into a cup of water. The sugar dissolves evenly throughout, much like how a chemical contaminant would dissolve into the water body from the sediment. However, if you added larger rocks (representing sediment), the sugar wouldn't pass through them easily (analogous to percolation).

Over a period of time, what can happen is you start with this big spill on the surface and over a period of time, this spill can spread. Over a period of time it can spread, there is no chemical, there is no NAPL here, this is just a spread, this is like a plume again.

As time progresses after a spill, the concentration of chemicals will change. Initially, the chemicals may be concentrated directly at the surface, but due to dissolution and diffusion, they can spread out in the water, forming a 'plume' similar to how smoke disperses in the air. This plume represents the boundary where the concentration of chemical is decreasing.

Consider a drop of ink in a glass of water. At first, the ink is concentrated in one spot but as it sits, the ink spreads through the water forming a lighter, more diffuse color. Similarly, contaminants spread from their source and can affect water quality.

At that point in time, the system now will now look like such that, this is water and there is a large section of contaminated sediment that is sitting inside here.

A contaminated sediment refers to sediment that has absorbed chemicals from the water, resulting in two potential outcomes: the pure chemicals may be gone from the water, and their concentration now exists in the sediment. Over time, these contaminants may continue to leach back into the water, perpetuating the cycle of pollution.

Think of how sponges soak up water. Once the sponge is full, it can release water back into the environment. Similarly, contaminated sediment can release pollutants back into the water, thereby maintaining contamination even long after the original spill.

When we invoke the word history, it means that very long back, we are saying 2 decades, 3 decades and all that. So the consequence is that sometimes when something may have been contaminated 30-40 years back and it is still there and it is causing an effect now, there is an aspect of liability.

Understanding the long-term history of contamination is essential, as pollutants can remain in the environment for decades. This brings to light the concept of liability for past contaminations—those responsible may no longer be in existence, yet the consequences of their actions linger, complicating cleanup efforts and community health.

Consider a factory that illegally dumped waste into a river decades ago. Although the factory has long been out of business, fish and plants near the river may still be affected by the pollutants, creating health risks for nearby communities that rely on that river for fishing. This highlights the importance of historical context in environmental studies.

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