6. Complexity of Sediment Transport

We were looking at the exchange between the sediment-water. So, the mechanisms of release what we had talked about one is by diffusion.

So it is diffusion inside the sediment plus conductive mass transfer at the interface. The second thing that we talked about is resuspension. This is where material can get re-suspended, so solid particles will get into the water and this entire cloud of suspended particles will go downstream.

When this cloud of particles goes downstream several things happen. The contaminated solids are suspended into water and while they are there, the desorption of chemical from the water occurs. Then this particle also settles back.

So, when there is a sudden gust of energy, it picks up material and goes into bulk. So, you will see this turbidity or the TSS all increases in the water and then it goes downstream and then it will also deposit back into the sediment.

This chunk discusses the basics of sediment resuspension and its effects on water quality. When solid particles from the sediment are disturbed, they re-enter the water column and can carry contamination with them. As these particles are suspended, chemical contaminants can desorb into the water, increasing pollution levels. Over time, these solids settle back to the bottom, influencing sediment composition and potentially polluting water sources.

Imagine stirring up a jar of sand and water. As you stir, the sand particles (sediment) become suspended in the water, making it cloudy (or turbid). Just like how the sediment carries pollutants, the sand may also carry any substances it has absorbed, leading to water contamination.

In order to estimate the effect of this, one needs to estimate what is the resuspension that is happening? How much of resuspension happens? That is not in the core purview of this course. What that means that is more of sediment transport, sediment hydrology, so that depends on the energy, that process is called a scouring.

This chunk emphasizes the importance of understanding resuspension in sediment transport studies. Scouring refers to the removal and suspension of sediment due to energy input, often from events like storms or high water flow conditions. Therefore, accurately estimating the amount of resuspension is crucial, as it influences the dynamics of water quality and sediment composition.

Think of a river during a heavy rainstorm. The increased energy from the flowing water can erode riverbanks and stir up sediment on the riverbed, causing clouds of silt to mix with the water—this is similar to how resuspension happens in nature.

There is a third mechanism by which chemical can get into the water which is not a very well known process, what is called as bioturbation. As the name suggests bioturbation is the transport caused by biological agent. This transport of chemicals in the sediment is due to biological agents, but what do you mean by biological agents?

Bioturbation refers to the mixing and transportation of sediments and chemicals caused by organisms living in or on the sediment surface. These biological agents, such as worms and crustaceans, play a crucial role in enhancing chemical transport by disturbing the sediment, making it easier for chemicals to move into the water column. They alter the structure and properties of the sediment and can speed up the release of contaminants.

Consider how earthworms aerate and enrich soil. Similarly, in aquatic environments, creatures like worms can mix sediments, bringing previously settled contaminants back into the water, akin to stirring a pot of mixed ingredients, where each stir reintegrates previously settled particles.

The action of bioturbation itself is different things, so people have observed this action about, how does it affect chemical transport in the top layer? ... if you want to model, you apply our previous model to this, our previous model was very simple system.

We have, this is our domain, this is contamination, this is sediment here and there is water. We are now simply looking at all of this entire region has the same property, so we are looking at diffusion.

This chunk introduces how models are used to understand the effects of bioturbation on chemical transport in sediments. By applying mathematical models to these systems, we can observe how chemicals diffuse and how biological activities influence these processes. The behavior of contaminants can change significantly based on factors like sediment disturbance and biological activity.

Imagine a sponge soaking up water. The sponge's structure allows it to absorb water quickly. When bioturbators disturb the sediment, it’s like squeezing the sponge to release that water back out—this models how disturbances in the sediment can rapidly alter the transport dynamics of the chemicals contained within it.

So, these are all very complex issues related to the biogeochemistry of the region, okay. If you look at sediments in coastal regions along seacoast, there is very little organic carbon, it is all mostly sandy. If somebody dumps, in India large number of industries are located on the coast... Dissolutions are very different, profiles are very different from if it is in a river or a lake which has a lot of organic carbon and a lot of clay, which affects its fate and transport into the sediment as well as its release from the sediment later on.

The final chunk addresses the variability of sediment systems based on location. Different environments (like coastal vs. freshwater systems) present challenges in understanding how sediments interact with contaminants. Factors like the organic content and sediment composition greatly influence how chemicals behave and are transported in these environments, highlighting the need for region-specific studies in sediment transport.

Consider two types of sponges: a kitchen sponge that's very porous (like sandy coastal sediments) versus a bath sponge that's dense and absorbent (like clay-rich river sediments). Each behaves differently when exposed to water or contaminants, emphasizing that where chemicals are deposited influences how they will be managed and transported.

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