1.2. Other Mechanisms of Chemical Release From Sediments – Part 2

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 exchange of chemicals between sediment and water occurs through two primary mechanisms: diffusion and resuspension.

• Diffusion is the process where substances move from an area of higher concentration to an area of lower concentration. In sediment, this means that chemicals will naturally move from the sediment into the water if the water has lower concentrations of those chemicals.
• Resuspension involves solid particles breaking free from the sediment and entering the water column, often due to physical disturbances (like storms). When sediments are re-suspended, they create clouds of particles that not only travel downstream but also carry chemicals into the water, impacting water quality significantly.

Imagine stirring a spoon in a glass of sugar water. Initially, the sugar is concentrated at the bottom. When you stir (like the resuspension), the sugar dissolves (like chemicals getting into water) and spreads throughout the liquid, affecting the taste of your drink (similar to how water quality is affected).

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.

To understand the impact of resuspension, it's crucial to estimate how much sediment is being resuspended. This estimation falls outside the primary scope of the course and delves into sediment transport and hydrology, which are heavily influenced by energy in the environment (like waves or currents). The term 'scouring' refers to the physical process where sediments are removed due to forces, leading to changes in sedimentary landscapes.

Think of a river during a flood. The strong current picks up dirt and mud from the riverbed and carries it downstream (the scouring effect), which can change the river’s appearance and quality of water, much like the mechanical stirring in the previous example.

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 okay. As the name suggests bioturbation is the transport caused by biological agent...

Bioturbation is the process where living organisms in the sediment, such as worms and crabs, disturb the sediment and affect the transport of chemicals. These organisms create channels and holes as they burrow, which can facilitate the movement of chemicals into the water more efficiently than diffusion alone. This biological activity not only reworks the sediment but also enhances its porosity, leading to faster transport of chemicals.

Consider how earthworms aerate the soil when they burrow. Just as their movement allows air and nutrients to circulate within the soil, bioturbation allows water and chemicals to move more freely through sediment.

There is a fourth mechanism which can also affect, it is called as advection. Now, advection is bulk flow of material coming out...

Advection refers to the bulk movement of water or sediment that carries chemicals along with it. In sediments, this process is less common than in soils because sediments are usually saturated and at static equilibrium. However, certain reactions, like the production of gases such as methane, can create bubbles that disturb sediments and carry chemicals upwards. Advection can thus affect the release of chemicals from sediments into the water column.

Imagine a soda bottle. When you shake it, gas bubbles form and create pressure, which, when released, carries liquid with it (the advection effect). Similarly, in sediment, gas production can disturb the sediment and push chemicals up into the water.

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