2.1. Mechanisms of Release

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.

The mechanisms of chemical release from sediments primarily involve diffusion and resuspension. Diffusion refers to the process by which molecules move from an area of high concentration to an area of low concentration within the sediment and water interface. Resuspension occurs when solid particles are lifted from the sediment and mixed into the water column, possibly carrying contaminants with them.

Imagine stirring a spoon in a cup of sugar water. Just as the sugar slowly dissolves and spreads through the water (diffusion), when you stir the cup harder, sugar crystals might get lifted from the bottom and suspended in the water (resuspension).

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.

The process of resuspension leads to several outcomes. Contaminated solids become suspended in the water, and while they remain in suspension, contaminants may desorb or release from the solids into the water. Eventually, these particles may settle back to the bottom, potentially affecting sediment quality and water clarity.

Think of a plastic bag filled with sand being picked up by a gust of wind. Initially, the sand is dispersed into the air, becoming airborne. As the wind subsides, some sand falls back down while others float away.

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.

To assess the impact of resuspension on water quality, it is essential to quantify how much sediment is being resuspended and transported. Although quantifying resuspension intricacies is complex and typically requires advanced study in sediment transport or hydrology, recognizing the fundamentals is crucial for understanding sediment dynamics.

Consider a beach after a storm; the waves can stir up sand and sediment. Scientists may use observations of cloudiness in the water to estimate how much sand was resuspended.

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.

Bioturbation refers to the disturbance and mixing of sediments by living organisms, such as worms or crabs. These organisms can burrow through the sediment, compacting and loosening it, which can enhance the movement of chemicals from the sediment into the water, thus playing a significant role in sediment chemistry.

Envision a gardener using a spade to turn soil. Just like the spade loosens and aerates the soil, allowing nutrients to mix and be more accessible to plants, bioturbating organisms mix sediments, helping release chemicals into the water.

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.

Bioturbation can enhance chemical transport in sediment. Through various actions, like burrowing or feeding, organisms increase sediment porosity, enabling better diffusion of chemicals. This process can speed up the transfer of contaminants from sediment to water, impacting overall water quality.

Imagine a sponge that gets squished; it pushes out the water trapped inside. Similarly, when organisms burrow into the sediment, they change its structure, making it easier for chemicals to move into the water.

There is a fourth mechanism which can also affect, it is called as advection.

Advection refers to the bulk movement of fluids that can carry dissolved contaminants along with them. While sediment is generally not very mobile, certain conditions, such as the formation of gas bubbles or changes in water flow, can create channels that facilitate the upward movement of contaminants through advection.

Think of a straw; when you sip a drink, you're not just moving the liquid but also all the flavors and nutrients mixed in. Advection works similarly, as it transports dissolved materials along with the water flow.

Within diffusion, there are very minor things, DOC facilitated transport.

Dissolved organic carbon (DOC) can affect the movement of chemicals in sediments. As organic matter changes due to environmental reactions (like pH shifts), it can release attached contaminants into the pore water, facilitating their movement. This process relies on the behavior of colloids and how they interact with other chemicals.

Imagine a sponge soaked in juice. If you squeeze it gently, the juice seeps out. Similarly, when conditions change in the sediment, organic materials can release trapped chemicals into the surrounding water.

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