4. Modeling of Chemical Transport

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.

Resuspension involves solid particles from the sediment being lifted into the overlying water column, forming clouds of suspended particles. As these particles travel downstream, they carry associated contaminants with them. This movement can impact water quality significantly as the particles can re-settle back into sediment after traveling some distance.

Imagine stirring a glass of muddy water. As you stir, the mud (solid particles) gets mixed into the water, creating a cloud. If you then stop stirring, the mud may settle again at the bottom. Similarly, resuspension in water bodies allows contaminants to move and disperse, but they can eventually settle back down.

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.

Estimating resuspension involves understanding various factors, including the energy conditions in the environment, such as during storms or high flow conditions. It is a complex analysis that might be more aligned with sediment transport studies rather than purely chemical transport.

Think of how wind can lift dust off the ground, making it airborne. Similarly, in water bodies, powerful currents or waves can lift sediments, but understanding and predicting how much sediment gets lifted involves studying environmental conditions.

The contaminated solids are suspended into water and while they are there, the desorption of chemical from the water occurs.

Once suspended, the contaminants attached to solid particles can begin to desorb or release into the water. This process can lead to higher concentrations of contaminants in the water than what was originally present, thereby impairing water quality. Essentially, the act of resuspension can temporarily increase pollutant levels in the water column.

Imagine pulling a sponge out of a bucket of soapy water. When you pull it out, the soap (chemical) clings to the sponge (solid particle). If you then squeeze the sponge, soap can get released back into the water, showing how contaminants can move between phases during resuspension.

Once a particle is suspended, it can desorb, it can transfer from the solid phase to a liquid phase, when it comes to liquid phase, it can also go to the atmosphere.

Once contaminants are released into the water from suspended particles, they can dilute and spread further, potentially even into the atmosphere under certain conditions. This broader dispersal makes it crucial to monitor and manage water quality, as contaminants can affect aquatic life and human health.

Consider how a spray bottle works. When you spray water, the liquid is dispersed into the air in tiny droplets. Similar mechanisms can occur in water bodies where chemicals can move from liquid to gas phase after resuspension, contributing to environmental pollution.

The other 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 process of biological organisms, primarily worms and other creatures, moving through sediment. This activity can disrupt the sediment structure and promote the movement of chemicals and sediments into the water column. Biological agents essentially enhance the transport of materials compared to simple diffusion.

Think of how gardeners till the soil by turning it with a shovel. This disturbance allows air and nutrients to mix in better, enhancing plant growth. Similarly, organisms like worms enhance chemical transport by mixing the sediments, facilitating contaminant movements.

Bioturbators are unconsolidating the sediment, so making it fluffy, so that from a transport point of view if you apply your diffusion equation through this.

The activity of bioturbators increases the porosity of the sediment by making it less consolidated. This change in sediment structure impacts how easily chemicals can diffuse through the sediment, making the transport more efficient. A 'fluffier' sediment allows for quicker movement of pollutants than a compacted one.

Imagine how fluffy cotton candy allows air to flow through it easily compared to a solid brick. In sediments, bioturbators help create a lighter, more aerated structure, facilitating greater movement of dissolved chemicals.

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