This group of processes destroys the transported substance. It includes radioactivity decay of radionuclides and either chemical or biological degradation of unstable substances in the subsurface environment.

The group of processes you're referring to is generally known as "attenuation processes". These processes reduce the amount, concentration, intensity, or quality of a substance as it moves through the subsurface environment. Here's a step-by-step breakdown:

1. Radioactive Decay: This is a natural process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. This changes the radionuclide into a different element, often a more stable one. For example, radium-226 decays to radon-222.

2. Chemical Degradation: This involves the breakdown of unstable substances through chemical reactions. These reactions can change the physical and chemical properties of the substance, often making it less harmful. For example, iron can chemically degrade certain contaminants like chlorinated solvents.

3. Biological Degradation: This is the breakdown of substances by living organisms, usually microorganisms like bacteria or fungi. They do this by using the substance as a source of energy or nutrients, converting it into less harmful substances as a result. For example, certain bacteria can biodegrade petroleum hydrocarbons in oil spills.

These processes are important in fields like environmental science and waste management, as they can help reduce the impact of pollutants and waste materials.