This form of groundwater contamination is called residual contamination . Many processes can affect how contamination spreads and what happens to it in the groundwater, potentially making the contaminant more or less harmful, or toxic. Some of the most important processes affecting hazardous substances in groundwater are advection, sorption, and biological degradation. Advection occurs when contaminants move with the groundwater. This is the main form of contaminant migration in groundwater.
Sorption occurs when contaminants attach themselves to soil particles.
Sorption slows the movement of contaminants in groundwater, but also makes it harder to clean up contamination. Biological degradation happens when microorganisms, such as bacteria and fungi, use hazardous substances as a food and energy source. In the process, contaminants break down and hazardous substances often become less harmful. Cleaning up contaminated groundwater often takes longer than expected because groundwater systems are complicated and the contaminants are invisible to the naked eye. This makes it more difficult to find contaminants and to design a treatment system that either destroys the contaminants in the ground or takes them to the surface for cleanup.