Which of the following best describes the drain field's role in a septic system?

The drain field, also known as a leach field, plays a crucial role in the septic system by filtering and treating wastewater before it is released into the groundwater. After wastewater leaves the septic tank, it enters the drain field, where it is distributed through a network of perforated pipes buried underground. As the wastewater percolates through the soil surrounding these pipes, various natural processes help to filter out contaminants and break down harmful pathogens.

The soil acts as a natural filter, providing additional treatment as the wastewater moves through it. Beneficial microorganisms in the soil further assist in decomposing organic materials and neutralizing harmful substances. This process significantly reduces the risk of groundwater contamination, making the drain field essential for the proper functioning and environmental safety of the septic system.

The other options describe functions that are not aligned with the drain field’s primary purpose. For instance, storing wastewater is a role of the septic tank, while separating solids and liquids is also a function of the tank itself. Collecting excess rainfall runoff does not pertain to the septic system's treatment process and is not a function of the drain field. Thus, option B accurately captures the important function of the drain field in treating wastewater.