What is the main feature of the annulus in well construction?

The main feature of the annulus in well construction is that it serves as a protective barrier. The annulus refers to the space between the well casing and the surrounding borehole. This space is crucial for maintaining the integrity of the well structure. It prevents contaminants from entering the well, thereby protecting the quality of the groundwater.

By having an annulus, which is often filled with grout or other sealing materials, the well is safeguarded against surface water infiltration and other pollutants that can potentially compromise the water supply. This barrier is essential in ensuring that the well water remains safe for consumption and use.

Other choices such as containing voluminous water or providing a space for installing pumps do not accurately describe the primary function of the annulus. While the design of the well may include ways for water flow and pump installation, these aspects are secondary to the protective role of the annulus. Similarly, connecting multiple wells is not a feature of the annulus, as each well typically stands alone in its construction unless specifically designed otherwise, usually for different purposes.