What material is recommended to be installed under a filter layer in an infiltration basin?

The recommended material to be installed under a filter layer in an infiltration basin is typically stone or other media. This choice is effective because it allows for optimal drainage and filtration of stormwater before it enters the underlying soil layers. Stone or other media provide a stable structure that prevents the filter layer from becoming clogged with sediment and organic material, thereby maintaining the infiltration capacity and overall functionality of the basin.

By using stone, proper void spaces are created to facilitate water movement while also serving as a barrier to keep larger particles from passing through to the subsoil. This design helps in protecting the infiltration system from potential clogging, which is crucial for the long-term performance and maintenance of the infiltration basin. Additionally, using stone or similar materials can enhance the aeration of the system, supporting microbial activity that further aids in treating stormwater.

In contrast, the other materials listed, such as pea gravel, coarse aggregate, or sand, may not provide the same level of structural integrity or filtration efficiency when used as the foundational layer in an infiltration basin. Each of these alternatives might lead to challenges related to sedimentation or stability that could compromise the system's effectiveness over time.