Which soil type classified by the Unified Soil Classification System is the best aquitard?

The best aquitard, according to the Unified Soil Classification System, is classified as CL, which denotes clay soils with low plasticity. An aquitard is a geological formation that restricts water flow, allowing minimal movement of groundwater. Clay soils, particularly those with low plasticity, possess small particle sizes and high moisture retention abilities. These characteristics lead to tight packing of soil particles, creating small voids that significantly impede the movement of water.

In contrast, the other soil types listed—such as SP (poorly graded sand), ML (silty soil), and SW (well-graded sand)—do not serve effectively as aquitards. Sand has larger particle sizes and greater porosity compared to clay, allowing water to move more freely through the soil structure, thus making them unsuitable for retaining water as an aquitard. Silty soil also allows more permeability than clay, further diminishing its effectiveness in serving as an aquitard. Therefore, CL clay is the most effective choice for serving this particular role in the Unified Soil Classification System.