Which statement regarding uniform grains is true?

Uniform grains refer to sediment or soil that consists of particles with similar sizes. This characteristic tends to contribute to high porosity, which is the measure of void spaces in a material. High porosity indicates that there are many small pores or spaces between the particles, allowing for greater capacity to hold water or air.

In contexts such as drainage and soil percolation, high porosity in uniform grains facilitates the movement of water through these grains, making them a suitable choice for various on-site sewage facilities. Consequently, this increased porosity can improve the efficiency of wastewater treatment systems, as it enhances microbial activity due to better aeration and access to nutrients.

The other options, while they may relate to different characteristics or effects of soil properties, do not accurately describe uniform grains. For instance, low porosity would imply the opposite of high porosity, which does not apply to uniform grains. Similarly, soil fertility and water retention capabilities must be properly assessed in relation to specific soil mixtures and conditions rather than just the uniformity of grain size.