Where should geotextile fabric be installed in a drainfield?

The installation of geotextile fabric in a drainfield is primarily intended to separate the porous media—such as gravel or sand—from the backfill material. By placing the geotextile fabric between these two layers, it functions to prevent the backfill material from migrating into the pores of the porous media, which can clog the drainfield and disrupt the proper flow of effluent.

This separation serves several vital purposes: it maintains the structural integrity of the drainfield, ensures that water can move freely through the porous media for effective filtration and treatment of wastewater, and helps to prolong the life of the drainfield system by minimizing the potential for blockages.

The other choices do not provide the functional benefits that occur with this particular placement of the geotextile fabric. For example, installing it above or below the porous media may not adequately prevent migration of backfill particles, potentially compromising the drainfield's performance and efficiency.