It's the job of the drainfield to disperse a septic tank's effluent. Many drain systems have a series of trenches that branch out from a distribution box. Some have a single, larger bed; a seepage pit; or a similar means of distributing tainted water back into the ground (the right one for your home depends on local codes, conditions, and practices).

The type and layout of the drainfield is designed according to the absorption qualities of the soil. To gather information on how readily the soil will absorb water, a soils engineer or septic contractor conducts 'perk' (percolation) tests by digging holes in several places in the yard and filling them with water.

A conventional gravel-and-pipe drainfield begins with a level-bottom trench located from 1 to 3 feet beneath the ground, but at least 2 feet above the groundwater table. A perforated drainpipe is centered along the trench over 6 to 12 inches of gravel and then covered with a few more inches of gravel. A silt barrier'a synthetic fabric'covers the gravel and pipe to help keep out silt and soil, then the trench is backfilled with soil. Once effluent reaches the drain system, the gravel and soil act as a natural filter to strain and remove harmful bacteria, viruses, and other toxins so the water is clean by the time it reaches groundwater sources.

A newer type of drainfield system, made by Infiltrator Systems, utilizes a series of lightweight plastic chambers instead of pipe and gravel; these are easy to use and treat more water with greater efficiency. A 3-foot-wide trench is dug, then the 3-by-6 1/4-foot ribbed sections are fitted together along the trench. A special end plate is added to the end of each run, the inlet pipe is inserted, and the trench is backfilled.