A curtain drain is a drain used to intercept water moving though the soil. The term "perimeter drain" describes a curtain drain that surrounds an area, making a complete circuit. Perimeter drains are sometimes installed around septic systems in order to remove outside water influences from the septic area to prevent a septic system from failing. The determination to use a curtain drain stems from the mottling in the soil found at the time of the site evaluation. Mottling is a term describing a change in the soil; spots or blotches of different color or shades of color interspersed with the dominant color of soil. This means little gray color mixed into the darker color soil and shows that water stays in the soil.