A subsurface wastewater infiltration system (SWIS) is defined as “an underground system for dispersing and further treating pretreated wastewater.  The SWIS includes the distribution piping/units, any media installed around or below the distribution components, the biomat at the wastewater-soil interface, and the unsaturated soil below.”1  Out of necessity, leaching tile fields or other SWISs must sometimes be located in the proximity of seasonal or onsite groundwater.  This creates an operational problem as groundwater saturation will impede the normal aerobic function of a soil-based disposal system.  “Dissolved oxygen is virtually absent in saturated soil, making degradation of effluent below the water table an anaerobic process.”2  When this potential exists, “curtain drains, vertical drains, underdrains, and mechanically assisted commercial systems can be used to drain shallow water tables or perched saturated zones.  Of the three, [subsurface drainage systems], curtain drains are most often used in onsite wastewater systems…”1

Although curtain drains have been the most widely used subsurface drainage systems, similar devices are termed perimeter drains, underdrains, vertical drains, French drains, collector drains, interceptor drains, footer drains, overflows, wet weather drains and gradient drains.  Unfortunately, some confusion exists about how each of these systems function. Many industry references, and even some regulations, use one or more of these terms interchangeably.  However, the USEPA has defined specific applications and provided guidance in the use of several types of subsurface drainage systems.  In order to properly apply any “artificially drained system”, the source of the groundwater being drained, its particular flow characteristics, the soil characteristics and the topography of the site must first be determined.  The correct system can then be selected and applied to address the specific site problem.

Each application of subsurface drainage systems relies on proper functioning of the SWIS and a thorough evaluation of the groundwater in the vicinity.  “From a functional point of view, subsurface drainage falls into two classes: relief and interception drainage.  Relief drainage is used to lower a high water table which is generally flat or of very low gradient.  Interception drainage is to intercept, reduce the flow, and lower the flowline of the water in the problem area.”3

The discharge of a subsurface drainage system is usually directed to a surface waterway or other off-lot discharge.  This makes it critical that regulators, designers, installers, inspectors and service providers be properly trained and possibly licensed.  If the drainage system is incorrectly designed or installed, bleed-through or outright bypass of the treatment process can result in the discharge of partially treated or untreated wastewater directly off-lot and will pose a threat to the environment.  Because of this potential, some regulatory agencies believe that subsurface drainage systems should never be allowed under any circumstances.  However, the key to effective use of subsurface drainage systems is management.  Periodic service and inspection will characterize any off-lot discharge and potentially avoid any point-source pollution of the environment at its origin.

Curtain Drains

Curtain drains are interception drainage systems used in hilly terrain where a water table is permanently located or “perched” above and separated from the normal groundwater table due to an impermeable soil layer above the normal permeable soil.  The curtain drain is designed to intercept the lateral movement of the water table and direct the groundwater to a location where it will not affect the continued operation of a SWIS.  Prior to installation of a curtain drain, the depth of the impermeable layer should be determined by site evaluation.  “If the restrictive layer that creates the water table is thin and overlies permeable soil, vertical drains may be used.”1  However, if the thickness or composition of the impermeable layer turns out to be prohibitive, a curtain drain should be installed.

Vertical Drains

Vertical drains are interception drainage systems used if a water table is perched above a thin impermeable layer and the SWIS is at a lower elevation, located in permeable soil.  Having the thin impermeable layer allows the vertical drain to extend down into the permeable soil below.  A vertical drain located around a portion of, or the entire perimeter of the disposal system will intercept the perched water table if it moves laterally and direct it down through the impermeable soil layer where it can leach into the underlying soil, and eventually into the groundwater table, outside the area of the absorption bed.  A vertical drain is the only subsurface drainage system that retains the flow totally onsite.


Underdrains are relief drainage systems used where groundwater exists close to the surface in permeable soil.  An underdrain provides lowering of the free water table on a continuing basis and directing the groundwater to an off-lot location.  Usually, multiple drain trenches are used to lower the free water table not just around the perimeter, but in the total area of a disposal system.  “Underdrains must be located to lower the water table to provide the necessary depth of unsaturated soil below the infiltrative surface of the soil absorption system, and to prevent poorly treated effluent from entering the drain.”1

Perimeter Drains

Perimeter drains are another type of relief drainage system used by some references that differentiate between a seasonally high water table and a continually high water table requiring underdrains.  A perimeter drain provides lowering of the free water table only when the water table is elevated, and directing the groundwater to an off-lot location.  Unlike underdrains, perimeter drains would be anticipated to be in operation only several months of the year.  As such, drain trenches are primarily used only around the perimeter of the disposal system.

Other Off-Lot Discharges

The different types of subsurface drainage systems used in conjunction with SWISs, constitute off-lot discharge.  While not always obvious, even a roadside ditch can be an off-lot discharge.  A roadside ditch is actually a relief drain, which carries surface and subsurface water.  Agricultural field tile is another type of off-lot discharge that conveys groundwater from beneath the surface of a large area.  Certainly, the direct discharge of effluent from various types of aerobic or advanced treatment units would fall into a similar category.  This would be true whether they are self-contained (pretreatment, aeration and clarification in a single tank) or a series of components such as a recirculating sand filter.  Even a passive system such as pretreatment followed by constructed wetlands would likely be an off-lot discharge.  As enforcement of regulations becomes more widespread, all of these systems will likely require compliance with the National Pollutant Discharge Elimination System (NPDES) program and the recently implemented Storm Water Rules.

Phase II Storm Water Program

Storm Water Rules promulgated by the USEPA call for phased NPDES requirements for storm water discharges.  Phase I of the new rules focused on discharges from municipalities of 100,000 population or more and the final rules were published in August 1995.  Phase II Rules were published in the Federal Register on December 8, 1999 and became effective March 10, 2003.  Phase II Rules were designed to cover small municipal systems and construction activity disturbing between one and five acres of land.  Many people notice that rural areas frequently use one acre as the required lot size for an individual home site.  Even though the general stance of the USEPA is not to require permits of individual homeowners, “the Rule [Phase II] allows for the exclusion of certain sources from the national program…as well as inclusion of others based on a higher likelihood of localized adverse impact on water quality.”4


For decades, subsurface drainage systems have been used to guarantee that the movement of groundwater does not interfere with the treatment process occurring in a soil-based disposal system.  Even though some regulatory officials disagree, the use of curtain drains and other subsurface drainage systems may not be detrimental to the environment, as the USEPA still references them in the February 2002 update of the 1980 Onsite Wastewater Treatment Systems Manual.  Independent field studies have shown that the use of a properly engineered and installed subsurface drainage system can provide a safe, effective method of insuring that groundwater will be directed off-lot and will not have a negative effect on the treatment system operation or the environment.  In one such study, county and state regulators, in cooperation with agricultural engineers, concluded: “In using a poorly drained Henry soil as the initial worst case situation, we found that with rainfalls at or near average, groundwater could be drained by a simple manner and at a reasonable cost.”5

  1. U.S. Environmental Protection Agency. Onsite Wastewater Treatment Systems Manual. (1980/2002).
  2. R. E. Hermanson, Extension Agricultural Engineer, Washington State University and R.L. Den-Herder, Area Engineer, U.S. Soil Conservation Service. Design of Interceptor and Curtain Drains. (1982).
  3. U.S. Department of Agriculture Soil Conservation Service. Drainage of Agricultural Land. (1973).
  4. Federal Register, Vol. 64, No. 235, Rules and Regulations, U.S. Environmental Protection Agency. 40 CFR. (December 8, 1999).
  5. K. E. Earlywine, Jefferson County Health; M.W. Jones, Arkansas Department of Health; D.K. Shoemaker, University of Arkansas; C.L. Griffin, University of Arkansas. Experimental Groundwater Draw Down Using a Curtain Drain on a Henry Soil in Jefferson County, Arkansas. (1988).

Background Information

For years, the most popular onsite wastewater treatment system was a “passive” system, composed of a septic tank (for pretreatment) and a tile field (for soil treatment).  This process was thought to be a surefire method to prevent pollution of the environment.  In simplest terms, whatever waste is generated onsite, is treated onsite.  As increased population density has required the use of previously undeveloped areas, seasonal or onsite groundwater has periodically resulted in problems with the design or operation of leaching tile fields and other subsurface wastewater infiltration systems (SWISs).  To solve this problem, various types of curtain drains and other subsurface drainage systems have been designed and developed.

The use of subsurface drainage is the intersection where onsite treatment meets off-lot discharge.  While not intentionally designed to be an off-lot discharge of untreated or partially treated wastewater, some professionals are convinced that either by their very design or by improper application, this is exactly what they are.  Properly designed and installed curtain drains and other subsurface drainage systems are a valuable tool in the design and use of onsite wastewater treatment and disposal systems, and they usually carry only groundwater.  USEPA has endorsed their use for decades, both in the 1980 Onsite Wastewater Treatment Systems Manual and in the 2002 update of the same reference.  Whether installed and working properly or not, one thing for certain is that three of the four most popular subsurface drainage systems constitute an off-lot discharge according to the new Storm Water Program Rules being implemented by the USEPA.  This means they will soon be subject to many of the same monitoring and management requirements as any other off-lot discharge.  As such, it is imperative that subsurface drainage systems be understood in their design and function, and managed in their operation.