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Use of Off-Lot Discharges
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
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
SUMMARY
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
Refer to the Norweco Technical Bulletin ONSITE WASTEWATER TREATMENT
AND THE USE OF OFF-LOT DISCHARGES for detailed information on the
use and application of off-lot disposal.
REFERENCES
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).
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