How to test a septic system: septic system inspection, septic dye testing, & diagnosis procedures
Septic tank inspections, septic drainfield inspections, septic piping inspections
Septic inspection worksheets, dye and load test procedure details
Septic tank pumping - when and how to pump a septic tank when testing a septic system
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In articles listed here we explain in complete detail how to buy, inspect, test, diagnose maintain and repair septic tanks and all other components of septic systems.
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shows the start of a septic dye test.]
Inspecting a home or commercial septic system means finding evidence that the onsite wastewater disposal and treatment system is working properly. The procedures used must cope with the difficulty that a septic system consists of buried components, involves major costs to replace, and may involve serious health and life safety risks as well. The major steps in septic system inspection and diagnosis are discussed at:
Below we provide links to more detailed septic system inspection, diagnosis, repair, design, and installation. Home buyers of a property with a septic tank should be sure to read Buyer's Guide Home Buyer's Guide to Septic Systems Inspection & Testing - What to Do, Step by Step.
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Books on Septic Systems to Buy Online, our list of recommended books on septic system maintenance, design, and alternative designs available at Amazon.com - Septic System Education Materials
Buyer's Guide Home Buyer's Guide to Septic Systems Inspection & Testing - What to Do, Step by Step.
Media Filter Septic Systems types of alternative septic systems using sand, peat, textile, foam cube or other filter media, single pass vs. re circulating media filters, how to identify them.
Septic Tank Inspection Procedure - How to Inspect Septic Tanks and evaluate the septic tank condition, baffles, sludge levels, damage, evidence of septic failure, etc.
Septic Tank Leaks can prevent a valid septic loading and dye test and can damage the drainfield
Septic Tank Location - How to Find the Septic Tank, how deep will the cover be, how to document its location
Septic Tank Pumping Articles - list of tank pumping and tank repair articles
Septic Tank Size: septic tank size and capacity vs. usage determine the required septic tank size, calculate size of an existing septic tank
Septic Tank Types: characteristics and inspection guide for steel, concrete, plastic, fiberglass, and home made septic tanks
Introduction to Septic Systems - Inspection, Testing, Maintenance
What is a Septic System
A "septic system," also referred to as a private, on-site waste disposal system, receives waste water and solids
from a Building's plumbing facilities (bathrooms, kitchens, shower, laundry), treats, and then disposes of the
effluent from this waste, by permitting it to absorb into soils at the property.
"Treatment" is accomplished by bacterial
action in the "septic" or "treatment" tank and it is mostly accomplished by bacteria in the soil around and below
the effluent absorption system, or "drain field." This bacterial action is needed to reduce the
level of pathogens in the effluent discharges from the waste system into the soil. The principal components of a private on-site waste
disposal system usually include the following:
piping connecting the Building to the treatment tank
a septic or treatment tank which retains solid waste
piping connecting and conducting clarified effluent from the treatment tank to a distribution box
a distribution box connecting the effluent line from the tank to the absorption system or "drain field"
an absorption system which permits effluent to drain to soils below
a bio-mat or bio-mass of pathogen-digesting bacteria which forms in soil below the absorption system.
Many variations on this general scheme are used, depending on local climate, soil conditions, available space, economy,
and available materials. Special equipment and systems may be designed for problem or difficult sites such as
rocky or wet ground, permafrost, or wet tropical marshlands.
Readers who are unfamiliar with what a private septic system is and the types of systems installed should
review articles and sketches of septic system components found at
The Septic System Information Website
Types of treatment tanks, adsorption systems, pumps, and other special
equipment are discussed in some further detail in this text, and are listed in the septic system inspection
checklist data. For a more detailed introduction you may want to read our Lockwood article
"What is a Septic System" and then return here to continue by using your browser's "BACK" button.
Safety Warnings for Septic System Inspectors
Providing inspection and diagnosis of on-site waste disposal systems is an extremely valuable public service
which helps protect people from expensive unanticipated septic system repair costs and helps protect public health
by assuring sanitary disposal of sewage and gray water waste from Buildings.
More importantly though, such
inspections may detect and warn about serious safety hazards at some properties. The strong warnings issued below
intend to reduce septic system safety hazards for inspectors and property owners/occupants, but it is not the author's
intention to dissuade inspectors from providing this valuable service.
Danger lurks at cesspools, open covers, tanks or tank covers in poor condition,
and from high levels of methane gas. These risk collapse, falling, asphyxiation, and other potentially
fatal hazards as well as risks of unsanitary conditions.
Septic and Cesspool Inspection Safety:
Collapse Hazards: Septic system inspectors face personal risks of
health and safety including possible fatal system collapse or asphyxiation.
Old steel tanks, thin, rusting steel or rotting home-made wood tank covers,
site-built tanks and cesspools, and recently-pumped cesspools are at particular risk of
collapse. Falling into a septic tank or cesspool is likely to lead to rapid asphyxiation from methane and
in cases of collapse, there is risk of becoming buried. The author has consulted in cases involving
such fatalities (homeowner fell into a site-built cesspool), and at one site inspection, walking near an
overgrown area the author himself stepped through a rusting steel septic tank top, surviving only
by throwing himself into a nearby clump of brambles! Beware of
flimsy, rusted, old-steel, home-made, or missing septic tank/drywell/cesspool covers
abandoned systems which may not have been filled-in
collapsed, or collapsing septic tanks or cesspools
possible presence of multiple components at a property, abandoned or in-use
Entering or Looking into Tanks: No person, except those licensed, equipped, and trained,
should ever enter a septic tank. Special equipment is needed. Tank inspection (or service) should not be performed alone.
Even leaning over to look into a (recently pumped) tank has been reported to cause asphyxiation.
Unsanitary conditions: Be alert for unsanitary conditions such as surface effluent or sewage backups into Buildings, events
which risk serious viral and bacterial hazards and which indoors, may require professional cleaning.
Issue appropriate warnings:
Septic system inspectors (and service personnel) must recognize and respond properly unsafe conditions
at a site, including issuing appropriate warnings and in some cases, marking off unsafe areas where, for
example, there is visual evidence of a risk of collapse hazard.
Avoid damaging septic system components or the Building: Improper septic testing procedures, such as flooding a dosing-system,
can damage the system. Also, remember to check for leaks into or under the Building being tested when running
water into the Building fixtures and drains. Don't leave water running unattended - at risk of flooding the
Building.
Don't do anything dangerous:
Inspectors should omit and report the omission of any planned inspection procedure which in the inspector's
judgment is unsafe or unsanitary.
Septic Inspector Qualifications/Licensing
If you perform septic inspections you are obligated to do so with proper information, training, procedures,
and in some communities a license is required. Some states (e.g. CA, CT, NJ, MA) have specific certification requirements for
inspectors of septic systems, as well as specific regulations regarding the
performance of the inspection itself. Be sure to obtain information pertinent
to your own state, usually from the state health department or state department
of environmental protection.
Septic System Warnings to Home Owners and Home Buyers
Septic system concerns for a Building owner start with safety. Here are some red flags:
Signs of collapse-possible fatal hazards: include depressions or "soil subsidence"
anywhere on or around the property. Any suspect area should be roped-off and absolutely no one
should walk over or even close to such a spot until it has been investigated by a professional.
Old or abandoned systems: such as site-built cesspools or drywells were often made with
a thin steel or wood cover which with age can collapse. If the history of the site or visual observation
suggests that there are or were old systems at the property, professional investigation is warranted.
Improper "abandonment" (failing to fill-in a pit) can lead to sudden collapses. Signs that there may
be old systems at a property might come from anecdotal evidence (ask a neighbor, ask the local
septic installing or service companies), or visual evidence such as seeing abandoned waste pipes at
basement or crawl space walls or floors. Don't assume that an old house which is now connected to the
public sewer didn't previously have an on-site waste disposal system.
Septic service by untrained workers: such as aerating, agitating, or pumping out an old
site-built cesspool, can lead to sudden system collapse. Prevent access over or near any such
systems.
Unsanitary conditions such as discharge of sewage effluent to the yard surface, to a nearby well
or stream, or previous septic backups into a Building deserve professional attention. Indoors special
cleaning may be needed to remove bacteria or other pathogens.
Septic testing by inexpert "inspectors" who may not follow an adequate procedure increases the risk
of a costly surprise.
Uninformed homeowners may not notice a danger or malfunction .Homeowners should review the
safety warnings listed above.
The information here is general in
nature. Since conditions and requirements vary widely at individual sites, the
you should obtain qualified expert advice pertaining to the specific system
about which you have questions, and should not rely on this general text for
costly diagnostic/repair/replacement decisions. In other words, I'll try to
give you some helpful information. In exchange, don't expect me to pay for your
new septic system.
The next chapter proceeds with inspection of the septic system components.
How Does Each Septic System Component Fail? - What to Look For During a Septic Inspection
This chapter discusses detailed "how to" steps instructing the investigator in
how to inspect specific septic components for signs of failure. The following section will discuss
types and causes of septic failure and will provide criteria that define "failure."
Before digging up your septic tank or calling a septic pumper, if you think the septic system is failed because of
drain blockage or drains backing up into the Building, you should to see
"Diagnosing Clogged Drains: Is it a blocked drain or the septic system? - A First Step for Homeowners".
If you link to that text, please return here using your browser's "BACK" button.
Inspecting Outside Waste Piping
Outside waste piping conducts sewage (black water and gray water) from the Building to the treatment tank or "septic tank,"
and from the treatment tank to the distribution box. These lines should be of solid, non-perforated material and need to be
protected from mechanical damage (such as by vehicles). Piping extending from the distribution box into drain fields
is normally perforated, though solid lines might be used if effluent is being processed by more specialized devices such
as seepage pits, galleys, or a sand-bed system.
House to tank
This line may become blocked by waste, damaged by collapse of a section, or invaded by roots.
Detection of these conditions is fairly easy by routing a snake or power snake from the Building drain to the septic tank.
An experienced power snake operator can often tell by "feel" that a drain line is collapsed, partially collapsed, or invaded
by roots. While you may make a temporary "repair" of such a condition by drain-cleaning, if the line is broken or
root-invaded, you should expect to have to excavate and replace it soon.
Tank to Distribution Box
The same failures can occur on this line as from house to tank.
Drain field piping
In a conventional "drain field" of perforated pipes buried in gravel-filled trenches,
a drain line may be invaded by tree roots. This is why experts advise keeping tree and shrub plantings away from drain
fields. Vehicle traffic can also collapse this or any outdoor waste piping, which is why experts advise against ever
driving over a drainfield or over any other septic system components.
Inspecting Septic Tank Condition
The purpose of the treatment tank or "septic tank" is to contain solid waste and to permit the beginning of
bacterial action to process sewage into a combination of clarified effluent, settled sludge, or floating scum in the
tank. An intact, un-damaged septic tank is normally always filled with these materials.
Only by pumping and visual inspection can actual tank capacity and condition be
completely determined. Probing in the area of a tank, without excavation, is
not recommended as the probe may damage a steel or fiberglass tank.
Steel septic tanks
Steel tanks typically last 20-25 years, then rust, and collapse. Before this time steel
baffles may rust off (damaging the drain field with sludge) or the tank top may become rusty and unsafe.
Since steel tank tops can be replaced while leaving the
old tank in place, the condition of the top itself is not a reliable indicator
of tank condition. See STEEL SEPTIC TANKS for details about steel septic tanks.
Concrete septic tanks
Concrete tanks at an existing septic installation are usually viable,
but might have damaged baffles or cracks
that permit seepage of groundwater in or septic effluent out around the tank. Occasionally we've seen
tanks made of poor-quality concrete (insufficient portland cement) which eroded badly. If the
tank outlet or absorption system have been blocked, examination of the tank interior may show
that effluent is or has been above the top of the baffles (see "baffles" below) thus indicating
a system failure discussed next. See CONCRETE SEPTIC TANKS for details about concrete septic tanks.
For detailed guides to inspecting different types of septic tanks see these articles:
Baffles in a septic tank are provided to keep solids and floating scum and grease
inside the tank. Baffles are provided at both the inlet to the tank (from the Building) and
the outlet from the tank (to the absorption system).
Broken baffles or high sludge levels can cause solids to flow out of the tank
and into the absorption system. The result is reduced absorption into
surrounding soil and eventual failure of the system. Floating scum thickness
and settled solids thickness can be measured through access ports into the tank
or cesspool. Finding solids at or covering the outlets or damaged baffles
should result in report of a very questionable adsorption system and possible
major repair cost.
If baffles are lost or damaged (rusted
off on a steel tank or broken off on a concrete tank), they can be repaired or replaced. For
example at a steel tank the contractor may simply insert a plastic piping "Tee" into the
tank inlet or outlet to create a new baffle system.
However, depending on how long the
tank was used without good baffles, the volume of solids and grease that moved from the tank
to the absorption system will have begun clogging soils there and will have reduced the future
life expectancy of the absorption system.
Baffle damage and repair, or even a complete tank replacement when the absorption system
has been left alone always lead the author to warn the Building owner that the future life
of the absorption system may be in doubt and that additional expense will be involved.
Inspecting the Level of Accumulated Solids, Sludge and Floating Scum in Treatment Tanks
Solids entering a septic tank are intended to remain there until pumped out during tank service. A large
portion of solids settle to the bottom of the tank as sludge. Grease and floating scum remain at the top of the
sewage in the tank. Baffles (discussed above) help keep solids, scum, and grease in the tank. Bacterial action
in the tank make a modest reduction in the solids volume and begin the processing of sewage pathogens, a step
later completed by soil bacteria in the absorption fields.
Net free area: If the sludge level becomes too high or the floating scum layer too thick, in addition to risking passage of
solids out of the tank (damaging the absorption system), the remaining "net free area" of liquid in the tank is reduced.
When the net free area becomes too small, there is insufficient time for waste entering the tank to settle out as
bottom sludge or top floating scum. That is, for an in-use septic tank with a small net free area, the frequent entry of
solid and liquid waste will keep the tank debris agitated, thus forcing floating debris into the absorption system where the
life of that component will be reduced (due to soil clogging).
The importance of keeping an adequate net free area in a septic tank is the reason that tanks need to be pumped
at regular intervals. Building owners who never pump a tank until it is clogged have already damaged the
absorption system.
See the Massachusetts Title 5 Septic Inspection procedure discussed below for details about
determining the level of floating and settled scum and sludge in the tank.
The distribution box (more than one may be in use) connects a single effluent line from the septic
tank to a network of absorption system components such as drainfield leach lines or to a network of seepage
pits or galleys.
In good system design the outlet openings from the distribution box to each drainfield
line can be adjusted to regulate the flow among the various absorption lines. If a distribution box becomes
tipped (or clogged) effluent may be routed to only a portion of the absorption system, thus overloading it and leading
to a "breakout" of effluent at the surface or to clogging and system backup.
An examination of the box interior
may show flood lines in the box if the drain field has been clogged or saturated in the past even if at the time
of inspection the box is not flooded. If the fields have been flooded you should be pessimistic about the remaining life
of the absorption system.
If the distribution box or D-box is tipped and/or effluent has not been uniformly distributed among the drainfield
lines (assuming they are of equal length and in equally good soils), only a simple adjustment of the outflow may be needed.
Round plugs with eccentric openings may be present or can be inserted in the D-box outlet openings to regulate flow
among the individual absorption lines.
Liquid level in the adsorption system
The absorption system or "drain field" has two jobs. First, it disposes of liquid effluent by permitting it to seep into
the soil below. Second, a "bio-mat" of bacteria which forms in the soil below the drainage field processes pathogens in
the septic effluent to make the effluent sufficiently sanitary as to avoid contaminating nearby ground water.
This
distinction
between successful "disposal" and successful "treatment" is important to avoid groundwater contamination but has not been
addressed by regulation in every municipality. Municipalities which require a minimum distance between the bottom of the
drain field trenches (or equivalent component) and the top of the seasonal high ground water table have recognized the
importance of a working bio-mat and the need to provide adequate dry soil for it to function.
Even in a well-designed drainage field, eventually the soil surrounding the drainfield device (perforated pipe in
gravel trench or other seepage system) becomes clogged with grease and debris.
Examining an excavated cross-section
of a failed drainfield will often display a black or gray band of sludge and grease of about 1" thickness at the inside
perimeter of the gravel trench. When this layer of soil becomes sufficiently clogged the passage of effluent into the soil
below is slowed and eventually blocked, leading to the need for replacement. Keeping a tank pumped so as to reduce the
passage of debris and grease into a drain field will extend its life.
This is the most expensive problem to correct. Look for septic effluent seepage to ground
surface in area of equipment or downhill from such equipment. Look for (illegal) drain field
line extensions to nearby streams, storm drains, or adjoining properties where the temptation
to "fix" a failing system by sending the effluent to an improper destination overwhelmed a previous
owner or repair company. In some areas
inspectors use septic loading and dye test. Seepage may be due to overloaded
tank, failed absorption system, or blocked/broken piping (may be less costly).
An excavator or septic contractor will often explore one or more drain lines (or similar components) by excavating a
portion of it to look for evidence of flooding or soil clogging. We've used a simple probe at the end and along a leach
bed to check for flooding of that component. (Be careful not to break or collapse old piping.)
Types, Causes, and Failure Criteria for Septic Systems: Defining "Failed" Onsite Wastewater Treatment or Disposal
This chapter discusses types of septic system failure
lists causes of each type of septic component failure, and lists the septic component failure criteria
or in other words
what conditions are defined as "failure"?.
The detailed "how to" steps instructing how to inspect specific septic components for signs of failure are
discussed in the text above.
In simplest terms, there are two visible disposal failures:
Effluent or sewage appears at the surface of the yard, or the neighbor's yard!
Septic odors may also indicate a system failure or an imminent failure. But such odors
may also be produced by defects in the plumbing vent system or other site conditions. Beware, sewer gas contains
methane and is explosive if it reaches a dangerous concentration inside a Building.
Typical causes range from things that are easy and cheap to repair, to a need for complete system replacement:
Clogged pipes
Broken pipes
Damaged tank
Tipped distribution box
Clogged/broken soil absorption piping
Clogged absorption soils (grease & solids)
Saturated soil absorption area
However there can also be treatment failures.
Effluent may not back up or appear on the surface, but if insufficiently
treated effluent reaches a private well or any stream or waterway, the
environment is being contaminated -- an unacceptable condition. Historically
many people have just worried about disposal. As the quality of drinking water
deteriorates in many areas and as population grows in many previously
thinly-populated areas, proper treatment has become the real concern for
everyone's health.
For example, if there is not sufficient soil
between the bottom of the soil absorption system trenches and the local
groundwater, the local environment is being contaminated.
Other causes of onsite wastewater disposal system failure:
Driving over the absorption system, leach field, drainfield - see Driving or Parking over Septic Components? is it ever acceptable to drive or park over septic system piping, tanks, or drainfields?
Paving over the absorption system
Flooding the absorption system with surface or roof runoff, or rocky, poorly-drained or under-sized sites may simply lack capacity
Improper original construction , especially on rocky, poorly-drained sites (pipes settle, for example)
Tipped or flooded distribution boxes, resulting in uneven loading of soil absorption system lines
Use of septic tank or drain field additives which claim to extend system life can generate so much activity in the tank that
solids are held in suspension and forced into the soil absorption system! Do
not add any treatments, chemicals, yeast, or other treats to a septic system. In general these treatments don't work, may ruin the system, and
are illegal in many localities. There is no magic bullet to repair a bad SAS.
Rusting steel tank covers can cause death! Rusted covers can collapse. I have reports of
children and adults who have died from this hazard, as recently as December 1997. In 2000 I consulted in a fatality involving an adult falling into a cesspool.
At a Building inspection I myself stepped through a hidden, rusted-through steel septic tank cover.
Falling into a septic tank, drywell, or cesspool is quickly fatal, either from being buried by falling soils and debris, or by asphyxiation. Septic gases are highly toxic and can kill in just minutes
of exposure. Even leaning over an empty (just pumped) tank has led to collapse and fatality of a septic pumper.
Concrete tank lids: can be damaged by vehicle traffic; heavy duty covers are available.
Steel tank baffles: rust out and fall off, permitting solids to enter the soil absorption system
Steel tank bottoms rust out permitting effluent to leak into soils around the tank, possibly giving a large void in tank at time of testing,
thus subverting a loading or dye test;
Concrete tanks can crack or sections may separate causing leaks with the same effects as just stated
Concrete tank baffles: may erode from chemicals, detergents, poor concrete mix, water flowing over top of baffles, or may be broken by improper pumping procedures
Houses clustered around a lake: often will have a marginal system as properties were crowded
together, built as part-time summer-camps, were built without code supervision, and often were built using amateur, marginal home-made systems.
Age: eventually even a well-maintained SAS will clog and have to be replaced.
Onsite Waste Disposal System Failure Criteria
Massachusetts Title 5 lists specific failure criteria and serves as
a good model for septic inspections anywhere.
Backup anywhere in the system
Discharge of effluent to the surface, stream, etc. regardless of
whether or not septic dye is observed
Static effluent level above outlet in the D-box
System has to be pumped more than 4x/year
Metal septic tanks (municipality dependent; note that in special
site conditions small metal tanks may be the "only" solution and
may be approved by local officials. An owner/buyer must be informed of the
implications of such installations.)
Soil Absorption System (or cesspool, etc) is at a depth exposing
it to the maximum groundwater level
Cesspool failures (MA)
Cesspools which, on inspection, have less than 6 inches of freeboard
Cesspools which, on inspection, have less than 1/2 day's storage
Cesspools which, on inspection, are located within100 ft. of a pond or dug well (surface water supply)
Cesspools which, on inspection, are located within 50 ft. of a private well (modern sanitary well)
Cesspools which, on inspection, are located between 50 and 100 ft from a private well if well fails bacteria test
in Massachusetts, cesspools located within Zone 1 of a public well are defined as "failed"
BOH evaluation is required if a cesspool is located within 50ft of any surface water
Soil Absorption System Failures (leach fields, drain fields, seepage pits)
Breakout of effluent observed (& I consider odors as well) in the drainfield constitute a failure
BOH evaluation in MA if the septic drainfield is located within 100 ft of surface water supply
if the septic drainfield is located within Zone 1 of a public well
if the septic drainfield is located within 50ft of a private well
if the septic drainfield is located between 50ft and 100ft of a private well if well fails bacteria test.
Levels of Septic System Inspection and Testing
LEVEL-0 Septic Inspections - Basic Visual, Loading & Dye Test
This level of inspection is typically provided during a "home inspection" for real estate transactions.
A VISUAL ONLY Septic System Inspection Includes: Basic visual inspection and reporting of information (Performed by
home inspector or other expert) (Some municipalities require this test be
performed only by specifically licensed septic contractors or engineers.)
A
VISUAL PLUS LOADING DYE TEST Septic System Inspection Includes: - Visual + Dye test and system loading. Warning: using an inadequate amount of
tracer dye or an insufficient volume of water for this test will make it meaningless. Therefore ordering a
"stand-alone" septic loading and dye test of a system should be expected to cost considerably more than
such a test which can be performed overlapped in time with other Building inspection services. Beware of
quick, minimal tests which place only a small volume of liquid into the system (perhaps 50 gallons over 10 or 15 minutes).
The minimum quantities of water and septic dye needed are discussed in a separate chapter at
Septic Loading and Dye Test Procedure Details -
a chapter of this text "Inspecting, Testing, & Maintaining Residential Septic Systems".
ADDITIONAL LEVELS OF INSPECTION for real estate transactions and for problem
diagnosis and bidding prior to actual repair work.
Level 1 Septic System Inspections - Visual, Open Covers, Inspect Equipment, Loading & Dye Test
A LEVEL 1 Septic System Inspection Includes: = Level 0 + Open accessible covers & inspect equipment. May include
loading and dye test, often does not. May include requiring tank pump out and
inspection. (Performed by home inspector or other expert)
Some municipalities
require this test be performed only by specifically licensed septic contractors
or engineers.
This is the Pennsylvania PSMA definition for level-1.
Level 2 Septic System Inspection Includes - Visual, Open, D-Box, Pump & Inspect Septic Tank
LEVEL 2 = Level 1 + Locate, excavate if needed, open, inspect tank and
distribution boxes, pump and inspect tank, baffles, distribution boxes;
determine system capacity, scum thickness, baffle condition, etc.
Performed by
septic pumping contractor or other expert.
Some municipalities require this
test be performed only by specifically licensed septic contractors or
engineers.
Absorption System Design & Maintenance Soil Perc Tests & Septic Drain Field Design & Maintenance
Soil Percolation Requirements and Soil Depth Requirements for Septic Absorption Systems / Septic System Drainfields
What is a septic system soil percolation test?
In specifying the size and type of absorption field (leach field, seepage pits, galleys, other) a septic engineer or health
department official will require that a soil percolation test or "perc" test be performed. You may hear it described as a
"deep hole test." The first time I participated in this procedure I found myself smiling with surprise at how low-tech the procedure
actually was (in New York State.)
After identifying the most-likely location on the lot for placement of a septic drainfield,
the excavator used a backhoe to dig a rough hole about 5 ft. deep. Happily no groundwater immediately filled in the hole (which would
have been bad news). Perhaps this is why builders try to have this test done in July which is the period of most-dry weather and lowest
groundwater table levels.
After digging this rough hole, the septic engineer poured a 5-gallon (joint compound) bucket of water into the hole.
In some cases a few buckets might be dumped therein. After that sophisticated move, the observers simply watched the rate at which the water disappeared.
a one-inch drop in water level in this hole in three minutes was considered very good. If the water was found still in the hole at no
drop in level the next morning, this was considered seriously bad and probably requiring some soil exchange or other special design measures.
What are the soil perc and other soil requirements for septic systems?
I like the Massachusetts Title 5 Septic Inspection criteria for defining a (at least possibly) functional drainfield, as the
text explains the role of the biomass below the absorption bed, sets soil depth requirements, and recognizes the importance of
keeping the bottom of the working biomass area in well drained soil sufficiently above the seasonal high water table.
Here is an example of soil requirements for a functional drainfield. This version is particularly clearly written and is for
residents of Ohio but the principles apply anywhere.
"In Ohio, soil absorption systems can be used in areas where the percolation rate of the soil is between 3 and 60
minutes per inch (soil permeability between 1 and 20 inches per hour). At least 4 feet of suitable soil is required
under the soil absorption system to provide adequate treatment of the septic tank effluent.
To accommodate the
construction of the system and provide adequate soil cover to grade, a minimum of 5 1/2 to 6 1/2 feet of suitable soil is
needed above the limiting layer. A limiting layer may be bedrock, an impervious soil layer (hardpan, fragipan) or a
seasonally high water table (gray soil or mottles).
The soil absorption system must be at least 8 feet from any drain
line on the lot, 50 feet from a water supply, and 10 feet from the property line, right-of-ways and the house. Septic
systems cannot be placed on the flood plain and are limited to areas with less than a 15 percent slope." http://ohioline.osu.edu/aex-fact/0743.html Ohio State University Fact Sheet "Septic Tank - Soil Absorption Systems"
How large does the absorption field need to be?
The size of the absorption field needed (in square feet of area, presumably
also unencumbered by trees, driveways, Buildings, etc.) can range considerably depending on the soil percolation rate.
A lot with a good percolation rate or "perc" of perhaps one inch of percolation
in three minutes might require about 4500 square feet for a typical three bedroom home. If the same home were
built where there was a poor a soil percolation rate of an hour per inch, 9000 square feet or more might be required
for the absorption area.
Drainfield size and location also have to take into account local zoning - setback requirements from property borders, setbacks from
streams, wetlands, wells, water supply lines, and other encumbrances.
Drainfield trench/line specifications
In the most common design of drainfield, perforated pipes are buried in gravel-filled trenches to form the drainfield. Pipes are
placed across the slope line of sloped property (so that all of the effluent doesn't simply rush down to and leak out at the end of the drain
line pipe). While some experts describe the bottom of these trenches as "level" in practice they are dug to slope slightly, perhaps
1/8" per foot or less.
A typical trench for a septic drainfield is 18 to 30 inches in depth, and 8 to 12 inches wide. The trenches are dug about 6 feet apart which allows,
in good design, space for a set of replacement trenches to be placed between the original ones when the first set fails.
The maximum length of a trench is typically about 150 feet but I've found installations that were three times that length.
Where lot space does not permit drainfield trenches such as I've just described, a septic engineer may specify that seepage pits
or galleys are to be installed. These fit in a smaller space since a single pit may be 6' to 8' in diameter. But the depth to which effluent is
being delivered (4' or more) means that the sewage effluent is unlikely to be fully treated by a biomass. These systems may successfully "dispose" of
effluent but they are probably not adequately "treating" it.
What destroys or shortens the life of drainfields?
It's easy to ruin or shorten the life of a drainfield:
install a drainfield in wet weather (which compacts the soil)
drive over the drainfield or build a parking lot over it (compacts soil, breaks pipes)
plant trees on the septic absorption field (roots enter pipes)
put a swimming pool in the middle of a drainfield - yes I've seen people do this!
forget to pump out the septic tank regularly (solids/grease are discharged into the fields, clogging the soil)
direct roof runoff or surface runoff across the drainfield or into the septic tank (flooding the system)
install the drainfield in an area of high seasonal water tables (flooding the system)
use the septic system to dispose of illegal oils, chemicals, fats, greases - one system in New York near the Taconic State Parkway
was connected to house in which was operated an illegal drug manufacturing operation. So much contaminant was flushed down house drains
that the workers contaminated their own well and poisoned themselves
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For general septic repair advice, contact your local health department, or experienced septic engineer or contractor in your area.
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