1.     Conducting a Site Investigation

The object of an on-site systems is to treat and dispose of wastewater generated on-site in a means which will protect public health and minimize environmental impacts.  To accomplish these goals a site investigation must determine the suitability of a location for a system and the type of system which may be installed.

 

The investigation must uncover clues to help designers and regulators understand the soil morphology and hydrology of the site.  This information will help predict wastewater flow through the soil and into subsurface materials.  This allows us to predict how and if a system will function and the probable flow paths of water from the site and the treatment received by the wastewater.

 

1.1     Preliminary Investigation

1.1.1     Key People and Entities

The first step in conducting a soil and site evaluation is understanding the owners expectations.  As a minimum the site evaluation should determine from the owner:

·         The intended use of the site

·         Building site preferences

·         How many occupants, bedrooms , employees or customers

·         Possible future land uses

 

As the site investigator is collecting this information from the owner the owner should be told about factors that may influence the location, size and type of system that may be feasible. 

 

The most critical site feature is the political climate, the relationships between the public, the local authorities, the client or owner and the consultants.  Most technical aspects of wastewater disposal can be overcome.  Local codes, authoritative dispositions, and other man-provided aspects of the on-site disposal must not be overlooked.  It is imperative the site investigator has a good understanding of the rules and regulations regarding on-site systems in any particular jurisdiction he/she is working. 

 

      

 

1.1.2     Background Documentation

 

After speaking with the owner it is a good idea to get as much site information available before making a trip to the actual site.   There are many materials available from many different sources.  The following are valuable sources of information regarding the site.  Not all listed items will be needed or available for each site.  Nor are the following items the only materials available to the site investigator.  It will rest upon the investigator to determine what materials will be required to conduct a thorough and complete investigation.

 

1.1.2.1     Subdivision Requirements

 

It is also important that the investigator understands requirements set fourth by homeowners associations, subdivision requirements and Codes, Covenants and Restrictions (CC&R's).  Utility companies also have codes and restrictions regarding their utility easements especially when it comes to setback requirements. 

 

1.1.2.2     City & County Governments

 

County Assessors Maps, which are available at the County Assessor Office, can give an accurate plan view of the site, as well as adjacent properties and can sometimes be used as an aid to locating the property.  If the property is located in an approved subdivision the final plat can be used to get very accurate dimensions of the lot and adjacent properties.  Typically the cover page of the final plat will have a vicinity map showing how to get to the subdivision.  The County Recorders Office has copies of all recorded subdivision plats.  The assessors map or final plat will not have contour information.  If available preliminary plats for subdivisions is the most accurate means of acquiring topography prior to the site visit.  Preliminary plats can usually be found at the County Planning and Zoning office or if the site is within the corporate boundaries of a city they can be found at the City Engineers Office.  If these offices do not have them the design engineer will probably have the preliminary plat on file at his/her office. 

 

1.1.2.3     USGS Maps

 

USGS Topographical Maps will have topography information at and around the site, however the contours are typically at such a large scale (40 foot contours are typical on 7-1/2 minute maps) they will provide little relevant information to the site investigator.  However they can be used to locate the property.

 

1.1.2.4     FEMA

 

The Federal Emergency Management Administration (FEMA) has done extensive flood investigations for most metropolitan areas and many outlying areas.  They have compiled FEMA maps which often show the 500 year and 100 year flood plains.  This information can be very beneficial to the site investigator.

 

1.1.2.5     Soil Survey Maps

 

Valuable information about a site can be collected by viewing soil survey maps prepared by the United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS)(formerly the Soil Conservation Service (SCS)).  The NRCS has conducted soil surveys for about 95 percent of the counties in the United States.  Soil survey maps are produced by NRCS upon the completion of field surveys conducted by staff members who are certified soil scientists.  There are two categories of soil survey maps: Reconnaissance and Detailed.  Reconnaissance maps cover wide areas and are based on soils sampled in the area.  Detailed maps are based on intensive review and sampling of the study area. 

 

 

Although soil survey maps may provide preliminary information they do have limitations.  Many surveys were conducted many years ago and do not reflect current conventions and practice.  They may not identify inclusions, which are soils of different types that appear in localized areas of another dominant soil type.  Transitional zones may not be marked well.

 

1.2     Equipment

 

After collecting the above information and having a good understanding of the owners intentions and expectations as well as the regulatory environment the evaluator is ready to go to the site.

 

Prior to going to the site the evaluator should sketch a plot plan.  The plan should Identify property corners and all  significant features such as roads, structures, water features etc….

 

The second step is to assemble appropriate tools and equipment.  Tools and equipment should include, but are not limited to:

·         Clipboard

·         Compass

·         Flagging

·         Stakes

·         Measuring wheel

·         String lines

·         Tape measures

·         Shovel

·         Camera

·         A device to determine grade.

 

1.2.1     Determining Grade

 

There is a wide variation of tools which can be used to determine the grade of a site.  These range from the extravagant to the simple.  The investigator should use equipment that he/she feels comfortable using as well as can determine the topography of the site to a suitable degree of accuracy.  Below is a list of some of the items available as well as the pro's and con's of these devices.

 

1.2.2     Hand Level

A hand level is a simple devise that looks like a telescope.  The operator looks through the eyepiece and lines the cross hairs on an object.  A bubble level can be seen through the eye piece so the operator can level the instrument.  The hand level is best used by having a second person holding a surveys rod.  The person looking through the hand level reads off various measurements as the rodman places the rod at all slope breaks.  The difference in readings is the change in elevation.  The hand level can be used by just one person by sighting in on various vertical objects on the site.  However this can be time consuming and there is a lot of room for human error.  The cost of a hand level is as little as $10.00 up to $150.00.

 

1.2.3     Clinometer

 

 

A clinometer is a hand held device used to measure angles of elevation or inclination.  It is used by holding the instrument to your eye and with both eyes open looking simultaneously through the lens and alongside the housing.  A horizontal sighting line will appear.  Raise or lower the clinometer (by tilting your head) to place the sighting line at eye level on an object that is upslope of downslope.  Read the number closest to the sighting line.  A clinometer is lightweight and can be used quickly and easily by one person, although just like the hand level a second person holding a surveyors rod is much more accurate.  The cost ranges from $100.00 to over $250.00.  Although a clinometer is more accurate than a hand level it is hard to determine accurate grades on lots that have undulating topography.

 

1.2.4     Abney Level

 

A Abney level is very similar to a clinometer in that it is used to measure the value of a slope by means of arc measure and grade percentage.  The cost of Abney levels range from $100.00 up to $250.00.

 

1.2.5     Rotating Laser Level

A rotating laser level sends out a laser beam which depending on the model can be read directly on a surveyors rod or by a receiver attached to a surveyors rod.  The laser quits rotating automatically if the instrument becomes un-level.  This item can easily by used by a single person and is very accurate.  The cost of a laser level can be very expensive.  Besides the laser level a tripod, surveyors rod and possibly a receiver are needed. 

 

1.2.6     Other

 

There are many other items which may be used such as Theodolites, Transits and Total Stations which can be used to accurately assess the grade of a lot.  As mentioned earlier the investigator should only use equipment he/she feels competent in.  If site conditions are complicated it may be prudent to hire a land surveyor to create a topography map of the site. 

 

1.3     Site Investigation

 

Upon arriving at the site, make a tour walking along property lines and noting on your site plan any items which may effect the siting of a system.  Look for signs of buried utilities and easements and confirm that buried utilities that were found during your preliminary investigation are either identified or are absent.  On your site plan note the following observations:

 

·         Aesthetic features such as trees or views

·         Buildings (on property and adjacent properties)

·         Driveways

·         Easements

·         Overhead power

·         Gas

·         Etc.,

·         Existing sewer lines (on property and adjacent properties)

·         Existing wastewater systems (on property and adjacent properties)

·         Existing water lines (on property and adjacent properties)

·         Percolation holes

·         Photo points (if photos are taken)

·         Proposed residence

·         Property lines

·         Property Corners

·         Roads

·         Road cuts

·         Rock outcroppings

·         Survey Monuments

·         Test holes

·         Vegetation

·         Wells (on property and adjacent properties)

·         Water-features

·         Dry washes

·         Live streams

·         Ponds or lakes

·         Swimming pools

 

 

 

Topography describes the sites surface contour.  Topography can be described according to four conditions:

 

·         Smooth - boundaries are flat

·         Wavy - boundaries have channels or alternating contours

·          

·         Irregular - boundaries have no distinct patterns

·         Broken - boundaries are disjointed

 

The location of a site in relation to the areas overall landscape is important because this affects the distribution and type of soils found below the surface.  For example, a sloping site may have more top soil near the summit and toeslope than along the backslope where soil may have been eroded by wind and precipitation.  Soils at the bottom of slopes tend to be wetter than soils on the summit because saturation usually occurs at the bottom where water collects.  On a sloped area soils parallel to the contour will tend to be more similar than soils perpendicular to the contour.

 

 

Your site plan should also always indicate North and a scale if necessary you may want to have a "key" or "legend" for symbols you may have used when drawing your map.  You should also make note of anybody on site who may have helped you identify site features.  The site plan which you produce must convey all necessary information regarding the site because you may not be available to translate ambiguous items on the site map.

 

2.     Soils

2.1     Soil Investigations

 

The treatment of on-site wastewater takes place in three areas: (1) the treatment unit itself, such as septic tank or aerobic treatment plant; (2) the discharge media, such as distribution rock, graveless pipe or chambered systems; (3) the soil it is being discharged to. 

 

The treatment unit and discharge media can be varied to achieve a desired effluent quality.  However, the soil is a fixed component of the wastewater treatment system.  Therefore the first two components must be designed to meet wastewater treatment needs not met by the soil component.  Soil and site evaluation and interpretation is the only way do determine the soil capability and is the most critical process in selecting and designing on-site wastewater systems. 

 

Several onsite system locations may have been identified during the site evaluation.  The next step is to determine soil conditions at one or more of these potential locations.  There are three primary methods used to conduct the soil investigation.

 

2.1.1     Probing

 

A soil probe is a hollow tube pushed into the soil by hand or power machinery.  When the probe is extracted a column of undisturbed soil may be viewed.  Probe diameters vary and extensions may be added to increase the length of the probe.  Probing is the quickest method of looking at a soil profile.  It also allows for undisturbed viewing of faint soil mottling or cemented layers.  The disadvantage of probing is the relatively small diameter of the probe and the inability of the probe to penetrate soils in rocky areas. 

 

2.1.2     Augering

 

An auger is a hollow cylinder with teeth at the bottom.  The cylinder is injected into the soil by twisting and using the teeth do dig down into the soil.  Auguring is labor intensive and slower than probing.  Augers typically are larger than probes so you can view a larger sample.  However, the sample is somewhat mixed and homogenized and may not reveal faint mottles, cemented layers or soil structure.  Auguring may also be ineffective due to rocks. 

 

2.1.3     Test Holes

 

Test holes or pits are the best method for viewing soils.  Test holes allow viewing of undisturbed soil and how the soil varies over the length of the test hole.  Test holes are the only reliable method to determine depth to bedrock.  The disadvantage of test holes in the necessity of a backhoe which adds cost and destruction of the site.  A backhoe is used to excavate a large enough hole so the evaluator can thoroughly study the soil conditions.  The test holes should be dug to a depth of at least four feet below the estimated depth of the intended disposal system.  The holes should be wide enough to allow sunlight to enter and shine on an exposed face of the hole.

 

2.1.3.1     Test Hole Safety

 

Working in a test hole presents many safety concerns.  Non are as serious as the risk of a trench cave-in.  When cave-ins occur they are much more likely to result in fatalities than other excavation-related accidents.  The Occupational Safety and Health Administration (OSHA) has safety standards for excavation and trenching.  Strict compliance with all sections of the standard will greatly reduce the risk of came-ins as well as other excavation-related accidents.  The March 5, 1990 revised standards are located in the appendices along with an OSHA Construction Safety and Health Outreach document on excavations.

 

2.2     Soil Principles

2.2.1     Soil Information

 

Soil is made up from disintegrated rock and decomposed organic materials known as humus.  Soil develops over time through the interaction of climate, topography and biological activity.  This is known as weathering.  Weathering can result from three processes: (1) mechanical disintegration , (2) chemical processes (3) biological activity.

 

Mechanical disintegration is the process of breaking rock into smaller and smaller particles.  It can result from climatic effects or through movement of rocks rubbing against each other.  Chemical reactions can be materials reacting with water, oxygen or organic material.  Materials may dissolve in water and/or have secondary reactions with other chemicals.  Chemical reactions of the original materials is the beginning of the process and can lead to new products and new reactions.  In the presence of oxygen, some materials oxidize to new products.  Some of these products may dissolve in water and be transported elsewhere.  Canter can bind directly with minerals in a process known as hydration.  Other minerals react with acids produced by living organisms.  Sequential reactions of products and byproducts create the character that is distinct for each soil.

 

2.3     Soil Components

 

Soil contains about 50% solid material and 50% pore space.  The solid portion is made up of organic matter and mineral matter discussed above.  The pore spaces typically contain equal amounts of water and air.  As wastewater is applied to the soil it moves through the soil through these pore spaces.  How easily the water moves is a function of the soil characteristics and is know as its infiltration rate. 

 

2.4     Soil Description Form

 

As you systematically collect information regarding the soil that information must be recorded so the information can be communicated and used by others.  Appendix 4.3 and 4.4 show examples of standardized forms used to collect, record and communicate the soil information.

 

2.5     Soil Charactieristics

2.5.1     Horizons

 

Soils are generally composed of layers called horizons.  The sequence of these horizons is called the soil profile.  Each horizon has a distinct character and must be characterized to determine how each horizon will react with the introduction of treated effluent. 

 

 

The O Horizon is an organic layer consisting of leaves, plant remains and animal droppings.

 

The A Horizon is commonly called the topsoil.  The A horizon is composed of soil minerals mixed with humus.  The A horizon results from the deposition of organic and inorganic materials.  The thickness of this layer typically ranges from 2 inches to 2 feet.

 

The B Horizon is called the subsoil.  This horizon is make of one or more layers of soils composed of the  minerals from the parent material below it and the material deposited above it.  B Horizons are usually dominated by certain minerals and elements.

 

The C Horizon is distinguished from the others by the lack of weathering. 

 

Sometimes a horizon develops directly below the A Horizon.  Water percolates through the A Horizon and dissolves the minerals in it.  This horizon is know as the E Horizon and is often called the zone of leaching.

 

2.5.2     Boundaries

 

Soil horizons vary in how dramatically they change.  Some horizons are separated by an easily visible change in soil characteristics, while others show gradual change.  Boundaries are measured in terms of transition width between adjacent horizons.  Each transition should be clearly marked in the test hole by using nails and ribbon.

 

 

 

2.5.3     Texture

 

Soil is a mixture of materials that can be organized by size.  The materials can be divided into rock fragments and fine earth materials.  Both  rock fragments and fine earth materials can be further characterized by Table 2.2.

 

Table 2.2 Soil Size Classification
Rock Fragments
Class	Size (mm)
Boulder	>300
Cobble	150 - 300
Gravel	2.0 - 150
Fine Earth Fraction
Class	Size (mm)
Sand	0.05 - 2.0
Silt	0.002 - 0.05
Clay	< 0.002

 

As site and soil investigators we are generally more concerned with the fine earth materials as long as the soil is not considered bedrock.  As a rule of thumb if the percentage of rock fragments exceeds 50 percent of the total soil mass the soil is considered bedrock.

 

Sand is loose and composed of single grains of minerals.  The minerals are predominantly quartz (SiO₂) and other minerals including feldspar, hornblende and mica. 

 

When dry, silt may appear cloddy but the lumps can be readily broken and when pulverized it feels soft and floury.

 

Clay is a fine textured soil invisible to the naked eye and sticks together when wet or dry.

 

Few soils consist wholly of particles of one size class.  Instead soils typically have percentages of sand, silt and clays.  Soil texture refers to the physical composition of soil in terms of proportional weight of each fine earth fraction.  The texture classes are defined in terms of particle size distribution of the fine earth materials.

 

The combination of silt and clay provides the greatest treatment variation in the soil.  The combination of sand, silt and clay is illustrated by the USDA Soil Triangle. 

 

 

The basic textures in order of generally increasing proportions of finer particles are sand, loamy sand, sandy loam, loam, silt loam, silt, sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay, and clay.  The classes of sand are shown in Table 2.3  Sand Classification.

 

Table 2.3 Sand Classification
Class	Size (mm)
Very Course	1.00 - 2.00
Coarse	0.05 - 1.00
Medium	0.25 - 0.50
Fine	0.10 - 0.25
Very Fine	0.05 - 0.10

 

2.5.4     Color

Color is the most obvious of soil properties and is easily determined.  It can be used to identify important soil properties.  The importance of soil color is greatest within microenvironments.  Some common relationships of color to other soil properties can serve as a basis for interpreting color.  Relationships of color that are observed repeatedly should be noted.  Often times specific color features can be associated with specific materials in specific environments.  

 

For example, darker colors may show the presence of organic matter.  This is because humified organic material is dark, however, raw organic material such as peat is not necessarily dark.  Where average temperatures are high soils that are high in organic material are usually less dark than soils in cooler regions.

 

Gray colors may indicate an absence of oxygen due to water saturation.  Alternate streaks of oxidized and reduced materials known as mottles  suggest periods of oxidation and reduction that may indicate seasonal saturation or poor drainage.  However some materials such as carbonic materials are gray and remain so even under oxidizing conditions.  It will take practice in your locale to fully understand the color relationships with soils. 

 

2.5.4.1     Munsell Color Standards

 

Soil color is determined by comparing the soil with a standard color chart.  The most commonly used chart is the Munsell Color System.  The Munsell Color System for soils consists of 175 different colors, or chips.  The Munsell System uses three elements of color (1) hue, (2) value and (3) chroma, to make up a specific color notation.

 

·         Hue identifies the quality of color registered by the eye as related to five principal hues (red, yellow, green, blue and purple).  There are another five intermediate hues that identify the midpoints between the principle colors (yellow-red, green-yellow, blue-green, pruple-blue and red-purple).  Hues are divided into four equal gradations between 2.5 and 10.  These gradations are prefixes used before the color or color pairs. 

 

 

·         Value is the lightness or darkness of a color on a scale of 0 (black) to 10 (white).  Medium gray has a value of 5.

 

·         Chroma is the intensity or purity of the color.  Chroma for soils is measured on a scale from 0 to 8.  0 being very neutral or dull and 8 being very pure. 

 

 

 

A Munsell color book can be used to judge colors accurately on sunny days when the sun is high.  Each page of the Munsell color book is 2.5 hue units, value increases by one unit as you look up the page and chroma increases by one unit from left to right.  Measurements of color are considered accurate if the color is reproducible by different individuals within 2.5 units of hue and 1 unit of value and chroma.

 

Each horizon may have a uniform color or it may be streaked, spotted and varied. organic matter, carbonates, iron and other substances commonly differ in color from the surrounding soil.  As mentioned earlier soils with poor drainage commonly are marked by a mixture of gray colors known as mottles.  Mottles can also be brown, yellow or red.  Typically a dominant color will be present.  Various color patterns in a soil horizon fall with under one of three labels.

 

·         Dominant color

·         Mottling

·         Physical state

 

2.5.4.2     Dominant Color

 

The dominant color is the color that occupies the greatest volume of the layer  For only two colors, the dominant color occupies more than 50 percent of the area.  for three or more colors, the dominant color occupies more of the area than any other color, but it may occupy less than 50 percent.  It is possible to not have a dominant color.

 

2.5.4.3     Mottles

 

Mottles are described in terms of quantity, size, contrast and color.  secondary properties such as shape and location may also be noted. 

 

Quantity is based on the percentage of observed surface that is occupied by mottles.

 

 

 

Size refers to the approximate dimensions of the face of the mottles.  If the length of a mottle is not more than two or three times the width, the dimension recorded is the greater of the two.  If the mottle is long and narrow the dimension recorded is the smaller of the two and the shape is described.

 

 

 

Contrast refers to the degree of visual distinction that is evident between the dominant color and the mottles.  Contrast should be described per Table :

 

 

The shape of the mottles may be significant and are often described by common works like streaks, bands or spots.

 

The mottles may not occupy the total height or length of the horizon.  If so this should be noted. 

 

The physical state of a sample is recorded as broken, rubbed, crushed or crushed and smoothed.  The term crushed indicates the sample is dry and the term rubbed indicates the sample is moist. 

 

2.5.5     Structure

 

Structure refers to the physical organization of a soil or the lumps it generally breaks into.  These physical characteristics persist through more than one cycle of wetting and drying in place.  An individual lump is called a ped.  Most peds are large enough to be seen by the naked eye. 

 

Some soils lack structure and are known as structureless or massive.  In structureless soils no peds are observable in place or after the soil has been gently disturbed. 

 

In soils that have structure the shape, size and grade of the peds have may have a marked effect upon the suitability of a soil for a system. 

 

2.5.5.1     Shape

 

Several basic shapes of peds are recognized in soils.  The basic shapes are:

 

·         Platy  - flat like plates and generally horizontal and overlapping

·         Prismatic - long and have flat vertical faces.  Their tops are generally flat

·         Columnar - similar to prismatic but tent to have distinct rounded tops.

·         Blocky - block-like or polyhedral.  Can be sub-angular blocky if their faces are rounded or angular blocky if their faces have sharp edges.

·         Granular - approximately spherical or polyhedral and are bounded by curved or very irregular faces that are not casts of adjoining peds.

 

 

 

2.5.5.2     Size

 

Size refers to the size of the peds.  Five classes describe the size:

 

 

If the peds are consistently larger than "very coarse" the actual size should be noted.

 

2.5.5.3     Grade

 

Grade refers to the adhesion between peds.  The determination of grade depends on the ease that the soil separates into discrete peds.  Three classes are used:

 

·         Weak - The peds are barely observable in place.  When gently disturbed the soil parts into whole and broken peds and a lot of material that exhibits no ped faces.

·         Moderate - The peds are well formed and evident in undisturbed soil.  When disturbed the soil material parts into a mixture of entire peds, some broken peds, and a little material that exhibits no ped faces.

·         Strong - The peds are distinct in undisturbed soil.  They separate cleanly when the soil is disturbed.  When removed the soil material separates mainly into entire peds.

 

 

2.5.6     Consistence

 

Consistence refers to the combination of soil properties that determine its resistance to crushing and its ability to be molded or changed in shape.  Consistence will vary with soil moisture so consistence is evaluated differently, depending on whether the soil is moist or dry. 

 

 

 

2.6     Application Rate

 

After soil information is collected and recorded this information will be used to determine the volume of water that can be delivered to a volume of soil over an infinite amount of time.  This is known as the soils application rate.

 

As wastewater is added to the soil, infiltration rates decrease from initial rates and remain at that level.  One of the primary factors in the decrease is the amount of pollution, organic materials (BOD) and suspended solids, in the water being discharged.  The more organics provided by the wastewater the more microorganisms which feed on the organics are produced.  These microorganisms produce slimes and physically fill the pore space's along with the suspended solids reducing the infiltration rates of the soils.  If the soil has a low infiltration rate to begin with, possibly due to texture, the wastewater applied to the soil must be treated to a higher degree to remove more organics and solids or a smaller volume of wastewater should be applied to a equal volume of soil.  Appendix 5.5 contains a flow chart that determines application rates based on soil texture, structure and consistence. 

 

3.     Percolation Testing

A second way to determine a sites application rate is the percolation test.  The percolation test is not a 100% accurate way to predict absorption rates of the soil.  It should be used in conjunction with other data such as soil classification and field observations mentioned earlier. 

The percolation test does not measure percolation or infiltration.  Percolation is the movement of water through a soil and infiltration is the movement of water into a soil.  The percolation test is an empirical procedure for observing water seeping into a hole in the ground.  It has been empirically related to application rates. 

 

3.1     Procedures

 

Following are procedures used for percolation testing in the State of Arizona.  These procedures have been taken almost verbatim form Engineering Bulletin 12 Minimum Requirements for the Design and Installation of Septic Tank Systems and Alternative On-Site Disposal Systems.

 

3.1.1     Digging the hole:

 

With hand tools dig a 12" square or 15" round hole.  If the soil collapses place a perforated pipe vertically into the hole and carefully pace gravel or some other supporting material between the pipe and the hole wall.  Perform the test within the pipe and adjust calculations to account for water displacement by the supporting gravel pack.  Perforated buckets may be used to support the sidewalls.  Fill void spaces between the sidewalls and the bucket with pea gravel.

 

3.1.2     Preparing the Hole:

 

Remove any smeared soil surfaces from the sides of the hole to provide as natural a soil interface as practical, to infiltrating waters.  Remove loose materials from the bottom of the hole.  To protect the bottom from scouring, add two inches of fine gravel.

 

3.1.3     Presoaking the Hole:

 

Presoak the hole by filling it with clear water to a depth of 12 inches above the bottom of the hole.  Determine the time for this amount of water to seep away.  If the water seeps away in 60 minutes or less, this procedure shall be repeated.  If a third test repeats the above result and the soil has a low clay content, 15% or less, and a low shrink-swell potential, the percolation rate test may proceed immediately.  If not, maintain a minimum water depth of 12 inches above the bottom of the hole for a 4-hour period by refilling as necessary or by use of an automatic siphon.  Water remaining in the hole after 4 hours shall not be removed.  Thereafter, the soil shall be allowed to swell not less than 16 nor more than 30 hours.  following the soil swelling period, remove any soil which has sloughed into the hole. 

 

3.1.4     Percolation Rate:

 

Fill the hole with clean water to exactly six inches above the soil bottom of the hole.  With a tape measure or float gauge and a timepiece, determine the time for the water to recede exactly 1 inch.  Refill immediately and repeat the process until successive time intervals needed for a one inch droop indicate that a stabilized rate has been obtained.  This in generally indicated when three consecutive percolation rate measurements vary by no more than 10%.  Report the stabilized percolation in minutes per inch.

 

3.2     Application Rate

 

Following is a table with Percolation rates and corresponding Application Rates.  The rates in Table 3.1 correspond to the equation

.  Which comes from the Ten State Standards with a factor of safety of 2.5.

.

Table 3.1 Application Rates vs. Percolation Rates
Percolation Rate MPI	Application GPSFP)
<1	0.00
1 to 2	1.40
3	1.10
4	1.00
5	0.90
7	0.75
10	0.63
15	0.50
20	0.44
25	0.40
30	0.36
35	0.33
40	0.31
45	0.29
50	0.28
55	0.27
60	0.25
>60 to 120	0.20

 

4.     References

Arizona Dept. of Environmental Quality, Engineering Bulletin No. 12 "Minimum Requirements for the Design and Installation of Septic Tank Systems and Alternative On-Site Disposal Systems, 1989

 

Arizona Dept. of Environmental Quality, Engineering Bulletin No. 12  "Arizona Statewide Technical Standards and Guidelines for Class 1 Onsite Wastewater Treatment Facilities" Prepared by the Arizona Ad Hoc Committee.

 

Burks, B; Minnis, M. Onsite Wastewater Treatment Systems Hogarth House, LTD. Madison, WI, 1994

 

Crites, R.; Tchobangolous, G.  Small and Decentralized Wastewater Management Systems.  McGraw-Hill, New York, NY 1998

 

Hammer, M.  Water & Wastewater Technology.  J. Wiley & Sons, Inc.  New York, NY, 1997

 

Tyler J.  "Soil and Site Evaluation for Onsite Wastewater Treatment Systems

 

Winneberger, T.  Septic Tank Systems "A Consultants Toolkit".  New York, NY Butterworth Publishing.  1984.