COVER LETTER

The intent of this paper is to develop a standardized soil percolation test based on the standard

percolation test in the U.S. Public Health Service Manual (1967). I relied heavily on works by

Hill (1966), Barbarick et al. (1976), Winneberger (1984), Kleiss & Hoover (1986), Kaplan

(1991), and fieldwork at Lincoln-Lancaster County Health Department. The methods and

materials section of this paper deals with the reasoning used to develop a standardized perc test.

A procedure of instructions for conducting a standardized perc test is the product of this effort

and is presented in the results section.

This paper is also a critical review of literature and research on the soil percolation test. I found

studying the literature cited in this paper to be an enlightening, educational experience. My

hands-on field experience in conducting perc tests has been invaluable in comparing and

contrasting fieldwork with literature. Organizing the discussion section of research results and

conclusions of studies cited in references was challenging. An inadvertent consequence of this

study is a proposed hypothesis that the perc test might measure both saturated and unsaturated

flow in soil around a perc hole depending on soil type. Of course, more research would be

needed to verify or falsify this notion.

TERMINOLOGY

The older literature is replete with the term “sewage disposal”. The updated “wastewater

treatment” terminology is used in this paper.

There are numerous terms to describe dispersal of liquid effluent from a septic tank into soil–

“subsurface absorption trench, bed or area”, “laterals”, “drainfields”, “filter fields”, “absorption

fields”, “dispersal fields”, and “leach fields”. The latter term “leach field” was selected because

it is the recent term reported in U.S. EPA Manual (2002), and because it is short. It also helps to

distinguish water of curtain drain fields from liquid effluent of leach fields.

REFERENCES

Nearly all references cited in parentheses in this paper refer to the previous one or two sentences.

Be Well,

Ron Marquart, M.S., R.S.

Lincoln-Lancaster County Health Department

3140 N Street

Lincoln, NE 68510

Email Address: rmarquart@ci.lincoln.ne.us

1

A STANDARDIZED PERC TEST —WHY NOT?

Ron Marquart*

Introduction

The soil percolation test (perc test) has been used worldwide to design onsite wastewater treatment

systems (Salvato, 1992). Many states still use the perc test to size the leach field absorption area of

onsite systems while some states have phased in soil morphology methods as a more reliable way to

size leach fields (Lewis, 1975). Elrick & Reynolds (1986) point out the unreliable variation of perc

rates because the procedure for conducting the perc test is not standardized. Supportive evidence

favors soil morphology instead of perc tests in determining the infiltration surface area of onsite

subsurface wastewater treatment systems. However, Brown et al. (1994) has developed a hydraulic

loading rate table based in part on an average of perc test results and soil type, and Barbarick et al.

(1976) found that percent sand, silt and clay in subsoil correlated significantly with perc rate.

Perhaps the commonly used perc rate tables can be modified to match the newly developed soil

morphology tables used in determining the infiltration surface area of a leach field. The

standardized perc test described in this paper (conducted by a qualified technician) might contribute

to modifying the perc rate table to a more acceptable loading rate in sizing leach fields. The

importance of a standardized perc test as one of many tools in evaluating soil for an onsite system is

supported in this paper.

An effort to standardize the universally used perc test in the U.S. Public Health Service Manual

(PHSM), 1967, 3rd Ed. was based primarily on books by Winneberger (1984) and Kaplan (1991), a

pamphlet by the Minnesota Extension Service (1995), papers by Hill (1966), Barbarick et al. (1976)

and Kleiss & Hoover (1986), the U.S. EPA Design Manual (EPAM, 1980), and fieldwork of

Lincoln-Lancaster County Health Department (LLCHD). The methods and materials section

explores the literature, describes LLCHD’s method and procedure, and explains the reasoning

involved in developing a standardized perc test. A procedure and instructions for conducting the

standardized perc test was the product of this effort and is presented in the results section. The

discussion section is a critical review of cited literature and research on the perc test. An inadvertent

consequence of this study is a proposed hypothesis that the perc test might measure both saturated

and unsaturated hydraulic conductivities in soil around a perc test hole depending on soil type. More

research would be needed to verify or falsify this notion.

Methods and Materials The procedure and instructions for conducting a standard soil percolation test are found on pages 4

to 6 of the U.S. Public Health Service Manual of Septic-Tank Practice, 1967. PHSM instructions are

found in Appendix A of this paper. All five numbered steps in the procedure of this manual need

modification and clarification in order to produce a more consistent standardized soil percolation

test. Relevant literature and reasons for modification and clarification are explained and presented in

this paper beginning with step one.

*Ron Marquart, M.S., R.S. works for the Lincoln-Lancaster County Health Department in Nebraska and has been

evaluating, witnessing and conducting soil percolation tests for the past 30 years. He can be reached at Lincoln-

Lancaster County Health Department, 3140 ‘N’ Street, Lincoln, NE 68510. Email address: rmarquart@ci.lincoln.ne.us.

Phone: 402-441-8030.

2

Step “1. Number and location of tests.”

Kaplan (1991) recommends a minimum of 4 perc test holes in a uniform soil type and references

work by Weibel et al. showing studies that from 6 to 22 perc test holes per site are necessary in

relatively homogeneous soil. Barbarick et al. (1976) recommends 4 to 6 test holes per site. The

EPAM (1980) recommends a minimum of 3 perc test holes per site and more tests if soil conditions

are variable. Gustafson & Machmeier (1995) recommend at least 2, and preferably 3, perc test holes

if soil texture is uniform over site. All authorities agree a priori that regardless of number of tests

used per site, holes are to be spaced uniformly over proposed absorption leach field.

Step “2. Type of test hole.”

The width of a hole affects the percolation rate of water seeping out of a hole. Narrow 4" diameter

holes yield faster perc rates than wide 12" diameter holes primarily because of the area (square) of

sidewall to volume (cube) of water ratio. A smaller diameter hole has more surface area per volume

of water than a larger diameter hole (NEHA, 1979; Salvato, 1992; Winneberger, 1984). Diameter

correction factors are needed for perc rates from holes more than 8" wide, which may be necessary

in very coarse textured soils (Kaplan, 1991). Barbarick et al. (1976) recommends using a slightly

less than 12” (30 cm) hole in Arizona soils. Their experimental results found that perc rates for 10

cm dia. holes were approximately 2.5 times faster than perc rates for 30 cm dia. holes. The EPAM

(1980) specifies a 6" dia. hole and refers to a 6" to 9" dia. hole for perc tests. Many states require 6"

to 8" dia. holes and Oklahoma is revising the depth of test holes to be 6” below the depth of

proposed subsurface trenches. All authorities agree a priori that holes need to be vertical, and most

agree that depth of holes are to be the same depth as proposed absorption leach field trenches ( 24"

is a goal recommended in EPAM, 2002).

Requirements for tests in frozen soil range from conducting a perc test in unfrozen soil as in South

Dakota to not conducting a perc test where frost is below the depth of proposed leach field as in

Minnesota. The cited literature in this paper is essentially silent on the effect of conducting a perc

test in wet soil above the plastic limit. (The discussion section covers more thinking about this

dilemma.)

Considering the above to arrive at a median number and size, a preferred depth of a perc test hole

and relevant conditions of soil, it seems reasonable that a standardized perc test procedure would be:

#1. in instructions of results section.

Step “3. Preparation of test hole.”

Digging or boring a perc test hole can damage the sidewall and bottom of hole. Digging can cause

shearing damage, and boring with a power auger can cause compaction damage of soil at the

sidewall and bottom of hole (Kaplan, 1991 & Winneberger, 1984). To remove this damaged soil,

Winneberger (1984) manually uses a rigid putty knife; Kaplan (1991) recommends a nail or tip of a

pocket knife to peel damaged soil away from the inside of hole. In their technique, a smaller width

hole is first dug or bored and then enlarged by peeling the sidewall to a 6" to 8" width. To peel soil

away from inside a hole the knife blade is inserted at an angle into the sidewall and a twisting

motion removes soil in small chunks. This method exposes a more undisturbed, natural soil-to-water

interface. During this process loosened soil falls into a dipper used to remove soil from hole.

3

LLCHD hands-on fieldwork conducting perc tests has found that enlarging a power auger bored

pilot hole of 5 ½" dia. to a 7" dia. hole with a hand auger works well, or use a hand auger only

(Barbarick et al., 1976). With this technique, loose soil of the power bored hole and peeled off soil

from sidewall of hand bored hole are more easily and quickly removed at the same time compared to

the time consuming, laborious, putty knife and dipper method. Several test hole comparisons of

these two techniques in the same soil and site revealed little difference in perc rates.

Kaplan (1991) reports that Van Kirk et al. found power auger bored holes had much slower perc

rates than hand auger bored holes because of severe compaction behind smeared soil from power

boring. LLCHD’s method of peeling a hole to a larger diameter with a hand auger would remove

both smeared and compacted soil caused by a power auger. Gustafson & Machmeier (1995), EPAM

(1980), and many states recommend the sidewall and bottom of a test hole be scarified or scratched

with a knife or nails driven into a board to remove smeared soil and thus help provide a natural soil-

to-water interface. LLCHD has found that nails driven into one end of a 1" x 2" board works well to

scratch the sidewall, and a 1 ½" ridged spatula fastened to the other end of the board works well to

peel the bottom of a hole. Compacted soil at the bottom area of a hole is removed by inserting

spatula blade into soil and using a twisting motion to remove chunks of soil. This procedure is

necessary to form a right angle of sidewall to bottom and to produce a flat bottom in a hole. The

loose soil is removed from bottom of hole manually using a small bread pan. This technique is less

likely to smear the sidewall and bottom of a hole than using fingers to remove soil.

Presoaking a hole with clear water prior to conducting perc test can cause a migration of fines

toward the sidewall and collapse of sidewall into the bottom of a hole (Hill, 1966). One can prevent

sidewall collapse by using a paper liner (Winneberger, 1984), strips of tarpaper (Derr et al., 1969), a

gravel filled hole (Hill, 1966), or gravel packed between a wire cylinder or perforated plastic pipe

and sidewall of a hole (Kaplan, 1991; PHSM, 1967; Winneberger, 1984). Hill (1966) found that

gravel pack prevented water turbulence when filling a hole and thus significantly slowed a migration

of fines toward the sidewall from surrounding soil. Migration of fines to the sidewall and bottom of

hole slowed perc rates. Both Winneberger (1984) and Kaplan (1991) emphasize that gravel packed

holes have much faster perc rates than holes without gravel pack. Kaplan (1991) explains why

correction factors are necessary for gravel packed holes and provides a table of gravel packing

correction factors that vary with the thickness of gravel pack. He also points out consequences of

not using a sidewall liner by referring to EPAM (1980) instructions that call for removing any soil

that sloughs into hole during the presoak which could compact the sidewall, plug soil pores, change

hole diameter and cause a real mess. All of the above methods place 2" of gravel on the bottom of a

hole to protect against scouring, puddling and sediment when filling hole with water.

LLCHD uses a permeable geotextile fabric instead of paper or a gravel pack to line the sidewall.

Geotextile fabric rolled into a double-walled tube is inserted into hole and positioned as a liner

against the sidewall to prevent or hold out sidewall collapse. The liner also helps prevent water

turbulence against the sidewall when filling the hole, thus reducing a migration of fines toward the

sidewall and into the bottom of hole. A mesh bag with 2" of clean gravel (1/4 to 3/4") holds the

bottom of liner in place. A nail holds liner in place at the top. Small amounts of soil that adhere to

the geotextile fabric and bag of gravel are easily washed off when perc test is completed. They can

4

be used again for future tests. It is important to point out that gravel used in a test hole as well as

aggregate used in a leach field trench must be clean. Fines from aggregates can clog the infiltration

surface and reduce water flow (White & West, 2003).

It should be apparent from this review of methods in preparing the perc test hole that proper

preparation is critical for consistent results. Evaluating the above methods, it seems reasonable that

a standardized perc test would be: #2 in instructions of results section.

Step “4. Saturation and swelling of the soil.” (presoak)

Most authorities agree that continuous use of an onsite system causes the void spaces (pores)

between soil particles surrounding leach field trenches to initially become partially filled and

eventually completely filled with water in fine or medium textured soils (prior to clogging mat

formation). The water filled pores produce a saturated soil, but clay loam requires more time for

water to intrude into and bind with individual soil particles to produce swelling that reduces pore

size and slows hydraulic conductivity. LLCHD has found that silty clay loam soils (and not in a dry

season) occasionally yield perc rates faster than 45 minutes per inch (mpi) whereas these soils

usually yield slow or 60 mpi (EPAM, 1980). Soil pit and bore hole examination of these soils

revealed well developed structure in the form of extensive small root channel pores (Bouma, 1975;

Kleiss & Hoover, 1986). A continuous presoak period of 24 hours or longer appears to increase

swelling to somewhat reduce small root channel pore size and somewhat slow hydraulic

conductivity to better simulate soil saturation in a leach field. A continuous presoak period of

several 24 hr. days and nights is necessary to shut cracks in some high clay content soils in a dry

condition during the summer (Lewis, 1975).

Authorities disagree on how effective a perc test presoak is in simulating the actual soil saturation

and swelling around an in-use leach field trench for all seasons of the year. Barbarick et al. (1976)

recommends a 6 hr. presoak for coarse textured soils and a 20 hr. presoak for fine textured soils.

Hill (1966) concludes that the presoak does not produce saturated soil conditions around the perc

test hole in coarse textured soil. Thus the perc rate is a measure of unsaturated flow by capillary

forces moving water away from a hole. He also found that perc tests conducted in early spring in

wet soil were slower and less variable, were faster as soil dries during early summer, and were

slower again if soil dries excessively by early fall. Healy & Laak (1973) found it is unlikely that the

presoak can effectively produce saturated conditions around a hole necessary to eliminate the affect

of capillarity in fine textured soils. They conclude that perc tests are unduly influenced by

capillarity as a function of rainy weather or a dry season and this results in a wide range of perc

rates. Gross et al. (1998) found that perc rates varied from season to season at the same site even

though the same presoak procedure was followed each season. (An analysis of their study is covered

in the discussion section.)

On the other hand, Tyler et al. (1991) tells us that perc tests do not directly measure unsaturated flow

but can give an average of saturated flow around a hole. Likewise, the EPAM (1980) tells us that

the perc test, when run properly, can give an approximate measure of a soil’s saturated flow. These

authorities lend support to the claim in the PHSM procedure that if presoak water is maintained in

the hole at a depth of no less than 12" over gravel for at least 4 hours, and preferably overnight, the

5

soil around the hole will approach the condition it will be in during the wettest season. The perc test

will then yield comparable results in a wet or dry season.

Kaplan (1991) reports that research by Weibel et al. showed that a presoak of 4 hours gave perc test

rates nearly twice as fast as perc test rates after an overnight presoak in the same soil. Another soil

was tested for 120 days and the perc rate continued to yield a slower rate until it “stabilized” at the

end of 120 days. On the other hand, Hill’s (1966) research showed that in coarse textured soils the

perc rate after a 4 hr. presoak was essentially no different than the perc rate after an overnight

presoak. However, continued testing for several days after the first perc test showed variable rates

from the first perc test rate. He thought these subsequent perc rates were influenced by dissolved air

and microbial activity in the water.

The origin of the use of the minimum 12" water level for presoaking appears to come from Ludwig

& Ludwig’s (1949) modification of the original “common” perc test conducted by Henry Ryon in

New York State in the 1920s. They modified the “common” perc test by filling a 1 ft. square, 18"

deep hole with about 12" of water instead of Ryon’s recommended 6" water level. They measured

the distance the water level dropped in 30 or 60 minutes, and refilled immediately to about 12" after

each time interval. This procedure was repeated and continued for 10 hours. It was discovered that

it takes about 6 hours for the water level drop to slow down to a stabilized water level drop per time

interval as the soil approaches saturated conditions. Their mathematical evaluation is based on the

observation that a ratio of time in minutes that water level drops 1" (perc rate) is constantly

increasing during the test, but the rate of increase is constantly decreasing. They concluded that the

perc rate is to be measured when the soil is at its most saturated condition in about 6 hours. A

standardized perc test instruction of filling siphon bucket at the end of a standard 4 hour presoak

extends saturation nearer to 6 hours.

It is important to clarify the ambiguous wording in the PHSM presoak procedure: “...it is necessary

to refill the hole...” One could interpret this to mean put more than 12" of water over gravel into a

hole, wait until water level drops to the bottom of hole and then refill it over 12" again, etc. for at

least 4 hours (Hill, 1966). The instructions for testing in sandy soils support this interpretation. On

the other hand, one could interpret the procedure to mean put a little more than 12" of water over

gravel into a hole and maintain this level so that no less than 12" remains in hole for at least 4 hours.

This latter interpretation is based on the wording: “...minimum depth of 12 inches ...” Thus, refilling

the hole means to keep the water level at least 12". The instruction to supply water with an

automatic siphon supports this interpretation. Hill (1966) suggested a modification of the standard

perc test by using a constant head water level in a test hole to prevent air from entering the pores in

the sidewall and slowing perc rates. Also, minimum could mean either 13" or fill with water to top

of hole (Kaplan, 1991). In the instruction “...and preferably overnight.”, another variable (How long

is overnight?) of longer presoak time is factored in before perc test begins 24 hours later.

LLCHD maintains a 12" to 16" presoak water level in the test hole for 4 hours for saturation and

swelling by means of a plastic tube siphon from a sealed 5 to 10 gal. bucket. (About 20 gal. of water

per 4 hr. presoak typically indicates a medium textured soil; considerably less than 20 gal. indicates

a fine textured soil; considerably more than 20 gal. indicates a coarse textured soil.) A rubber

stopper is removed from the lid to fill the bucket and replaced to produce an air-tight seal for the

water to automatically siphon to the proper level. A fitting is installed near the bottom of bucket

6

with 3/4" flexible plastic tubing that can be bent upward along side of bucket and held in place with

the bucket handle to prevent water from entering the hole during the test. The end of a 1/8" diameter

metal rod is attached to a stool tank (WC) float and passes through a 1/4" metal sleeve or screw eyes

attached to a 2x4 board solidly positioned at the opening of the hole. This structure provides a

floating reference point. The floating reference point is positioned at eye level for easy and accurate

water level measurements to the nearest 1/16".

The PHSM also contains this ambiguous instruction: “In sandy soils ...the test may be made as

described under item 5C, after the water from one filling of the hole has completely seeped away.”

This instruction indicates that after 12" or more of water has completely seeped away, one can

proceed to procedure in 5C and begin the test immediately and not have to wait 24 hrs. to begin the

test. In sandy soils 12" of water will seep away in a short time period. The instructions in 5C imply

that 6" of water will take longer than 10 min. to seep away, and thus a water level drop of less than

6" can be measured. Therefore, 12" of water would most likely take over twice as long or more than

20 min. to seep away during the start of the presoak. But, if 6" of water seeps away in less than 10

min., the water level cannot be measured in a 10 min. interval. In this instance 12" of water would

take less than 20 min. to seep away during the start of the presoak.

The EPAM’s (1980) presoak instructions for sandy soil is also ambiguous. The instructions state:

“If, after filling the hole twice with 12 in. of water, the water seeps completely away in less than ten

minutes, the test can proceed immediately.” This probably means that the hole is filled with 12" of

water that seeps away, then immediately refilled again with 12" of water and both 12" fillings or a

total of 24" of water seeps completely away in less than 10 min. One can interpret this instruction to

mean that it would limit the number of perc tests that could begin immediately because it requires

twice as much water (2x12 = 24") to seep away in less than 10 min. compared to the PHSM

instruction (implied) of 12" of water to seep away in more than 20 min. However, Kaplan (1991)

interprets this instruction to mean that if 12" of water seeps away in less than 10 min., refill with

another 12" of water. If this second refill seeps away in less than another 10 min., a total of 24" of

water seeps away in less than 20 min. In either case, the test is conducted immediately at 10 min.

intervals for 1 hr. with water at 6" above the gravel. Regardless of the interpretations, if 12" of water

seeps away twice in less than 10 min. or once in less than 10 min. twice, then most likely 6" of water

will seep away in less than 10 min. Moreover, in both cases there will in most instances be no water

in the hole to measure!

Evaluating the above literature research, finding the PHSM and EPAM instructions ambiguous, and

based on LLCHD fieldwork in attempting to maximize thorough saturation of soil around the perc

test hole, it seems reasonable that a standardized perc test would be: #3 in instructions of results

section.

Step “5.ABC. Percolation-rate measurement.”

The day following the presoak undoubtedly means to make measurements 24 hrs. after start of

presoak (a range of 15 to 30 hrs. is allowed in EPAM) except in sandy soils. The instructions in step

5A need modification: if water remains in test hole the following day, adjust depth over gravel to

approximately 6"; this one water level drop is used to calculate perc rate; from a fixed reference

point, measure water level drop. Research in the literature and fieldwork at LLCHD support

modifying these instructions to produce consistent results.

7

If water is added to hole during a 4 hr. presoak only, then swelling overnight would be for 20 hrs.

with no additional water added before test begins. If the instruction in step 4. “...and preferably

overnight,” is followed, then water would probably be added at most for 12 hrs. beyond the 4 hr.

presoak for a total of 16 hrs. (Hill, 1966). With this water in the hole, swelling overnight would be

for 8 hrs. with no water added before test begins 24 hrs. after the start of presoak. Obviously, the

latter presoak procedure would more likely have water in a hole than the former (in most fine

textured soils). Thus, water remaining in a hole before test begins might be because of the presoak

procedure. Winneberger (1984) points out examples of perc test holes with water in a hole before

beginning the test yielding passing perc rates the first 30 min. of test, but failing if tested until the

last 30 min. of 4 hr. test.

There are a host of additive precision errors that can accumulate to adversely influence the accuracy

of attempting to measure the water level in a perc test hole from a fixed reference point. For

example, accurately seeing contact of tape or ruler with water surface and allowing movement

through the meniscus, not holding tape or ruler vertical for each measurement, a bend in tape

measure, accidentally bumping fixed reference point, not avoiding parallax, and difficulty in

measuring to nearest 1/16 inch are all potential errors that could effect accuracy (Kaplan, 1991 &

Winneberger, 1984). A floating reference point similar to the diagram in EPAM (1980) is by far a

more accurate method of measurement than a fixed reference point.

The instructions in step 5B “...approximately 6 inches...” and “...refilling 6 inches over the gravel as

necessary” can be highly variable and significantly affect perc rates (Bouma, 1971; Derr et al.,

1969). Winneberger (1984) tells us that more “caprice” causes more variability in the test by these

instructions. (The origin of the 6" water level appears to come from Ryon’s original “common” perc

test.) Some technicians might interpret the instruction to mean refill hole to about 6" only if water

level drops nearly to the bottom. He reports that most technicians know that refilling a hole just

before it is nearly empty of water before the last 30 min. measurement will result in faster rates than

refilling hole to 6" after each successive measurement. He also reports that studies show refilling to

8" above gravel can increase perc rates by 1.5 to 3 times. NEHA (1979) states that a 2" to 8" water

level variation above the bottom of a hole can have considerable influence on perc rates. Barbarick

et al. (1976) recommends a 5⅞” (15 cm) water level above 2” (5 cm) of gravel and to measure the

time for water level to drop 1” as the perc rate. Kaplan (1991) instructs to fill test hole with water

to exactly 6" above gravel and to keep water level between 3 to 6 inches during final three

measurements. The EPAM’s instructions state that at no time during the test is water level to rise

more than 6" above gravel, and the water level is readjusted to 6" after each measurement. It is clear

that these authorities emphasize the importance of a water level near but not to exceed 6" above

gravel.

Instructions in step 5B also state that the water level drop in the last 30 min. of a 4 hr. test is used to

calculate perc rate. But it is important to consider the instruction stating that prior water level drops

provide information to guide possible procedure modification to suit local circumstances. The

EPAM and Kaplan (1991) have modified their procedures based on information from prior water

level drops. Their method allows the test to end before or after the end of a 4 hr. period depending

on when two consecutive water level drops vary by no more than 1/16" provided at least three water

level drops are measured. The last water level drop is used to calculate perc rate for a test hole.

8

Gustafson & Machmeier (1995) follow a similar procedure by calculating perc rates for each water

level drop and ending the test when three consecutive perc rates vary by no more than 10%. These

rates are averaged to represent the perc rate for a test hole. The total number of test holes per site are

not averaged; the slowest hole is used to represent the perc rate for the site (Minnesota Code, 1996).

Comparing the two procedures, one finds that the 1/16" method is equivalent to the 10% method in

allowing only about 6% variation (1/16 = .0625 = 6.25%) between two water level drops, and avoids

potential technician errors in calculating an average. Salvato (1992) reports that some authorities

prefer only a 3-5 % or less variation between measurements. LLCHD has found that in most perc

test holes the water level drop stabilizes after two hours into the test, and almost all holes will show

two consecutive drops varying 1/16" or less per 30 min. interval before or by the end of 4 hrs.

Instruction 5C implies that 6" of water will not seep away in 10 min. Then one can measure the

water level drops for each 10 min. interval for 1 hr. The sixth water level drop is used to calculate the

perc rate. What if 6" of water does seep away in less than 10 min? In this case, it would seem

reasonable to use a stopwatch (Gustafson & Machmeier, 1995). Start timing the moment the hole is

refilled to 6" and measure the amount of time it takes for water level to drop to 5". Repeat this

procedure six times, and the sixth time interval is the perc rate.

The parenthetical instruction in the EPAM is important in a perc test evaluation of soil: “(If tests in

the area vary by more than 20 min/in., variations in soil type are indicated. Under these

circumstances, percolation rates should not be averaged.)” LLCHD fieldwork experience supports

this instruction and extends it to included variation in soil structure within the same soil type. But

this instruction only tells what not to do and leaves what to do in question. The standardized perc test

instructions explain what to do in this case.

Finally, it is important to calculate the perc rate and find the average perc rate correctly. The EPAM

(1980) instructions are correct for calculating the perc rate, but incorrect for calculating the average

perc rate. The perc rate is calculated by dividing time interval in minutes by inches drop in water

level (i.e., 30 min. ½" = 60 mpi; 30 min. 3/4" = 40 mpi). Kaplan (1991) and Winneberger (1984)

correctly point out that it is mathematically incorrect to average perc rates (i.e., incorrect to add 60

mpi + 40 mpi = 100 2 = 50 mpi). One must average inches per minutes and then calculate perc rate

from this average (i.e., correct to add 1/2" + 3/4" = 1 1/4 2 = 5/8"; 30 min. 5/8" = 48 mpi). After

averaging many different combinations of perc rates, I have found that the mathematically incorrect

method usually always results in slower perc rates.

In conclusion, the perc rate upper limit has been traditionally set at 60 mpi for a conventional onsite

septic system. Perc rates slower than 60 mpi or faster than a lower limit of about 1 to 5 mpi are

considered unacceptable. Perc rates between about 1 - 5 mpi to 60 mpi are considered acceptable.

Soils with perc rates slower than 60 mpi are fine textured and too dense to adequately accept water.

Soils with perc rates faster than about 1 to 5 mpi are coarse textured and too sandy to adequately hold

effluent water for proper treatment. Winneberger (1984) reports that in the 1970s Florida’s local

jurisdictions with sandy soils set a perc rate upper limit at 15 mpi, and in California, localities having

dense clay loams, set the upper limit at 120 mpi. He concludes that in some localities, establishing

perc rate limits has more to do with the politics of a wealthy population’s growth and development

than with soils. He suggests that an upper limit of 80 mpi is workable provided the maximum leach

9

field size from a perc rate table at 60 mpi is doubled and the two fields are periodically alternated for

use. The EPAM (1980) reports that an upper limit of 120 mpi may be used provided extreme care is

exercised during construction and the soil is well below the plastic limit. The 60 mpi limit, however,

is generally accepted as the slowest perc rate for soils to satisfactorily accept hydraulic loading from a

conventional leach field.

A review of the above literature and LLCHD fieldwork indicates that it would be reasonable for a

standardized perc test to be: #4 and #5 in instructions of results section.

10

Instructions

1. Percolation test holes. Percolation (perc) tests are made in

at least 3 and preferably 6 or more vertical 6" to 8" wide holes

spaced uniformly over site and must be to the depth of

proposed leach field trench. Test holes in wet or frozen soil

may yield results inconsistent with the same soil in a moist or

dryer and unfrozen condition.

2. Preparation of test hole. Dig or bore a smaller diameter

hole 24" deep with a power auger and peel soil off sidewall

with a hand auger to a larger diameter 6" to 8", or use a hand

auger only. Use a 1½" ridged spatula fastened to a 1"x2" board

to peel off compacted soil and level bottom of hole by inserting

blade into soil and twisting off chunks of soil. Remove

smeared soil from sidewall by scratching the lower 18" of hole

with nails driven into a 1"x2" board. Remove all loose soil

with a small bread pan by hand instead of fingers to avoid

smearing soil surface. This technique helps provide a natural

soil-to-water interface for seepage. Roll an 18" high strip of

permeable geotextile fabric into a double-walled tube and

insert into hole as a liner to prevent and hold out sidewall

collapse. Place a mesh bag of clean gravel 2" deep inside tube

and form liner against sidewall to hold it in place at bottom of

hole. A nail holds liner in place at top. This helps prevent

scouring, sediment and puddling at bottom of hole, and

migration of fines to sidewall when water is put into hole.

3. Saturation and swelling of soil (presoak). Fill holes

gently with clear water to a depth of 12" above gravel for 10

min. If water seeps away, fill to 12" again for another 10 min.

If water seeps completely away again, this indicates a sandy

coarse textured soil that is difficult to saturate and if there is

little or no clay, soil will not swell and a presoak is

unnecessary. Test can begin immediately. Proceed to 4c. If

water remains in holes after 10 min. fillings, fill holes 12" to

16" above gravel and maintain this level for at least 4 hrs. A 5

to 10 gal. bucket with siphon adds water to proper level. Fill

bucket at end of 4 hr. presoak to continue saturation and

swelling.

a. In most fine and many medium textured soils, each hole

takes less than 20 gal. in 4 hrs. A 4 hr. presoak is sufficient

and these soils usually yield slow or 60 mpi. Begin test 24

hrs (20 to 28) after start of presoak. (If it is a dry season with

soil cracks, a hole may take more than 20 gals. Proceed to 3c.)

A 4 hr. presoak is sufficient for most coarse textured soils

Results

11

because saturated conditions are usually not achieved in soils

with little clay even with longer presoak time. Each hole

usually takes much more than 20 gal. in 4 hrs. Test can begin

after water empties from bucket filled at end of presoak. Test

may begin 24 hrs. (20 to 28) after start of presoak to allow for

swelling if some clay is present. Proceed to 4b.

b. If test is conducted in a fine textured soil (and not in a dry

season) and each hole takes more than 20 gals in 4 hrs. or

yields perc rates faster than 45 mpi, conduct presoak again for

24 hrs. or longer. Soils with about 25% or more clay taking

this much water indicate unusually well developed structure

(peds or root channel pores) and longer presoak time for

swelling might reduce pore size.

c. Soils with slow permeability or containing about 25% or

more clay may require a presoak saturation of 24 hrs. or longer

for thorough swelling to shut cracks in a dry season, or use this

method for some soils to extend saturation and swelling in all

seasons. (A trailer with a 500 gal. tank and garden hoses can

supply water to WC fill valves in holes to presoak for 24 hrs.)

4. Water level drop measurement. Measurements may be

made 24 hrs. (20 to 28) after start of presoak, after water

empties from bucket filled at end of presoak, or immediately at

beginning of presoak.

a. If water remains in hole after presoak, adjust water level to

6" or less (5 ¾ to 6") above gravel. Measure drop in 30 min.

intervals from a floating reference point to nearest 1/16".

After 2 hrs. of measurements, continue measurements until

two consecutive drops do not vary more than 1/16". After

each measurement, readjust water level to not exceed 6" (5 ¾

to 6"). The last water level drop is used to calculate perc rate.

The perc rate will usually be slow or 60 mpi.

b. If no water remains in hole after presoak, gently add 6" or

less (5 ¾ to 6") of water above gravel and measure drop in 30

min. intervals as described in a. above. The perc rate will

usually be <60 mpi.

c. In sandy soils, or other soils if 6" of water seeps away in

less than 30 min., use a 10 min. interval and run test for 1 hr.

The drop during last 10 min. of 1 hr. is used to calculate perc

rate. If 6" of water seeps away in less than 10 min., use a

stopwatch to time a 1" drop from 6" to 5" six times. The 6th

time is the perc rate.

12

5. Record information. On a paper tablet draw a lined table

and number holes on the left side and record time across top in

30 min. intervals. Record measurements to nearest 1/16".

Make a diagram of hole locations, use arrows to show same

sequence of tests for each hole, and record distance between

holes. Note wetness of soil and results of soil borings. Mark

location of perc test site on soils map and identify soil type.

Record all available soil morphology for site. Note

temperature, weather, recent rainfall, drought, and date of

conducting presoak and perc test. Record any other site

factors. The perc rate is calculated by dividing time interval in

min. by inches of last water level drop (ie., 30 min. ½" = 60

mpi). If test holes have perc rates that vary by more than 20

mpi, variation in soil type or structure is indicated; these holes

should not be averaged. More test holes showing a 20 mpi or

less variation over an area of a proposed leach field may be

necessary to locate uniform soil; these holes (at least 3) can be

averaged. If a ≤ 20 mpi variation is not achieved, a detailed

soil morphology evaluation should be conducted by a soil

scientist to justify averaging perc test results. (The correct

method of averaging is to find the average inches of drop for

all holes and then calculate perc rate.) A perc rate slower than

60 mpi or faster than between 1 to 5 mpi is not acceptable for a

conventional leach field.

Discussion Most states have phased out use of only the perc test to determine the size of infiltration surface

areas for the bottom of leach field trenches of onsite wastewater treatment systems (Gross et al.,

1998; Kleiss & Hoover, 1986; NEHA, 1979). Healy and Laak (1973) conclude that the soil

percolation rate should not be used for determining hydraulic loading in leach fields. Supportive

evidence indicates that proper evaluation of soil morphology and organic loading are more reliable

methods for sizing onsite leach fields (Brown et al., 1994; Kleiss & Hoover, 1986; Tyler et al., 1991

& 1994; EPAM, 2002). However, Conta et al. (1985) references McKeague et al. (1982) indicating

that soil morphology, by itself, does not appear to be a useable method in many soils. But many

states still use the perc test to size leach fields for septic systems. During the time it takes for other

states to phase in soil morphology methods, perhaps it is possible to modify existing perc rate tables

to be more consistent with soil morphology tables similar to Table 3 (which is based in part on an

average of perc test results and soil type) in the paper by Brown et al. (1994). Barbarick et al. (1976)

found that percent sand, silt and clay in subsoil correlated significantly with perc rate. A literature

review and field experience of LLCHD makes one point abundantly clear, perc tests by themselves

must not be used as the only means of evaluating the suitability of onsite systems. Perc test results

must always be used in conjunction with soil maps, soil morphology and site factors (Gross et al.,

1998; Gustafson & Machmeier, 1995; Kleiss & Hoover, 1986; Lewis, 1975; Salvato, 1992;

EPAMs,1980 & 2002).

13

There are two primary criticisms of the perc test. One criticism is that procedures for conducting

this test are subject to errors by technicians. These errors, together with inconsistent modifications

of procedure in conducting tests, cause variable perc test results in the same soil and site (Brown et

al., 1994; Derr et al., 1969; Kaplan, 1991; Kleiss & Hoover, 1986; EPAM, 1980; Winneberger,

1984). On the other hand, Hill (1966) maintains “In the percolation test, variation is a characteristic

of the soil and not the fault of the method.” Derr et al. (1969) states that “The variability associated

with results from percolation tests at the same site and on the same soil has long been one of the

questionable facets of the test.”, and explains that “The variability of percolation test results is

influenced by a complex association of soil and environmental characteristics...variability ranged

from 0 to 253% with a 73% average...at 250 sites...” Healy and Laak (1973) conclude “The

theoretical analysis of the percolation test indicates that in fine-grained soil the percolation rate is

dependent on the size and shape of the hole, the permeability, and the ambient capillarity of the soil

at the time the test is run and that long-term soaking does not eliminate the effect of capillarity.”

Obviously, there is not a consensus on causes of perc test variability. But it would seem reasonable

that carefully following a standardized perc test procedure as described in this paper by a trained,

qualified technician would help to eliminate most of the inconsistent interpretations and

modifications associated with the perc test instructions in the PHSM. Doing so would produce more

consistent perc test results.

The second criticism is that the perc test attempts to measure saturated hydraulic conductivity

(saturated flow) rather than unsaturated hydraulic conductivity (unsaturated flow) in soil (Bouma,

1971; Miller & Wolf, 1977; Tyler et al., 1991; EPAM, 1980). Unsaturated flow is the functional

condition of soil necessary for proper treatment of liquid effluent beneath the infiltration surface at

the bottom of leach fields. Saturated flow is an undesirable soil condition that does not provide

proper treatment in septic system leach fields (Kleiss & Hoover, 1986). Interestingly, Hill (1966)

also tells us “Water flows from a percolation test hole through unsaturated soil, and its movement is

governed more by capillarity forces than by gravity.”, and EPAM (1980) tells us “The most

commonly used test is the percolation test. When run properly, the test can give an approximate

measure of the soil’s saturated hydraulic conductivity.” The question arises whether it is saturated or

unsaturated flow that is measured around the perc test hole?

Hill’s statement that the perc test measures soil’s unsaturated rather than saturated conductivity begs

an explanation. The Merrimac sandy loam and Cheshire fine sandy loam soils in Connecticut

described in Hill’s research have characteristics similar to Dickenson and Judson fine sandy loam

soils (USDA soil maps, 1977) in Nebraska. They are all relatively coarse textured soils without

much clay content. They produce fast perc rates in the range of <10 to about 25 mpi. Whereas the

silty clay loam soils of Nebraska usually produce slow perc rates of about 45 to >60 mpi. Perhaps

there is a “break-point” perc rate for coarse textured soils with fast percs <45 mpi measuring

unsaturated flow, and for fine textured soils with slow percs >45 mpi measuring saturated flow.

Preliminary field perc test results by LLCHD, Hill’s (1966) research, and EPAM (1980) table of

estimated perc rates for soil texture suggest a possible arbitrary “break-point” of about 45 mpi for

approximating the measure of unsaturated and saturated flow in soil around the perc test hole.

Fritton et al. (1986) indicates that perc test results could be converted to saturated flow values, and

Conta et al. (1985) reports that in soils with <35% clay by weight the perc test is accurate and could

14

be used reliably to predict soil hydraulic behavior. Tyler et al. (1991) explains that measurement of

unsaturated flow in the soil is generally difficult, time consuming, and expensive. They maintain

that perc tests do not directly measure unsaturated flow, but can give an average of saturated flow

around the test hole. On the other hand, Conta et al. (1985) explains that the technique of measuring

saturated flow is time consuming, expensive, complex and not suited for routine use. They maintain

that all the problems associated with perc tests exist for saturated flow measurement procedure as

well, and that saturated flow tests should not replace perc tests. It appears that more research is

needed to find out if there is a point where fast perc rates approximate a measure of unsaturated flow

and a point where slow perc rates approximate a measure of saturated flow.

Even in states that have phased out the perc test for sizing leach fields, the perc test could still be a

useful tool to help measure hydraulic conductivity in leach fields. For example, similar to municipal

wastewater systems that bypass their treatment plant during flooding, onsite septic systems may have

to “bypass” the desirable treatment condition of unsaturated flow to an undesirable condition of

saturated flow during wet periods. A standardized perc test would be useful to evaluate soil

conditions that occur during periods of high soil saturation caused by heavy rainfall. In this instance

a perc test 60 mpi should help assure that water will move away from the leach field during wet

seasons of the year. It is important to point out, especially in soils with perched water tables, that

curtain drains constructed upslope from leach fields help shorten time of soil saturation around leach

fields after a rainfall event (EPAM, 2002).

Furthermore, there are two conditions of soil morphology evaluation the perc test could verify. If

soil morphology shows a suitable soil for a leach field, the standardized perc test would most likely

also verify soil suitability with a perc rate 60 mpi. If soil morphology shows an unsuitable soil for

a leach field, the standardized perc test would most likely also verify soil unsuitability with a perc

rate >60 mpi (Bouma, 1971; Lewis 1975; NEHA, 1979). In these two instances the perc test would

be a valuable tool to verify soil morphology. In the few instances where soil morphology and perc

tests do not agree, one should in most cases give priority to soil morphology and not the perc test

(Kleiss & Hoover, 1986).

There are natural and unnatural soil conditions, and areas where the perc test should not be used to

evaluate the suitability of a conventional onsite septic system. A floodplain is an area requiring

special engineer-designed onsite systems; codes unwisely allow development on this area (Kleiss &

Hoover, 1986; Mileti, 1999). Soils in the floodplain usually have seasonal high groundwater

problems. The perc test must not be conducted if the bottom of the hole is not at least two to four

feet above a limiting condition (such as seasonal water table, bedrock or other restrictive horizons).

Conducting a perc test too close to a limiting condition might produce a 60 mpi rate and thus give a

false assurance that an onsite system will function properly. However, the leach field will most

likely not properly treat wastewater in this soil (Kleiss & Hoover, 1986; EPAMs, 1980 & 2002).

Another natural condition is frozen soil. The perc test may not yield the same results in frozen and

unfrozen soil. The unnatural soil condition of reworked medium or fine textured soil fill material

cannot be reliably evaluated by a perc test. Soil morphology, especially structure, redistribution of

texture, and layering severely alters native natural soil conditions in disturbed soil (Kleiss & Hoover,

1986). The inevitable compaction of soil by dump trucks, exposure of bare soil to rainfall causing

puddling and migration of fines into soil pores in depositing fill soil material also adversely effects

15

infiltration of liquid effluent (Miller & Wolf, 1977). The months, years or possibly decades

necessary for fill soil to return to a “natural condition” is controversial (Salvato, 1992; LLCHD field

experience). A perc test in fill soil can produce a 60 mpi rate, but this satisfactory rate can be

misleading and is not comparable to a perc test conducted in native undisturbed soil. Engineer-

designed onsite systems would be a better alternative to the construction of a conventional septic

system in disturbed soils. These examples require comprehensive site-specific information before

proceeding forward with an onsite system.

Conducting a perc test during the wet season of the year presents a dilemma. A presoak for

saturation and swelling of soil during the wet season should produce conditions of high saturation

around a perc test hole similar to continuous leach field trench operating conditions more so than a

presoak during the dry season. On the other hand, boring a hole in fine textured soil above the

plastic limit (wetness) might smear sidewalls and bottom of a hole and adversely affect movement of

water into soil around a hole. Scratching sidewalls and bottom of a hole may not be effective in wet

soil. Consequently, water in a hole in wet soil might not percolate into soil around a hole as well as

a more natural nonsmeared, less moist soil below the plastic limit. The literature is silent on this

matter except for Hill’s (1966) comment about difficulty of scarifying the sides and bottom of holes

in wet Wethersfield soil.

Gross et al. (1998) found in their study that nearly half of perc test results conducted in the dry

season were faster than perc test results conducted in the wet season at the same sites. Nine of 13

sites had slower perc rates in the wet season. However, of those nine, three perc holes showed

negative rates (water level rising or flowing into holes). A question arises whether or not all holes

were an adequate distance above seasonal groundwater: all holes were 24" deep, with one site

having groundwater within 5 ½" of the surface; two other sites had seasonal water tables above the

bottom of their holes. Perc test holes two to four feet or more above the seasonal water table should

give more reliable results than holes too close to or in the water table during the wet season. It

would seem that groundwork for condemning or confirming the reliability of the perc test ought to

be based on tests done an adequate distance above a limiting condition (Kleiss & Hoover, 1986).

Furthermore, it is important to point out that four of the 13 sites had faster perc rates in the wet

season than in the dry season, and four other sites “...gave similar soil absorption areas for the

percolation test and the soil morphology approach.” Perhaps a research study conducting a

standardized perc test in soil sites an adequate distance above a seasonal water table would produce

an even more favorable correlation between perc test and soil morphology absorption areas. Under

these conditions, the authors’ conclusion that the morphological approach is more reliable than the

perc test in sizing leach field absorption area might not be as clear. It is interesting to note that Hill’s

(1966) research found the fastest perc rates in moist soil and slowest rates in both wet and dry soil.

A famous comedian has said many times, “I can’t get no respect!” The same can be said of the soil

percolation test. We have been kicking the perc test around ever since a New York State engineer

named Henry Ryon first proposed it in the 1920s. The standard perc test procedure in the PHSM has

been in use since 1957. A reading of perc test procedures from 10 different states reveals that many

did not even follow basic instructions of having no less than 12" of water above 2" of gravel for the

presoak, and 6" of water above gravel for conducting the test. Even some researchers in references

cited did not follow standard perc test procedures. We have been comparing apples with oranges

and we need to level the playing field. Over 75 years without a standardized perc test is too long.

16

Instead of kicking out the perc test, it is time to carefully follow a standardized soil percolation test

procedure and give it the respect it deserves.

Conclusion The science and art of designing and installing an onsite wastewater treatment system is a complex,

thoughtful process. Most lay people and some professionals in the onsite industry do not realize the

complexity and importance of proper onsite system design and installation for home owners. The

U.S. EPA reports that about 25 percent of households in the U.S. are served by onsite systems. This

is a big impact on our environment.

The onsite wastewater system industry is important business affecting many people. We need as

many valuable tools as possible to get the job done right. There may be a final test of the perc test

that will finally lay the issue of a reliable perc test to rest. It would involve a two step process. First,

the perc test needs to be standardized to produce more consistent results. Second, more research is

needed to find out if the perc test can reliably approximate unsaturated flow in coarse textured soils

with fast perc rates, and reliably approximate saturated flow in fine textured soils with slow perc

rates. The soil percolation test may turn out to be a more valuable tool than ever thought possible.

The challenge is clear. Site factors integrated with use of soil maps, soil morphology, hydraulic and

organic loading, and a standardized perc test would foster a comprehensive evaluation of suitability

of soils and sizing infiltration surfaces for onsite subsurface wastewater treatment systems. The

standardized soil percolation test presented in this paper would be a valuable tool to evaluate soil for

onsite systems.

ACKNOWLEDGEMENT

The author wishes to thank the LLCHD management for their support in conducting this study.

Francis Belohlavy, Soil Scientist and Jerry Hood, REHS provided invaluable information about

soils. Rick Hoopes, P.E. provided technical assistance and references. My colleagues Bryan Hurst,

M.S., R.S., Leon Marquart, REHS and especially Phil Rooney, PhD. contributed their editing skills

to this paper. A special thanks goes to the untiring patience of Diana Lane for entering the

manuscript on the word processor and fitting the text on a single-page standardized perc test

instruction sheet, and to Joyce Endres for fitting and labeling diagrams on the instruction sheet.

Many thanks also go to Lloyd and Terri Jakoubek for providing their equipment and time to help

conduct numerous perc tests.

17

Appendix A

1. Number and location of tests.— Six or more tests shall be made in separate test

holes spaced uniformly over the proposed absorption field site.

2. Type of test hole. — Dig or bore a hole, with horizontal dimensions of from 4 to 12

inches and vertical sides to the depth of the proposed absorption trench. In order to save

time, labor, and volume of water required per test, the holes can be bored with a 4 inch

auger. (See Fig. 2, page 7.)

3. Preparation of test hole. — Carefully scratch the bottom and sides of the hole with

a knife blade or sharp-pointed instrument, in order to remove any smeared soil surface

and to provide a natural soil interface into which water may percolate. Remove all loose

material from the hole. Add 2 inches of course sand or fine gravel to protect the bottom

from scouring and sediment.

4. Saturation and swelling of the soil. — It is important to distinguish between

saturation and swelling. Saturation means that the void spaces between soil particles are

full of water. This can be accomplished in a short period of time. Swelling is caused by

intrusion of water into the individual soil particle. This is a slow process, especially in

clay-type soil, and is the reason for requiring a prolonged soaking period.

In the conduct of the test, carefully fill the hole with clear water to a minimum depth of

12 inches over the gravel. In most soils, it is necessary to refill the hole by supplying a

surplus reservoir of water, possibly by means of an automatic siphon, to keep water in the

hole for at least 4 hours and preferably overnight. Determine the percolation rate 24

hours after water is first added to the hole. This procedure is to insure that the soil is

given ample opportunity to swell and to approach the condition it will be in during the

wettest season of the year. Thus, the test will give comparable results in the same soil,

whether made in a dry or in a wet season. In sandy soils containing little or no clay, the

swelling procedure is not essential, and the test may be made a described under item 5C,

after the water from one filling of the hole has completely seeped away.

5. Percolation-rate measurement. — With the exception of sandy soils, percolation-

rate measurements shall be made on the day following the procedure described under

item 4, above.

A. If water remains in the test hole after the overnight swelling period, adjust the depth

to approximately 6 inches over the gravel. From a fixed reference point, measure the

drop in water level over a 30 minute period. This drop is used to calculate the percolation

rate.

B. If no water remains in the hole after the overnight swelling period, add clear water

to bring the depth of water in the hole to approximately 6 inches over the gravel. From a

fixed reference point, measure the drop in water level at approximately 30 minute

intervals for 4 hours, refilling 6 inches over the gravel as necessary. The drop that occurs

during the final 30 minute period is used to calculate the percolation rate. The drops

during prior periods provide information for possible modification of the procedure to

suit local circumstances.

C. In sandy soils (or other soils in which the first 6 inches of water seeps away in less

than 30 minutes, after the overnight swelling period), the time interval between

measurements shall be taken as 10 minutes and the test run for one hour. The drop that

occurs during the final 10 minutes is used to calculate the percolation rate.

Source: U.S. Public Health Service (1967). Manual of Septic-Tank Practice, No. 526.

Washington, D.C.

19

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