8.4 How to determine infiltration and perform a percolation test?

A soil percolation test helps to determine the suitability of the site and establish the design size of the system. The length of time required to carry out such a test varies with different types of soil.

It is essential to make the tests exactly where the infiltration trenches will be dug and at the correct depth, which is the future level of the systems’ bottom. At least 3 measurements must be carried out. In addition, environmental observations and soil samplings should be conducted. The in-situ percolation test, with variable head, measures infiltration speed recognised as the most appropriate for sizing infiltration systems for on-site wastewater disposal systems. The test must be carried out according to a defined experimental protocol in order to provide values close to real operating conditions.

The following steps describe the procedure for a standard percolation test:

1. Dig a hole with a diameter of 15 cm to the recommended depth for the bottom of the infiltration system/trench (typically 0.8 m).
   The hole should be cylindrical with a minimum height of 30 cm in the designated soil absorption area.
   
2. Scratch the walls and the bottom of the hole to restore the natural soil texture.
   
3. Remove any excess soil.
   
4. Place a 5 cm layer of fine gravel (diameter of 1.2 – 1.8 cm) at the bottom of the hole.
   
5. Fill the hole with clear water to a minimum height of 30 cm.
   
6. Allow the soil to pre-saturate for at least 4 hours, preferably overnight. The water should be clear and free from organic substances or high sodium content.
   
7. Perform the measurement:
   
   • If water remains in the hole after the pre-saturation period, adjust the water depth to 15 cm. Then, measure the water level drop every 30 minutes. Continue the test until the last reading is the same as the previous one or after 4 hours.
     
   • If no water is left in the hole after the pre-saturation period, add 15 cm of water. Then, measure the water level drop at 30-minute intervals and adjust the water height to 15 cm by adding the missing water. Continue the test until the last reading is the same as the previous one or after 4 hours.
     
   • If the 15 cm of added water disappears before the 30-minute interval has elapsed, in this case, the time interval between measurements should be 10 minutes.
     
     
8. Calculations:
   
   Percolation Rate (min/cm) = Time (min) / Lowering in Water Level (cm)
   
   
9. At least 2, preferably 3, percolation tests are recommended to be conducted. One at each end of the proposed infiltration system location and at the installation depth of the system. They should be conducted under normal weather conditions, without rain or freezing.

A common misunderstanding is that the percolation rate obtained from a percolation test is equivalent to the infiltration rate. These terms have different meanings. Infiltration rate describes the velocity at which water enters the soil, expressed commonly in mm per hour. The percolation rate, measured in minutes per cm, reflects how quickly water infiltrates the soil. In this sense, the percolation test provides valuable information about the soil’s permeability but does not directly relate to the system’s daily capacity to absorb wastewater.

The direct measurement yielded by a percolation test tends to overestimate the infiltration rate (except in cases in which the testing procedure applies similar dimensionality to the testing hole). The so-called Porchet Method (Inverse Borehole Method) can be used to determine the infiltration rate:

Where,

Parameter	Description	Unit
I	Infiltration rate	mm/h
ΔH	Height difference	mm
r	Radius of test hole	mm
Δt	Time interval	min
Havg	Average head height over the time interval	mm

Where,

Parameter	Description	Unit
DT	Total depth of test hole	mm
D0	Initial depth to water	mm
DF	Final depth to water	mm

The percolation test measures nothing but a gross relative permeability and only provides an approximation of a soil’s percolating capacity. When sizing an infiltration system, other factors such as design flow, soil texture and effluent strength need to be considered. The following table lists estimated acceptable percolation and infiltration rates for different soil textures and a corresponding effluent infiltration rate approximate for effluent originating from households. A safety factor of 1.5 or more is recommended for effluents of other origins.

Soil Texture	Percolation Rate (min/cm)1	Infiltration Rate (mm/h = l/m2/h)1	Effluent Infiltration Rate (l/m2/d)2
Gravel, Coarse Sand	<0.4	>1,440	Not desirable
Sand	0.4 – 1.6	1,440 – 360	50
Fine Sand	1.6 – 2.1	360 – 288	42
Silty Sand	2.4 – 5.6	252 – 108	32
Sandy Silt	5.6 – 16.7	108 – 36	25
Clayey Silt	16.7 – 23.8	36 – 25	19
Sandy Clay	18.5 – 23.8	32 – 25	12.5
Silty Clay	23.8 – 47.2	25 – 14	8.5
Clay	>47.2	<14	Not desirable

Suppose the infiltration rate is above 1440 mm/h or below 14 mm/h. In that case, the infiltration is not recommended because of too quick infiltration (and the increased risk for groundwater contamination) or too slow infiltration (needing extensive infiltration areas).

Side Note: Methods to Determine Infiltration Rate

While percolation tests are primarily used in designing on-site wastewater disposal systems, infiltration tests are common for stormwater management to help reduce flooding, control erosion and improve water quality.

In this sense, infiltration tests assess how quickly water penetrates the ground’s surface, typically at or near the ground’s surface.

The most common method to determine the infiltration rate is a field test using a cylinder or ring infiltrometer. The following schema shows the single- and double-ring infiltrometer that can be used.

Exercise:

The bottom of a proposed infiltration basin would be 1.5 m below the natural grade. Percolation tests are performed within the boundaries of the proposed basin location, with the depth of the test hole set at the infiltration surface level (bottom of the basin). The percolation test data sheet is prepared as the test is being performed. After the minimum required number of testing intervals, the test is complete. The following data was collected at the final interval:

Parameter	Description	Value	Unit
∆T	Time interval	10	minutes
DT	Total depth of test hole	150	cm
D0	Initial depth to water	30	cm
DF	Final depth to water	35	cm
r	Radius of test hole	10	cm

Determine the infiltration rate and investigate if the soil texture is suitable for infiltration.

Solution:

Step 1: Determine the change in height over the time interval.

Step 2: Determine the average head height over the time interval.

Step 3: Determine the tested infiltration rate.

An infiltration rate of 1.22 cm/h or 12.2 mm/h is relatively low and may be considered insufficient for effluent infiltration.