Determining Measurement Range and Other Important Technical Specifications for Aardvark Permeameter.

Aardvark Permeameter is a constant-head borehole permeameter. Aardvark does not use a Marriott Bubble Tower for establishing a constant head height at the bottom of borehole. Instead, it uses float valve technology. Float valve technology eliminates “bubble noise” (which is a common issue related to using Marriott Bubble Tower technology) and therefore it is expected to add to measurement accuracy. However, in soils with extremely low permeability, small amounts of leaking (from float valve), or even evaporation may distort measurement results. 

Considering significant differences between Aardvark methodology and permeameters based on Marriott Bubbler, there is a need for determining important technical specifications (e.g. measurement range, suitable reading intervals, etc) for Aardvark.

Over head pressure has a significant impact on Aardvark water supply (from reservoir into borehole). Aardvark water supply rate was measured under minimum practical overhead pressure (7 kPa or 1 psi) and maximum nominal applicable pressure (34 kPa or 5 psi). Results were used for determining Aardvark operational rage.

It is not possible to determine Aardvark leaking (from float valve) rate in soil. The reason is that there is no way for distinguishing between valve leakage and soil percolation rate. Therefore Aardvark system was installed in laboratory and in a clear pipe with 10 cm (4 inch) diameter (recommended borehole diameter for Aardvark). The pipe was completely impermeable to water. Therefore it was possible to detect very small amounts of water loss (due to leakage or evaporation) from Aardvark system. Evaporation from borehole was measured using another identical cylinder and Aardvark measurements were corrected for evaporation.

Overhead pressure above Aardvark Permeameter Unit was adjusted on 41 kPa (6 psi). It was 7 kPa (1 psi) more than maximum nominal pressure (34 kPa or 5 psi) for Aardvark to get reliable results. Aardvark software application was used to perform automated readings (every minute) for more than 20 hr. Reading procedure was repeated 15 times. Reading data was used for simulating leakage rate and evaporation rate at any time increment.

A mathematical model was created to calculate Saturated Hydraulic Conductivity (K) based on four methods of calculation and due to different scenarios. Calculation methods include Glover solution, Reynolds and Elrick solution, Radcliff and West method and Earth Manual method. For each level (order of magnitude) of K, maximum and minimum of Flow Rate were calculated. Measurement Error was calculated using Aardvark Scale accuracy and resolution and also system water loss due to leakage and evaporation.

A mathematical model was created for simulating Aardvark leakage rate at any time increment (1 minute) after opening Aardvark Reservoir valve. Since leakage Error reduces over time, Reading Start Time was defined as time duration between opening Aardvark Reservoir valve and starting “reliable” (less than 5% Measurement Error) readings (due to small Leakage Errors). Using Flow Rate, Leakage Rate and Measurement Error (less than 5%), Reading Intervals and Reading Start Time were optimized for each level of K.

Aardvark practical operational range is 10^-4 > K > 10^-9 m/s. A 1-min Reading Interval is well enough for soils with K > 10^-6 m/s. Reading Interval in the range of 10^-7 m/s is 1 to 15 min. Longer Reading Intervals (15 min to 8 hr) are needed for soils in range of 10^-8 m/s. Reading Intervals for K values in range of 10^-9 m/s are very long (8 to 24 hr). However, using Aardvark automated reading feature, it is still possible to measure these soils.

Reading start time for soils with K values bigger than 10^-5 m/s is 5 min. For soils in range of 10^-6 Start Time is between 5 to 10 min. For soils in range of 10^-7, Start Time is between 10 to 35 min. Start time for soils in range of 10^-8 m/s is 35 min to 4 hr. In extremely slow soils (K value in range of 10^-9 m/s) Start Time can vary from 4 to 48 hr.

Aardvark was capable of performing reliable and repeatable measurements automatically and for long periods of time which makes it a suitable instrument for measuring saturated hydraulic conductivity in laboratory and field condition.