Methods Used to Determine Hydraulic Conductivity By Josh

Methods Used to Determine Hydraulic Conductivity By Josh Linard

Background • Hydraulic Conductivity, K, is essential to understanding flow through soils. – Darcy’s Law – Richards’ Equation – Advection-Dispersion-Equation • Soil characteristics that determine K – Particle size – Porosity – Bulk density

More about K • K is a function of pressure or moisture content – low matric potential = high moisture content = high K • Want to know either – Saturated hydraulic conductivity, Ks, or – Unsaturated hydraulic conductivity, K.

Other considerations • What should the sample size be? • Where to conduct experiment? • How is the water applied? • Sample size – Contemporary soil core devices. – Representative Elementary Volume (REV).

Experiment location • Field – Advantages • Soil is undisturbed. – Disadvantages • Can’t control the environment. • Logistics. • Laboratory – Advantages • Highly controlled environment. – Disadvantages • Sample can be aggravated during transport. • Facilities

Water Application • Ideally, the soil should be wetted from the bottom up. • Should use a deaerated 0. 005 M Ca. SO 4 solution to limit air retention. • What volume of water is required and what volume is available.

Determining Ks • Laboratory Methods – Constant head – Falling head • Field Methods – Test basins • Note: for each method…. – good contact must be made at the lateral boundaries of the core. – Evaporation must be measured.

Constant Head Method • Wet the column from the bottom up. – Can be a problem depending on sample size. • Add water until it’s at the desired height. – Hydraulic gradient = 1 (Figure 10. 1 a) – Macropore collapse? Need a different gradient. – (Figure 10. 1 b) • Capture the outflow, when it’s rate becomes constant Ks is obtained.

Constant Head Apparatus

• L is length through the soil • y is the height of ponded water • x is the height of water required to lower the gradient so that y can be maintained. • Note: if the gradient is 1 then Ks = q as per Darcy’s Law.

Falling Head Method • Wet the column from the bottom up. • Fill a burette to above the height of the soil column and allow it to drain. • Drain until the rate of head loss is constant. • (Figure 11. 1)

Falling Head Apparatus

• a is the cross-sectional area of the burette • A is the cross-sectional area of the soil column • t 2 – t 1 is the time required for the head to drop from H 1 to H 2.

Test Basin Method • Isolate a column of soil – Usually much larger than a core to be used in the laboratory. • • Seal the lateral faces of the column Ensure the column is saturated Apply a constant head of water at rate P. Obtain Ks using a mass balance approach: I = P - E where, Ks is equal to I since the soil is saturated.

Ks Method Summary • The constant head method is used for soil with a high Ks (> 0. 001 cm/s). • The falling head method is used for soils with lower Ks (10 -3 - 10 -6 cm/s). • Laboratory experiments can obtain Ks in each dimension.

Determining Unsaturated K • Field methods – Ring infiltrometer. • Laboratory methods – Instantaneous profile method. • Note: ensure that all instruments make good contact with soil.

Ring Infiltrometer • Used either in the field or laboratory. • Can use either one or two rings. – Scale dependent on ring size. – 2 rings allows vertical K to be isolated. • Can measure K when the matric potential, ym, is > 0. – When ym is 0 a surface crust of a known potential can be used.

Ring Infiltrometer Method • Isolate soil column as in other methods. • Place the infiltrometer on the soil, ensuring good contact. • Water is ponded on the soil and the infiltration rate recorded. • Unsaturated K is determined using the Richards’ equation.

Ring Infiltrometer Water Supply Double Ring

Instantaneous Profile Method • Uses tensiometers and gamma ray absorption to measure matric potential, f, and moisture content, q, respectively. • Pond water until the outflow is constant and then start the experiment when the last of the water has entered the soil. • K is obtained using

Instantaneous Profile Method Tensiometers Gamma Ray Detector TDR’s Gamma Ray Emitter

Unsaturated K Method Summary • Ring infiltrometer – Different sample sizes require different rings and sometimes infiltrometers. – Water can be hard to provide depending on the sample size. – Have to ensure good contact with soil. • Instantaneous profile method – Expensive to operate and hard to set up. – Have to ensure good contact with the soil.

Conclusion/Recommendations • Methods described allow for determining K in most settings. • It’s hard to account for macropore flow. • There is no method for determining horizontal K in situ. • Scales of measure are subject to criticism.