Lab 2
Bulk Properties of Water and Movement of Water Through Soils - Part 1
OBJECTIVES
In this laboratory, you will be getting hands-on experience with the measurement of
water in soil. You will learn soil bulk density calculatations, different expressions of water
content, and behavior of saturated and unsaturated flow of water through soil.
INTRODUCTION
Water flows from greater potential to lesser potential. In soils, water is usually at
negative potentials and moves from areas of less negative matric potentials to areas of more
negative matric potentials. In other words, water generally flows from a zone which has more
water to an zone that has less water (assuming homogenous texture). This can be considered as
nature’s striving to reach an equilibrium condition, although in reality, equilibrium is rarely
achieved. Water potentials can be 0 or greater in cases of saturated soil or ponded water on the
soil, respectively, and the water moves downward (if possible) in response to gravity, lowering
its potential.
Unsaturated water movement in the soil is the result of capillary forces that cause water
to move toward areas where the water will have a lower free energy (i.e. water moves toward
areas where the soil particles will hold it with the greatest degree of tension). A capillary is any
small tube or pore. We can measure the strength of capillary forces by observing the height to
which pores will draw water above a free-water surface. Capillary movement is the result of two
forces which have components acting in the same direction; namely (1) adhesion - attraction
between the water molecules and the pore material (glass wall of tube or soil particle surface)
and (2) cohesion - the attraction of water molecules for one another. A special case of cohesive
forces plays a major role. This is the surface tension caused by the unbalanced forces of
cohesion at the liquid-air interface. As adhesion attracts the water to the capillary walls, surface
tension acts like a rubber membrane to “pull-up” the body of water. The upward movement
continues until the capillary forces are balanced by the weight of the column of water drawn up.
The capillary forces and hence the height of rise are affected by the radius of the pore and the
surface tension of the liquid. This relationship is expressed as follows:
h = 2γ cosα / rρg
(1)
Where :
h = maximum height of rise (m)
γ = surface tension (0.073 N/m for water)
α = angle between the liquid and solid surfaces
ρ = water density (1.0 Mg/m3 or 1000 kg/m3)
g = gravity (10 m/s2)
r = radius (cm) of the capillary pore (not of the soil particle).
For water under normal conditions in glass capillary tubes, the above formula can be simplified
to:
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h = 0.15 /r [when h is expressed in centimeters]
(2)
When a series of glass tubes are placed in water, water will rise to a height determined by
the radius of the tube: higher in tubes of smaller radius than in tubes of larger radius. Also
the flow rate, Q, (expressed in m3/s) is directly related to the to the 4th power of the radius (Q ∝
r4), implying that water will move MUCH slower in tubes of smaller radius than in tubes of
larger radius.
In soil, pores are assumed to roughly behave as a series of different size capillary tubes.
Certainly soil pores are not uniform and straight like glass capillary tubes, however the basic
principle is the same. The interconnectiveness (network) of pores also distinguishes soil.
Soils of different texture have dissimilar pore size distribution and therefore varying
ability to cause capillary rise. Finer textured soils have smaller pores on average, therefore, water
will rise to greater heights in these soils. In addition to the small volume of transmission and the
adhesive force of pore walls, the tortuosity of unsaturated soil pathways also offers greater
resistance to the movement of water.
Practical applications of water movement in unsaturated soil:
Water Supply to Plants - To a certain extent, finer soils should be able to "wick"
water from a water table closer to the plant rooting depth so that plants may get to it.
Whether that water is actually available depends on the tension at which it is held.
Water Table - Finer textured soils over a water table may cause wet soil (“capillary
fringe”) closer to the soil surface than in coarser textured soils.
Arid Soils - Water containing dissolved salts are “wicked” to the surface by capillary
rise. The hot, dry climate evaporates the water, leaving behind the salt. Such
phenomena may lead to salt-affected soils.
Hydraulic Conductivity (K) can be defined as the capacity of the soil to conduct water
in response to potential gradients and is quantified by measuring rate of water movement. In the
capillary rise experiment, you observed water at its equilibrium point after water wetted the soil
from below and moved upward into dry soil having more negative potentials, but you did not get
to observe the rate of water movement. Rates of water movement in unsaturated soil values, are
difficult to determine for defined conditions.
Most K's that are reported are saturated K (Ks) values. It is relatively easy to establish a
constant head (positive water potential) apparatus and measure gravitational water. Saturated
hydraulic conductivity is related to both texture and structure because of their effect on pore-size
distribution. Again, remember that in filled tubes, flow rate is proportional to the 4th power of the
radius (Q ∝ r4), which implies that K is proportional to the square of the radius (K ∝ r2). Coarse
textured soils have larger pores and offer less resistance to water flow; such soils have a higher
Ks value. Finer textured soils generally have smaller pores and more tortuous paths, and so Ks is
smaller (even though total porosity is usually greater). Stable granular structure in soil
contributes to greater Ks than unstable structural units because pores between aggregates are
generally larger than pores within the matrix.
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The vertical flow of water in a saturated porous medium can be described by Darcy’s
Law:
Q = V/T= A Ks [H /L]
Where:
(3)
Q = flow rate (cm3/s)
V = volume of water in cm3 (ml)
T = time in s.
A = area perpendicular to flow
Ks = saturated hydraulic conductivity in cm/s
H = hydraulic head in cm
L = depth of soil in cm
The hydraulic conductivity accounts for the properties of the soil that affect water
transport and yet are difficult to assess individually. The other factors in Darcy’s equation are
controlled and/or measured, allowing us to calculate Ks. Notice that flow rate (Q) is proportional
to the hydraulic head (potential gradient), the cross-sectional area of soil sample, time, and
hydraulic conductivity, and inversely proportional to the depth of soil.
The water that drains from soil is considered gravitational water, in excess of the
retention capacity of the soil, (“field capacity”), and flows primarily through macropores. Such
water drains from the soil profile because the downward pull of gravity on the water mass in
these pores is greater than the adhesive/cohesive forces. In contrast, matric potentials exerted by
micropores can overcome the downward pull of gravity.
There are techniques to measure saturated hydraulic conductivity in the field and in the
laboratory. There are advantages and disadvantages to each. Field soils have intact structure,
particles > 2mm, stones, roots, etc. which presents the most realistic situation, but may lack in
complete saturation, control of lateral flow, etc. More controlled conditions in the laboratory
(should) allow more precise measurements, but the soil structure may be disturbed and samples
are less representative of reality. Hydraulic conductivity measured in the laboratory will often
differ from that measured in the field.
Practical Applications of soil hydraulic conductivity:
Landfill clay caps – should have Ks values less than 1x10-6 cm/s. A very slow Ks is
desired in this situation because air and water contact with the buried material is to be
minimized.
Landfill clay liners – should have Ks values less than 1x10-6 cm/s. A very slow Ks is
desired in this situation to prevent movement of waste leachates to groundwater.
Septic system leach fields - Optimum Ks values should fall in the range of 8.5x10-28.5x10-3 cm/s. Too slow, liquid may pond on the surface and anaerobic conditions
prevent proper biotic processing of waste. Too fast, proper filtration is not achieved.
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Procedure
Glass Tubes
1.) Measure the height of rise of the liquid in each of the capillary tubes. Measure up from the
surface of the liquid. Record your measurements.
2.) Calculate the inside diameter of the glass tubes using; h = 0.15/r.
Sand Columns
1.) Measure the height of rise of liquid in each of the sand columns (from the water surface up).
The sand should appear darker in color where the water has risen. Record your measurements.
2.) Calculate the effective pore diameter of the sand in the columns using h = 0.15/r.
Results
Glass Tubes
Height of rise
Inside diameter
Tube #1
Tube #2
Tube #3
Sand Columns
Particle size
0.5 - 1.0 mm
0.25 - 0.5 mm
0.10 - 0.25 mm
< 0.1 mm
Adelphia soil (particle size
not provided)
cm
cm
cm
Height of rise
cm
cm
cm
Effective pore diameter
13
cm
cm
cm
cm
cm
cm
cm
cm
cm
cm
Saturated Hydraulic Conductivity
Perform this exercise for 2 soils.
1) Measure the height (H) and diameter (D) of the cylinder
2) Place a filter paper in the cylinder and wet the filter
paper.
3) Add approximately 150 g of soil and tap to distribute
evenly. Measure soil height to get L. Cover soil with
filter fiber to prevent displacement of soil.
4) Fill the cylinder to the lip with tap water and allow to
drip for 15 min. Keep the cylinder filled to the lip to
maintain constant H. Discard water collected after the
first 15 minutes. (Note: the first 15 min is needed to
saturate soil). You can begin setting up for the second
soil now.
5) Collect the water that passes through the soil for 2 more,
separate, 15 min. periods.
6) Measure the volume of water in a graduated cylinder to get V.
7) Calculate the hydraulic conductivity using an average of the two water flow measurements
(V1 & V2). Rearrange Darcy's equation (Eq. 3) and solve for Ks.
DATA:
Soil 1 Designation:______________
H = ____________cm,
D = _____________ cm,
S = ____________cm
V1 = ____________ cm3
T1 = _____________s
V2 = ____________ cm3
T2 = _____________s
Avg. V = ____________ cm3
Avg.T = ____________s
K = ______________cm/s
Soil 2 Designation:______________
H = ____________cm,
D = _____________ cm,
S = ____________cm
V1 = ____________ cm3
T1 = _____________s
V2 = ____________ cm3
T2 = _____________s
Avg. V = ____________ cm3
Avg.T = ____________s
K = ______________cm/s
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Measurement and Calculation of Soil Water Content
1) Obtain a soil core ring and measure its volume by closing one end with plastic wrap and a rubber
band. Fill a graduated cylinder to 100 mL with water. Pour water from the graduated cylinder to
fill the core. Calculate the volume of the core by subtraction. Record the volume of the core
below. (Note: at room temperature, 1 mL of water occupies one cubic centimeter of space).
Compare to the volume calculated by V = πr2h, where r is half of the inside diameter and h is
height.
2) Empty water from ring, remove the plastic wrap and rubber band, dry off the core ring, and
cover one end with filter paper and 2-3 layers of cheesecloth held in place with a rubber band.
Make sure rubber band is strong and securely in place. Weigh the ring + cheesecloth.
3) To this ring, add enough dry, sieved (< 2 mm) soil to fill ring to top. Allow soil to settle
naturally. Tamp the soil LIGHTLY to make sure there are no large crevices or voids. Record
weight of ring and soil.
4) Place soil core in a pan and slowly fill the pan with water almost level to the upper rim of the
ring. By wetting from the bottom, we hope to minimize entrapment of air in the pores. The soil
will quickly saturate with water. You may add additional soil to compensate for settling, but
keep track of weight of soil added. (Start with known weight of soil + beaker; use this soil to add
necessary amount to ring; reweigh beaker; subtract.) Allow core to equilibrate with water while
observing a demonstration.
5) Place your soil core in the oven (105 °C). Allow it to dry to constant weight. This will take 24-48
hours. Make arrangements to come back to weigh your dry soil core.
6) Using your results and the list of formulas following, calculate θ at field capacity (both by
volume and by weight), bulk density, and % pore space of your soil. Refer to example problems
if necessary.
Data:
Soil 1:
Measured Volume of core:
_______ cm3
Weight of ring + cheesecloth (tare):
Weight of tare and soil:
Weight of soil (air dry):
Weight of wet soil and core:
Weight of wet soil:
Weight of gravitational water:
Weight of dry soil and core:
Weight of dry soil:
_______ g
_______ g
_______ g
_______ g
_______ g
_______ g
_______ g
_______ g
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Calculated Volume of core: _______ cm3
Weight of additional soil:_______ g
Soil 2:
Measured Volume of core:
_______ cm3
Weight of ring + cheesecloth (tare): _______ g
Weight of tare and soil:
_______ g
Weight of soil (air dry):
_______ g
Weight of wet soil and core:
_______ g
Weight of wet soil:
_______ g
Weight of gravitational water:
_______ g
Weight of dry soil and core:
_______ g
Weight of dry soil:
_______ g
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Calculated Volume of core: _______ cm3
Weight of additional soil:_______ g
This diagram shows a general relationship
between soil permeability and soil texture.
The texture of the least permeable horizon
must be considered. Remember, however,k
that soil structure is also very important in
determining permeability.
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