Porosity, Permeability, & Water Retention Lab

*Problem: To what extent does changing one aspect (you choose) of a soil's composition affect it porosity, permeability, and water retention?

Research: Groundwater originates as surface water, melting snow, or rain that infiltrates down into the ground. Groundwater accumulates in either sediments or bedrock and flows slowly through the subsurface. A layer of sediment or rock that yields sufficient water for human use is called an aquifer. Good, producing aquifers have two distinct qualities: they are porous and permeable.

Porosity: Porosity is the percentage of a body of sediment or rock that consists of open spaces, called pores. Porosity determines the amount of water that sediments or rocks can hold. Thee are many kinds of pores – ranging from pores in sediments, to pores in volcanic rocks, to cavities in soluble rocks, to fractures in any rock. And many of these pores can be filled to differing degrees by cement. The porosity is expressed as a percentage of the total volume of material:

% Porosity = Volume of pore space in ml / total volume of sediment in ml x 100

%Water Retention = ((Vol H2O ml soil holds - Volume H2O ml poured out of soil) / Vol H2O ml soil holds) x 100

Permeability: Permeability is the ability of soil, sediment, or rock to transmit fluid. Material with low porosity is likely to have low permeability as well, but porosity does not necessarily mean high permeability. In order for pores to contribute to permeability, they must be (a) interconnected, and (b) not so small that they restrict flow. For example, clay commonly has high porosity, but clay grains are so broad in proportion to their microscopic size that the molecular force between clay particles and water restricts the flow. The permeability of aquifers is usually expressed as a velocity in meters/day.

(Permeability = distance traveled in cm / elapsed time in sec).

Description of your soils: Observe your soil for the following qualities. Color, darkness, particle size, mix of particle sizes. To measure particle size composition, separate your soil into piles by particle size using the classroom sifter.

*Hypotheses: Please make your H1 hypothesis. You really don't need a null hypothesis because we are not going to do statistics on this data.

Porosity

A) H1:

Water Retention

B) H1:

Permeability

C) H1:

**Experiment:

**Materials: list all materials you are instructed or choose to use

**Procedure: Design and write procedures to test your 3 hypotheses. Watch your instructor show you how to measure porosity and then write a porosity procedure. Then watch your instructor measure permeability, then you write a permeability procedure. Do the same for % water retention.

**Data:

Qualitative Soil Data:

Soil
Color
Darkness

Relative Particle Size %s

Grav CSand EFSand FClay UFSiltClay
1
    
         
2
    
         

Quantitative Soil Test Data (from "rapitest soil test kits"):

pH
N
P
K
       

 

Quantitative Porosity & Retention Data Table:

Soil
(A)
Volume (ml)
of water
(B)
Volume (ml)
of Sediment
(A / B)* 100
% Porosity
(C)
Volume (ml) of
water drained
from soil
% Water Retained
((A - C) / A) * 100
1
          
2
          

Quantitative Porosity & Retention Data Table:

Soil
A
Height of Soil
Column (cm)
B
Time elapsed from pouring
until reaching the Bottom (sec)
A / B
Permeability
(cm / sec)
1
      
2
      

**Graphs: Please make three separate bar graphs: 1) two soils % porosity 2) % water retention & 3) permeabilities

Discussion: Please writeup a formal discussion of your results (all 5 paragraphs).

* write this section both in your lab notebook and formal lab writeup
** write this section only in your lab notebook