Grain Size Distribution Analysis
METHODOLOGY & PROCEDURES FOR DETERMINING SEDIMENT GRAIN SIZE DISTRIBUTION

PURPOSE
To provide an overview of the principles used to determine sediment grain size distribution and to detail the procedures used in a grain size distribution analysis of sediments obtained from the Rio Panuco basin of east-central Mexico.  Specifically, a wet sieve technique combined with a hydrometer analysis will be examined.
The sediment samples that were processed using the procedures described below were obtained from the channel and floodplain of the Rio Panuco and its major tributaries.  Field research trips to the region were led by Paul F. Hudson in the summers of '99 and '00 and the winter of '99.  Paul F. Hudson and his graduate students seek to understand fluvial geomorphic processes (i.e. floodplain sedimentation rates, flood variability) and the paleohydrology of this unique system that spans a number of climatic regimes, from dry-temperate in the west to moist-tropical at the foot of the Sierra Madre Oriental.

MAP OF RIO PANUCO BASIN

Figure 1 : Aerial Photograph of the floodplains of the Rio Moctezuma and Rio Panuco

Figure 2 : Rio Moctezuma at El Higo

• THEORY OF PRACTICE
A grain size distribution analysis provides the researcher with the percentages of certain grain size classes (i.e. fine sand, silt, clay) within a given sediment sample.  The silt/clay fraction is determined through the use of a hydrometer, while classes of sand are separated with sieves.
The hydrometer method is a type of mechanical analysis that is based on the specific gravity of a sediment/fluid mixture.  Because the specific gravity of water fluctuates with temperature and the hydrometer is calibrated to read 1.00 at 20º C, a control temperature and specific gravity must be maintained throughout the analysis.  The equation used to measure settling rates of differently sized particles is :
n = 2r2g(rs-rl)/(9h)
where:
n    =     velocity of fall
g    =    acceleration due to gravity
rs    =     particle density
rl    =     liquid density
h     =     fluid viscosity

This equation is referred to as Stokes' Law.  Stokes' Law makes a number of assumptions.  They include:
-  terminal velocity is attained as settling begins
-  settling and resistance are due to fluid viscosity
-  particles are smooth and spherical (which is not the case with natural sediment grains)
-  no interaction between grains in solution
Although some of these assumptions are not always met in natural conditions, Stokes' Law still provides an accurate base with which a hydrometer analysis can be performed.

• EQUIPMENT NEEDED
- 400 mL beakers
- 600 mL beakers
- 1000 mL beakers
- squeeze bottle
- digital scale
- small pestle and mortar
- spoon
- drying oven
- milk shake mixer w/ containers
- sodium hexametaphosphate ((NaPO3)6) in sealed flask
- distilled water
- sink
- hydrometer
- Bouyoucos tube
- synthetic stopper (that fits Bouyoucos tube orifice)
- thermometer
- timer (watch) w/ seconds
- data sheets
- sieves : U.S. Standard Mesh #18
U.S. Standard Mesh #35
U.S. Standard Mesh #60
U.S. Standard Mesh #120
U.S. Standard Mesh #230

Figure 3 : Hydrometer                                                                                            Figure 4 : Stack of U.S. Standard Sieves

Hydrometer and wet seive procedures

Figure 5 : Geo-archaeology and Applied Geomorphology Lab : UT Dept. of Geography

(Each of the samples should have its own data sheet to record all measurements.)
Example of Data Sheet

Sample Preparation
1.  Pour ~ 100g of sediment from the sample bag into a 400mL beaker and label beaker with the sediment number or naming convention.
2.  Allow ~100g sample to dry in oven at 105º C or air dry over the course of a couple days.  Complete dryness is necessary for an accurate mass (explained
in Step 5).
3.  Using a small pestle and mortar, thoroughly grind dried sediment till all large particle clumps are separated into individual grains.
4.  Using a digital scale, weigh one 600mL beaker for each sample.  Make sure the beaker is completely dry and labeled to ensure the correct sediment
is placed in this beaker.
5.  Using a spoon and being careful to spill sediment only into beaker, pour exactly 50g of sediment out of the mortar and pestle into the beaker.  The beaker
should be resting on the digital scale during this step and care must be given to not spill excess sediment onto the scale itself, ...only in the beaker.  The
remainder of the sediment can be added back to the original sample bag, if desired.
6.  Add 250mL of distilled water to the 50g of sediment.
7.  Add 100mL of sodium hexametaphosphate ((NaPO3)6) to the sediment and distilled water.  This chemical is a deflocculating agent, which aids in the
separation of clays into individual particles.
8.  Using a spoon, stir mixture thoroughly.
9.  Pour sample into a milk shake container.  Use a squeeze bottle of distilled to ensure all sediment is transferred from the 600mL beaker to the container.
(From now on, use the squeeze bottle of distilled water during each transfer to ensure all sediment is accounted for.)  Mix with a milk shake mixer on low
power for 5-7 minutes.

Figure 6 : Milk Shake (Sediment) Mixer

10.  Pour back into 600 mL beaker (or 1000mL beaker if more room is needed).  Again, make sure the beaker is labeled correctly.  Let sample sit overnight
to allow for chemical deflocculation.

Hydrometer Analysis
1.  Transfer sample from 600mL beaker to a 1000mL Bouyoucos tube.  Fill with distilled water to the fill line.  The top of the meniscus should reach the fill
line.
2.  Fill another Bouyoucos tube with only distilled water.  Insert a thermometer into the water and record the control temperature.  Using the hydrometer
that will be used for the analysis, place it (large end first) into the tube.  Allow the hydrometer to stabilize and record the specific gravity.  The
measurement should be read at the top of the meniscus.  This is the control measurement and it should be taken periodically throughout the hydrometer
analysis.
3.  Have a watch with seconds and the hydrometer close at hand.
4.  Use a synthetic stopper to cap off the Bouyoucos tube with the sediment sample. Turn the tube upside down and again right side up, using both hands at
each end of the tube and continue this for 1 minute.  This ensures complete mixing of the sediment sample.
5.  As soon as the Bouyoucos tube is placed on the table, the watch should be read and all time intervals at which a hydrometer reading is
required will be extended from this time.  It is a good idea to make sure the tube is placed down at a hh:mm:00 time (i.e. 3:45:00).
6.  At each measurement time, the hydrometer should be slowly placed into and slowly removed from the solution.  The hydrometer should be read at the
following times after the tube is set down :
45sec, 1min, 90sec, 3min, 5min, 10min, 15min, 30min, 45min, 1hr, 2hr, 4hr, 6hr, 1day, 2day
Once again, control temperatures and specific gravity readings should be periodically read during the analysis, especially as the time intervals are spread
further apart.

Figure 7 : Bouyoucos tubes with sediment (Notice control tube to right)

Figure 8 : Insert and remove hydrometer slowly                                                     Figure 9 : Top portion of hydrometer stem exposed in Bouyoucos tube

Wet Sieve Analysis
1.  Obtain 5 sieves of the following mesh #'s:
U.S. Standard Mesh #18   (0 phi) ; very coarse sand
U.S. Standard Mesh #35   (1 phi) ; coarse sand
U.S. Standard Mesh #60   (2 phi) ; medium sand
U.S. Standard Mesh #120 (3 phi) ; fine sand
U.S. Standard Mesh #230 (4 phi) ; very fine sand
U.S. Standard Sieve Sizes
Wentworth Particle Size Classes
2.  Stack sieves on top of one another in a sink (#18 on top and #230 on bottom)
3.  Pour contents of Bouyoucos tube into the sieves.  Again, make sure all sediment is accounted for.  The silt and clay will pass through the bottom sieve.
4.  Turn on faucet (no longer distilled water) and gently press and glide fingers across top sieve while water is running into sieve.  This ensures that all grain
sizes smaller than the mesh will be able to pass through.  When complete, remove top sieve.
5.  Obtain and label 400mL beaker.  Label should include sample number and sieve number.  Weigh dry beaker on digital scale.  Remember to record
this mass on the data sheet.
6.  Using a squeeze bottle of tap water, remove all sediment from top sieve into the pre-weighed 400mL beaker.  Be thorough.
7.  Repeat steps 4-6 for the remaining sieves.
8.  Place all 400mL beakers with sediment into an oven and dry at 105º C till all water is gone.
9.  Weigh dry beakers with sediment on a digital scale.  The difference in mass is the mass of the sediment.

Figure 10 : Stack of sieves with sediment ready for                                                              Figure 11 : Using a squeeze bottle to wash sediment into beaker
wet sieve analysis

• SOURCES OF ERROR
Sources of error exist throughout the procedures outlined above.  These sources of error are described below:
1.  Loss of sediment mass during the procedure.  This can be observed after using the milk shake mixer (sediment left on the shaft and blade) or
on the stopper after turning over the Bouyoucos tube.  Some grains are also lost if wedged in the mesh of sieves.
2.  Inaccurate hydrometer reading.  This can especially occur immediately after the tube is placed down and foam or froth hides the top of the
solution's meniscus.  Also, it is necessary to insert and remove the hydrometer slowly in order to minimize disturbance of the sediment settling processes.
3.  Evaporation.  Water will evaporate out of the Bouyoucos tubes over the period of a day or two.  Evaporation will serve to increase water density,
and will therefore underestimate sediment settling and overestimate the hydrometer reading.

• GRAPHICAL REPRESENTATION

• CONCLUSIONS