Background

Grain size distribution is one of the most important characteristics of sediment. This is true because grain size is a powerful tool for describing a site's geomorphic setting, interpreting the geomorphic significance of fluid dynamics in the natural environment, and distinguishing local versus regional sediment transport mechanisms as well as because grain size is a dominant controlling factor in sediment geochemistry. Cations derived from mineral weathering and pollution sources are preferentially adsorbed onto clay, which has the highest surface area to volume ratio of any particle size class.

The grain size distribution of a sediment sample is determined by any one of a number of techniques or combination of techniques depending on the range of sizes present in a sample. For this procedure, "fines" are any particle smaller than 63.5 microns. Sand is 63.5 microns to 2 mm in diameter. Gravel is larger than 2 mm.

Purpose

To determine the grain size distribution of a sediment sample..

Universal Materials

All methods described below require the following materials.

0.5 % Sodium Hexametaphosphate (SHMP), which is a dispersant that helps prevent particles from aggregating. To make this, first make a STOCK solution of 5 % SHMP that will last you a while. To do this, measure out 50 g of SHMP on a mass balance and pour it into a 1 L bottle (that is cleared marked with a black line at the 1 L level) that already contains 500 ml of distilled water. Cap the bottle tightly and shake for one minute to aid dissolution of the powder. Add distilled water to bring the total volume up to 1 L. Shake again and leave over night. To generate the 0.5 % solution from the 5 % stock solution, pour 100 ml of the stock solution into a 1 L bottle and then fill the bottle up to the 1 L line with distilled water.

Procedure
Method 1: Separating sizes in samples that are predominantly gravel and sand

Materials: Mass balance with (0.001g resolution), drying oven, 63.5-micron sieve (#230 mesh), 0.35 mm sieve (#45 mesh), 2-L plastic bottle with a black line at the 1 L level, large plastic funnel, 0.5 % SHMP, squeeze bottle with 0.5 % SHMP, distilled water.

1. Estimate largest size class (sand or gravel). If gravel is present, homogenize sample and subsample ~500 g. If no gravel is present, homogenize sample and subsample ~100 g.
2. Place subsample into plastic or tin can.
3. Add 0.5% SHMP until sediment is under a few inches of fluid.
4. Mix and wait for ~5 minutes.
5. Decant fluid through the 0.35 mm and 63.5-micron sieves into a pan. The larger sieve retains large clasts and helps you remove macro organics like twigs and roots. ·Repeat until fluid runs clear.
6. Carefully empty bottom pan periodically through funnel and into a 2-L plastic bottle.
7. Some sand will end up on the sieve. Use a squeeze bottle of SHMP solution as needed to help flush the sands on the sieve clear of fines.
8. Once fines are out of the can, rinse remaining sand fractions in can and on sieve with distilled water to remove all SHMP residue. Flush all sand into a pan and dry sample at 80°C in oven.
9. Use Method #3 for particle size analysis of sand and gravel.
10. Use Method #4 for particle size analysis of fines.

Method 2: Separating sizes in samples that are predominantly fines

Materials: Mass balance with (0.001g resolution), drying oven, sieve set, 500-ml plastic bottle with a line on the outside at the 500-ml level, 0.5 % SHMP, 30 % H2O2, squeeze bottle with 0.5 % SHMP, distilled water, plastic or metal spatula, 2-L plastic bottle with a black line at the 1 L level.

1. Weigh out ~30 g of moist sediment (DO NOT DRY SAMPLE)
   If sample is from a core, take material from along the whole length of the interval, as homogenization of cohesive sediment is essentially impossible at this stage.
2. Place sample in a 500-ml plastic bottle with a wide-mouth lid.
3. Add about a teaspoon of 30 % H2O2 to the sample. Using a spatula, carefully break up sample to allow the H2O2 to mix with it and react with any organics present to remove them. H2O2 reacts slowly, so allow it time to react. Once the reaction is going, the sample will foam up. Use the spatula to stir the sample and knock down bubbles to avoid a spill. If sample spills, you'll have to clean up the spill carefully and start all over. Keep adding H2O2 in small amounts until no more reaction occurs. Leave sample over night with lid off to insure that the reaction is complete.
4. Weigh out 2.5 g of SHMP and add it to the bottle. Recall that 2.5 g / 500 ml = 0.5 % SHMP.
5. Fill bottle with distilled water up to the 500-ml line. Tighten lid on bottle and shake. Leave sample for several hours or preferably over night for SHMP to fully dissolve.
6. Shake bottle for 2 minutes and follow Method #1 beginning at step 5.

Method 3: Grain size determination for sands and gravels

Materials: Mass balance with (0.001g resolution), 0.5 phi sieve set, large plastic funnel.

1. Retain all particles larger than 8mm (or whatever the largest sieve is) as one class and weigh.
2. Dry sieve sands and gravels using 0.5 phi sieve set.
3. Any fines retained in the bottom pan should be rinsed into the 2-L bottle of fines for that sample using 0.5 % SHMP and the large funnel.
   
4. Weigh the fraction retained on each sieve and record on datasheet.
5. Calculate the total weight of the coarse fractions.
6. Divide each fraction by the total to get the relative weight percent.

Method 4: Grain size determination for fines

Two alternative approaches are possible. One uses the Laser Granulometer while the other uses the Pipette method.

If the Laser Granulometer is going to be used:

1. Shake the 2-L bottle vigorously to homogenize sample.
2. Use a 2-L graduated cylinder and smaller cylinders as needed to determine the total volume of the sample.
3. Pour sample back into the 2-L bottle and shake bottle vigorously to homogenize sample.
4. Pour subsample into a pre-labeled 20-ml vial for later analysis on machine.
5. Pour another 20 ml into a pre-labeled and pre-weighed vial with a line indicating the exact 20-ml level. Place that sample into the oven and let it evaporate for several days. When it is dry, weigh it. Subtract out the weight of the vial as well as the weight of the SHMP (0.5 % · 20 ml = 100 mg) to obtain the total weight of sediment per 20 ml. Divide weight by 20 to obtain weight per unit volume (i.e. concentration). Multiple concentration by total sample volume to obtain the total sediment weight of the fine fraction.

If PIPETTE procedure is going to be used:

Materials: Mass balance with (0.001g resolution), thermometer, X 1-L graduated cylinders (with X<10), a 10-ml and a 50-ml graduated cylinder, two 20-ml pipettes with sharpie lines at 5, 10, and 20 cm depth, 2 pipette bulbs, 7·X pre-weighed and pre-labeled 20-ml vials (each set of seven vials should be labeled on their bottom with a letter designation for the set and a number designation for the sample (e.g. A1…A7, B1…B7, etc), digital clock with seconds, squeeze bottle with 0.5 % SHMP, cylinder plunger, vial rack, drying oven.

1. The procedure is scheduled to begin at 4:30 pm and end at ~8 am so that it can run over night with a minimal waste of time and maximal potential for eating and sleep.
2. If 2-L bottles with samples have less than 1-L of fluid, add 0.5 % SHMP to bring sample up to just under 1L. if there is more than 1 L, then be prepared to determine the additional volume later.
3. 30 minutes before start determine the temperature of the room at a resolution of 0.5°C.
4. Select the pipette analysis spreadsheet and sample withdrawal table appropriate for the temperature.
5. Pre-weigh vials and recorded on pipette analysis spreadsheet.
6. 5 minutes before start time shake the first sample vigorously and carefully pour it into 1-L cylinder until cylinder is exactly full with 1-L of sample.
   ·If there is less than 1 L, add 0.5 % SHMP carefully to bring solution up to 1 L.
   ·If there is more than 1 L, pour excess into other cylinders to determine their volume, record the actual total volume on the pipette analysis spreadsheet and be sure to carry that total volume through all calculations. Dump excess after volume is determined.
7. Put the cylinder plunger in the 1-L cylinder and gently pull it up and down to keep fluid mixed.
8. At start time, remove plunger carefully.
9. Follow the sample withdrawal table by using the pipette to withdraw fluid from the appropriate depth in the cylinder at the appropriate time. Expel fluid into vial and place vial in vial rack in oven to dry. Mastery of the pipette bulb will speed the rate of the procedure and enable more samples to be processed by 1 person.
10. When all pipetting is finished, properly dispose of the remaining fluid in the cylinders and properly clean the clyinders.
11. Wait over 1-2 night for all vials to be fully dried out and then weigh subsample + beaker and enter onto pipette analysis spreadsheet.

Safety Issues

Drying oven settings do not specify temperature, so you have to be careful not to set the dial too high. Check the thermometer in the top of the oven to make sure the temp is <100°C before you use it. If you overheat the oven, the thermometer will break and make a big mess that you will have to clean up. Thus, we do not use a mercury thermometer. See H2O2 MSDS info.

See Sodium Hexametaphosphate MSDS info…