Soil Sediment Sampling

EXECUTIVE SUMMARY

With the support of the World Bank in Tanzania, Ramani Huria (RH) and JBA Consulting partnered in October 2018 to develop a surface soil sediment dataset for the greater Dar es Salaam region of Tanzania. The data will support a geomorphological assessment for erosion and flood risk studies. A national-level soil profile had existed for Tanzania prior to this effort, but contained only a single sample from Dar es Salaam. This was not sufficient to analyze erosion potential across the city. The JBA team in consultation with Ramani Huria decided to use a 2km grid, which resulted in 731 sampling points being pre-established throughout the city. A team of 10 field mappers and 4 office technicians—all Tanzanian youth participants in the Ramani Huria project—were trained in sample collection and analysis (sieving). A total of 643 points were sampled and sieved; 88 sites were inaccessible for one or another reason. The 643 samples were analyzed by sieving each to separate the sediment by particle size and weighing the resulting fractions.

Scope/Application

The procedures contained in this document are to be used by investigators when collecting and handling samples in the field and when preparing sampling equipment for soil sediment sampling field investigations.

METHODOLOGY

Pre- Field Preparation

• Prepare sample interval in advance (use QGIS or ArcGIS) and assign ID number to each sample to avoid double allocation of points during assigning site work or any inconveniences during analysis.  
• Prepare data recording application (preferably OpenDataKit’s Android application - ODK Collect) and develop a form/survey with the following guiding question;

• A GPS point

• Selection from a list of the Region, District, Ward, and Subward

• The ID number of the sample site (from the numbering of the 2km grid)

• A photograph before, during, and after the collection

• Information on accessibility

• A note if the weather was wet or dry

• Characteristics of the site (loose or consolidated sediment, low, medium, or high vegetation

cover, rural or urban setting)

• A photo facing outward from the site in each cardinal direction

You are advice to use Kobo Toolbox instance as the back end (server) for the data collection.The survey form will be uploaded to this server, allowing all investigators to download the black survey form, fill it out offline on their phones, and upload data back to the server.

Field Sample Collection

Collect two samples of 0.5kg of sediment were collected in the field at—or if inaccessible, within 500m of—each of the pre-established sites. Collect one sample near the surface (after discarding 3cm of overlay), and the other at a depth between 7 and 15cm.

Record a set of data at each site using OpenDataKit’s Android application ODK Collect.

Materials

Use the following materials to collect samples in the field

• Trowel or small shovel

• Plastic “ziploc” bags with 1kg capacity

• Android phone pre-loaded with ODK Collect

• A separate maps and navigation application, maps.me, pre-

loaded with the locations to be visited (the 2km grid)

• First aid kit

• Marker pens

• Permission letter for the sampling activity from the municipal authorities (Depend on the system where you are working)

• Tape measure

Field Sampling Procedure

Use the following procedure prior to commencement:

1. Navigate to the pre-defined sampling location.

2. Check the site if is safe. If not, move on to the next site, and record the reason using

ODK Collect.

3. If a sample cannot be taken (there may be a building or water body preventing access) the following procedure should be used:

• a suitable point within 500 meters of the defined sample location should be found by moving in a northerly direction. In some

cases, points may simply not be accessible. In this case a reason should be recorded.

4. Once a suitable sample location has been identified use ODK Collect to record the relevant information by completing the survey form.

5. Use ODK Collect to take four photographs of the typical site surroundings (facing north, east, south, and west).

6. Begin taking the surface sediment sample using the following procedure:

(a) Identify the area where you will take the sample. This should correspond approximately to the sample point location, however it does not need to be exact.

(b) Take a photograph of the ground before the sampling takes place

(c) Remove the top layer of surface sediment using a pre-cleaned trowel or scoop (3 cm should be removed and discarded)

(d) Collect one bag of sediment a depth between approximately 3cm - 7cm (TOP sample) and another from a depth approximately between 7cm and 15cm (BOTTOM sample). A tape measure should be used to measure the depths.

(e) Place the collected sediment into plastic bags and tightly seal the them. If the bag appears at risk of breaking open the sample should be double bagged.

(f) Each bag should be labeled with the unique site ID, either TOP or BOTTOM sample and the date using an indelible marker pen.

(g) Move on to the next sample point.

Laboratory Analysis

The pair of samples—top and bottom—from each site is to pass through a set of progressively finer-meshed sieves, which will result in nine separate fractions. Each fraction has to be weighed. The resulting measurements, which represent the proportion of each sediment

particle size at each site, has to be recorded

Materials

The following materials are to be used:

• Set of metal sieves

• Scales

• Handwash station and gloves

• Brush, cloth, and towel for cleaning sieves

• Android phone with ODK Collect and sieving survey

Sieving and Data Entry

The following are the procedure for the sieving:

1. If samples are moist, they should be dried before analysis by being placed in the sun— though not exposed to wind—in a clean dry dish.

The following steps should be performed for both samples, beginning with the TOP sample.

2. Weigh about 500g of sediment from the bag into a weighing dish.

3. If sediment particles are lumped or conglomerated, crush the lumped particles until they become loose.

5. Determine the mass (weight in grams) of each sample before sieving.

6. Prepare a stack of sieves. Those with larger opening sizes are placed above the ones with smaller opening sizes. The lowest on the stack is a simple pan which retains the sediment that passes through the finest sieve.

7. Make sure all sieves are clean—if any particles are stuck in the openings poke them out using the brush.

8. Weigh all empty sieves and the pan separately. Record the results in ODK Collect.

9. Pour the sediment into the stack of sieves from the top and place the cover on top. Shake the sieves carefully so the sediment filters through—this may need to be done for 3 to 5 minutes.

10. Stop shaking and measure the mass of each sieve + retained sediment. Record the results in ODK Collect.