National Water-Quality Assessment (NAWQA) Program
Guidelines for Collecting and Processing Samples of Bed Sediment for Analysis of Trace Elements and Organic Contaminants for The National Water-Quality Assessment Program
By Larry R. Shelton
U.S. Geological Survey Open-File Report 94-458
Summary of July 1995 revisions to the published report;
Any reference to the use of nitric acid for cleaning should be changed to hydrochloric acid (HCL).
Change the 'Equipment and supplies list' (table 2) on page 7 from 'Nitric acid, high purity' to 'Hydrochloric acid, trace element-free, 5 percent by volume'.
Delete the second and third lines on page 11, paragraph 3, and ADD 'Fill the 500-mL plastic wash bottle with a 5 percent (by volume) solution of hydrochloric acid (HCL). Rinse all the equipment used for trace element collecting and processing with a minimum amount of acid.'
Change table 3, page 12, from 'Rinse with nitric acid solution' to 'Rinse with HCL solution'
The following is an expanded procedure allowing for collecting organic bed material.
PROCESSING ORGANIC BED MATERIAL
Study Units that have coastal drainage systems or wetland areas may have difficulty collecting traditional depositional zone bed samples. Wetland areas typically do not have erosion which transports and deposits silt and clay. However, vegetation and algae moving through the system dies, settles to the bottom, decays, and becomes a mat of organic muck. This organic material can be used for the Occurrence and Distribution Assessment phase of the National Water Quality Assessment Program as an indicator of contaminates transported and present in the drainage system. However, when collecting bed samples that are predominately organic material, the processing guidelines must be modified. Bed samples rich in organic material will not pass through a 63 micron sieve, therefore a 2 millimeter sieved fraction will be used for trace element analyses. This procedure requires the results be stored under a different set of parameter codes. Also, larger amounts of sieved material are required for analyses because organic material has less weight per volume then silt and clay. For National Synthesis, the interpretation of the analytical results of the organic deposit can be normalized to the results of the silt and clay analyses by comparing total organic carbon (TOC) values. The Guidelines for Collecting and Processing Samples of Stream Bed Sediment, OFR 94458, should be followed except for these modified instructions.
In addition to the original equipment, a 2.0 millimeter (mm) nylon mesh sieve, 12 inches in diameter, (Gilson, NMA010) will be needed to process samples for trace element analyses of the organic material.
Follow the Guidelines in OFR 94458 and composite cores from a 100 meter stream reach and from the steam cross section. However, subsample at least 25 times to a depth of 10 to 15 centimeters using a slotted Teflon spoon, scoop, or spatula. Carefully remove the organic material from the stream bed, discarding the excess liquid, without losing the fines. Composite 2 to 3 liters of organic material into a large glass bowl.
TESTING FOR ORGANIC BED MATERIAL
Attach the 63 micron mesh sieve cloth to the plastic frame and assemble over the funnel and 500mL plastic jar as usual. Place the additional 2.0 mm nylon mesh sieve material over the 63 micron cloth sieve frame. Place an aliquot of the composite sample on the 2.0 mm sieve. Carefully work the sample through the two sieves with a Teflon policeman, and by shake aggressively. Process several aliquots of the sample then inspect the liquid that has been collected in the jar. If the liquid is very cloudy it may be possible to achieve a traditional bed sediment sample of silt and clay. Analyzing only the <63 micron fraction for trace elements is the preferred procedure when enough silt and clay can be found in the stream bed. So, if possible process the sample following the original Guidelines (OFR 94458, page 17). However, if the liquid in the jar is clear or contains minimal fine sediments, continue with this modified procedure. Accurate field judgement is required to determine which procedure will produce the best results to achieve the project criteria. Continuity within a basin or Study Unit might also be important. If it is important to know the ratio of silt and clay to organic material request the laboratory to do a percent ash analyses.
TRACE ELEMENTS (Less Then 2 mm Fraction)
If it is determined that the trace element sample will be the <2.0 mm fraction, continue to process the sample through only the 2.0 mm sieve and collect all of this fraction in a 500mL wide mouth plastic jar. The desired sample can probably be achieved by aggressively shaking the sieve. Discard any material not passing through the 2 mm sieve. For the trace element analyses 10 grams of dry material is required. That will require filling the 500mL jar about onefourth (120 mLs) full of wet organic material. As described in OFR 94458, page 17, allow the fine sediments to settle for several days then decant the liquid leaving only wet sediments. Label the sample so that the Central Laboratory will realize that this is a <2.0 mm fraction for trace element analyses.
ORGANIC CONTAMINATES AND PARTICLE SIZE
The samples for organic contaminates and particle size analyses will be processed according to the original Guidelines in OFR 94458 through a 2.0 mm stainless steel sieve. However, 25 grams of dry material is required for each analyses. Therefore, the glass 500 mL jar should be about twothirds full (300 mLs) of the wet organic material for the organic contaminate analyses and fill the plastic 500 mL jar with 300 mLs for the particle size determination. Sieving this large amount of material may generate more liquid then is desirable, if so, allow the fines to settle then decant the excess liquid.