Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 5.7
SEDIMENT SAMPLING

5.7 SEDIMENT SAMPLING

5.7.1 SEDIMENT SAMPLING EQUIPMENT

A brief overview of sediment sampling approaches and equipment is provided below, primarily for shallow sediment sampling scenarios. This section will be expanded in the future to include more detailed information. Well-thought-out Decision Units and investigation objectives are required for sediment investigations in the manner as done for soil investigations. Multi Increment samples should be used to characterize sediment DUs (see Section 4). Random, small-scale variability limits the reliability of data for small discrete sediment samples. This can be especially critical for vertical characterization of contaminants in sediment.

Figure 5-37. Sediment Sampling Tube Center

left photo: Ten-foot long PVC pipe used as a pole handle, cut into five 2-foot lengths for easy air transport and assembly in the field (1-inch diameter, Schedule 40 PVC).
Center right photo: Screw ends attached to cut PVC, with solid cap on end of one piece, to keep the water out.
Bottom left photo: Two-foot long, one-inch aluminum sampling tube (thin-walled towel holder) attached to the bottom of the PVC pole.
Bottom right photo: Sampling tube attached to bottom PVC piece with two metal hose clamps. The bottom piece of PVC is sealed to keep mud and water out. (Examples in Figure 5-37 made by Weston Solutions, not patented)

Figure 5-38. Collection of Sediment Increments from a Drainage Canal

Upper left photo: Increment collection point accessed.
Upper right photo: Sampling tube pushed into sediment to target depth.
Bottom left photo: Increment core pushed out of tube using disposable 3/4-inch wooden dowel. Tilt the tube slightly backward before pushing out sample in order to drain excess water, being careful not to lose the sediment.
Bottom right photo: Increment collected on disposable plate and placed into sampling container (e.g. one-gallon freezer bag carried in clean bucket). Note cylindrical shape of increment.

Additional procedural information on sediment sampling is available from many sources including Superfund Program, Representative Sampling Guidance, Volume 5: Water and Sediment (USEPA, 1995c), USGS National Field Manual for the Collection of Water-Quality Data (USGS, 2005), and Field Sampling Procedures Manual (NJDEP, 2005). These documents focus on older discrete sample collection methods but still include useful considerations for Multi Increment sampling approaches.

With proper planning and equipment, the collection of Multi Increment samples from sediment in relatively shallow water (e.g., <15 ft deep) does not involve significantly more effort than required for the collection of surface soil samples. Various types of sampling equipment are available for collecting sediment samples. Consider the type and characteristics of the water body associated with the sediment to be sampled when selecting sampling equipment. Factors such as the width, depth (especially if tidally influenced), flow, and bed characteristics of the water body are important.

Increments collected from sediment should be core-shaped (see Section 4.2.5.2). Consider a tube-shaped sampler for collection of increments to ensure cylindrical-shaped increments (see Figures 5-37 and 5-38).

Figure 5-39. Alternative Scoop-Shaped Samplers for Thin Sediment Layers

Left photo: Collection of increments from a canal with a thin sediment cover using a scoop sampler (scoop made by Tetra Tech EMI).
Right photo: Flat-bottom scoops with upright, flat sides to help avoid a bias toward the upper layers of sediment

If the sediment cover is exceptionally thin (e.g., < 4 inches) then use of use of a cup-type device or a flat-bottom, scoop sampler might be most practical (Figure 5-39).These devices might also be required for the collection of increments from coarser-grained sediment or other situations where use of a core catcher for the tube sampler is not practical. A flat-bottom scoop with upright square sides (Figure 5-39) will also help to avoid bias to the upper portion of the sediment.

Figure 5-40. Collection of Multi Increment Sediment Increments from a Drainage Canal

Marked, floatable rope used to assist in increment spacing and collection

Figure 5-41. DU Designation and Collection of Multi Increment Samples from a Sugar Mill Drainage Canal

Tape measure placed along DU boundary to assist in increment location and collection

Marking sediment increment collection locations in a stream or canal can be challenging. Consider placing a long, floatable rope (or tape measure) with a marked spacing within the DU (Figure 5-40). A long tape measure with increment positions marked by pins or flags can also be placed along the edge of the waterway to guide sample collection (Figure 5-41). Refer to Section 4.2.5.1 for guidance on increment spacing in long, narrow DUs.

Take care to minimize disturbance and loss of an increment as the sampling device is being lifted. If sediment fines are preferentially lost during increment collection then the resulting Multi Increment sample will not be representative. Decant excess water from collected sediment Multi Increment sample by waiting several minutes and then carefully pouring excess water out of the container. Use a cellulose paper filter to catch and re-place fine sediment back into the container as necessary. Note that the collection of undisturbed, anaerobic sediment samples for geochemical analysis, if required, might require alternative methods. This should be discussed with the HEER Office prior to sample collection.

Alternative tools and methods are required for the collection of sediment samples from larger and/or deeper water bodies. Figure 5-42 depicts a core sampler used to collect Multi Increment sediment samples from the large estuary pond depicted in Figure 3-32 of Section 3. Twenty-four Multi Increment samples consisting of 30 increments each were collected from 6 DUs designated in the pond over a 4-day period, including triplicates from one of the DUs.

A clear two-foot tube is attached to the end of the extendable push rod. A small boat and GPS can be used to maneuver to pre-established, increment collection locations within the pond. The coring device was manually forced into the sediment to the targeted depth and retrieved. A valve at the top of the sampling tube helps to hold in the sediment as the core is extracted from the increment location. As the core is being from the water, the base of the sampling tube is capped to prevent sediment loss. The sampling tube is then removed from the valve/push rod, capped on the other end and stored.

Figure 5-42. Use of Core Sampling Device to Collect Sediment Increments
Left Photo: Core sampler attached to valve and push rod
Middle Photo: Manual collection of sediment core increment from skiff
Right Photo: Individual core increments collected from a sediment DU

Once all cores (e.g. 30) for a DU have been collected, the individual cores containing multiple DU layers are extracted from the sampling tubes and the respective DU layers are combined to form bulk MI samples(Figure 5-43). The base cap is removed from the bottom of the tube and the tube placed on a plunger. The upper cap is removed and the tube is progressively forced downwards. Targeted DU layers are removed and placed in containers specific to that DU layer to form a bulk Multi Increment sample.

Bulk Multi Increment samples can either be sent to the lab for processing or, if needed, processed in the field to reduce mass. In the latter case, the sample is placed on clean, plastic sheeting and spread out to a thickness of about ½ inch (Figure 5-44). A flat edge spatula is then used to collect subsamples from the sample in a systematic, random fashion. A minimum of 30 subsamples is recommended. The initial bulk sample and/or split samples can be submitted to the laboratory for further processing and testing in accordance with incremental sampling methods.

Figure 5-43. Removal of Sediment Core Increments from Sampling Tubes

Upper Photo: Target DU layers in a core increment.

Middle Photo: Removal of increment by forcing sampling tube downward on a plunger; target DU layers removed from core and placed in dedicated container for combination with increments from other cores collected from the DU.

Lower Photo: initial bulk Multi Increment samples prepared by combination of core increments for each DU layer (layers representing 3 different sediment depth intervals in the DU).

Figure 5-44. Field Subsampling of a Sediment Multi Increment Bulk Sample

Upper Photo: Sample spread out into a 0.5 inch layer; large rocks and debris removed.

Middle Photo: Flat-edge spatulas used to collect subsamples in a systematic, grid fashion.

Lower Photo: Bulk Multi Increment sample prepared by representative subsampling, for shipment to the laboratory.

Alternative types of sediment samples are available for deeper or otherwise less accessible sediment. A Vibracore sampling tool was used to evaluate the feasibility of collecting Multi Increment samples from deep-water (50 ft) in a field study carried out by HDOH in 2013 (Figure 5-45). A small pontoon boat equipped with a GPS was again used to locate pre-established increment location points. A 30-increment Multi Increment sample from the upper foot of sediment in a DU was collected over a period of five hours using the approach described below.

A metal sampling tube with an inner liner is fitted to the base of an electric motor. The device is lowered to the sediment interface using a small wench. The motor is then used to vibrate the sampling tube into the sediment. The depth of penetration is monitored at the surface using a tape measure attached to the top of the device. A sediment catch is connected to the base of the tube to retain the sediment core when collected. To help speed the collection effort, an 8 foot long collection tube was used to collect up to 6 to 8 one-foot deep increments (target layer depth was 0-1 ft below sediment surface in the DU) without bringing the vibracore aboard the boat between collecting those increments. After the first increment was collected (and held in the tube by the sediment catch) the vibracore was lifted off the sediment floor several feet and slowly moved to the next increment location for another 1-foot sediment collection in the same sampling tube.

The sampling device is then retrieved to the surface. The liner and sediment core are removed from the sampling tube. An attempt to collect subsample increments to reduce the mass of the larger volume of sediment collected in the 8 ft core by cutting wedges or regularly spaced plugs had limited success (Figure 5-46; see also Subsection 5.4.2).The repeated vibration action of the vibracore during the period that multiple DU increments were collected in the same sampling tube (as well as the nature of the sediment in that location) tended to cause the sediment in the tube to disaggegate and run or mix together. If sediment collected under these circumstances is to be representatively subsampled, the sediment from the entire core would need to be collected, spread to a thin layer on a large flat surface, and systematic random increments collected, as illustrated in the example in Figure 5-44.

An additional concern noted in the use of the Vibracore was the loss of some very fine sediment when the sample tube was placed horizontally on the pontoon boat for core liner extraction. If the sediment in the core liner can be removed from the core tube in a vertical orientation (or other technique to limit loss of fine sediment particles), that would result in a more representative sample for contaminant evaluation.

Figure 5-45. Use of a Vibracore Device to Collect Sediment Samples in Deep Water

Left Photo: Vibracore sediment coring device; electric vibrator attached to top of sampling tube.
Right Photo: Vibracore lowered to top of sediment at increment collection location.

Other devices used to collect surface sediment samples include center pivot grabs, clamshell pivot grabs and drags, sleds, and scoops. Sediment core-type samplers are capable of collecting sediment samples at depths comparable to benthic grab samplers and when feasible are more suitable for MI samples. Sediment core samplers have the ability to retain the integrity of sediment horizons with minimal disturbance and allow the collection of unbiased core-shaped increments.

Figure 5-46. Removal of Core Increment from Vibracore Sampling Tube and Subsampling

Bulk Multi Increment sample prepared by combination of increment subsamples from all cores collected within the targeted DU.

Several types of sediment core samplers and benthic grab samplers are described in the following documents, among other sources.

  • New Jersey Department of Environmental Protection (NJDEP) Field Sampling Procedures Manual. August 2005.
  • United States Geological Survey (USGS), National Field Manual for the Collection of Water-Quality Data, Book 9, Handbooks for Water-Resources Investigations, Chapter A8, Bottom-Material Samples. Version 1.1. June 2005.

For sediments that consist primarily of <2mm particles, consider subsampling Multi Increment samples in the laboratory for extraction and analysis without drying, in order to reduce sample preparation and analysis time. Drying and sieving are carried out primarily to remove large particles. A sediment moisture content analysis is also necessary if the laboratory subsamples are collected without first drying the bulk MI sample(s), in order to report the laboratory data on a dry weight basis.

If both sediment and surface water samples are collected in the same location, collect the surface water sample first. If several sediment samples are collected from a streambed, collect the most downstream sample first with subsequent samples collected while proceeding upstream. Follow appropriate sample handling and analysis procedures, which are described in Section 11.