Sediment samplers

Fig. 2 Cable suspended sediment samplers in operation at MEMS (Fig. 1), lower Ebro, during floods (a) US DH-74 depth-integrating suspended sediment sampler (b) US DH-59 depth-integrating suspended sediment sampler (c) 152 mm intake Helley-Smith sampler (d) 76 mm intake Helley-Smith sampler. See text and [25] and [31] for operational details...

Evaluations of various soil and sediment samplers have been reported [56, 57]. The sediment shovel proved highly practical, but was limited because small particles tend to be lost when the shovel is lifted [56]. A cryogenic sediment sampler was less convenient to use, but allowed the collection of almost undisturbed samples. Houba described a different device for the automatic subsampling of soil, sediment and plant material for proficiency testing [57]. In another study, Thoms showed that freeze-sampling collects representative sediment samples, whereas grab-sampling introduces a bias into the textural composition of the 120 mesh fraction, due to washout and elutriation of the finer fractions [58]. 

To obtain as much data as possible, the system needed a continuous sampling capability. Also, to fulfill the requirement of rapid data analyses while at sea, nondestructive elemental analysis was selected for the method of shipboard analysis. A sampling and analysis system called the continuous seafloor sediment sampler (CS ) was developed from this work. 

The CS system is made up of three major components a seafloor sediment sampler, shipboard sample processor, and nondestructive elemental analysis instrumentation. It was described in detail previously (8). Design requirements for the seafloor sediment sampler are that it be in constant contact with the seafloor while being towed at speeds up to 6 knots, agitate only the upper surficial seafloor sediment and create a plume, contain a pumping means to sample the sediment slurry plume, and be capable of transporting the sediment slurry to a surface ship. These conditions were achieved by designing a towable sled that contained, within its structure, a submersible pump that was hose-connected to the surface ship. 

Sediment sampling of the seven stations using the CS equipment was carried out by running transects with the survey vessel parallel to, and as close as possible to, the marker buoys. The CS underwater seafloor sediment sampler was pulled at a speed of three knots and, when abreast of each buoy, the sediment collected was recorded as being from that station. The sediment wafers prepared aboard ship from the collected slurries were immediately analyzed by XRF for three elements (Mn, Fe, and Ti) and were stored for further land-based analyses of other elements. A comparison of the elemental content of the sediments collected from the seven stations by box coring and with the use of the CS equipment constituted the basis for ground-truth evaluation of the CS system. 

Steinmann, C. R., E. Veit, S. Grosser, and A. Melzer. 2001. An in-situ mobile interstitial water and sediment sampler (MISS). Hydrobiologia 459 173-176. 

Extreme low flow where use of the sediment sampler is impractical. Samples may be collected by immersing the bottle by hand. 

Sediments from the bottom of streams, rivers, lakes, estuaries, and oceans are collected with a bottom grab sampler or with a corer. Grab samplers are equipped with a pair of jaws that close when they contact the sediment, scooping up sediment in the process (Figure 7.5). Their principal advantages are ease of use and the ability to collect a large sample. 

Disadvantages include the tendency to lose finer-grained sediment particles as water flows out of the sampler and the loss of spatial information, both laterally and with depth, due to mixing of the sample. 

The method above, however, is not suitable when one needs a precise study of the vertical distribution of pesticides. Generally, the concentration of pesticides in paddy sediment is highest at the surface. Special care is required to avoid contamination with surface soil when the sediment is collected. The sediment core should be collected in two stages. First, a pipe with a diameter greater than that of the core sampler is inserted in the sediment and then water inside the pipe is removed gently with a syringe, pipet, etc. Next, a layer of surface soil (1-3 cm) is taken with a spatula or a trowel and then subsurface soil is collected with a core sampler to the desired depth see also Figure 4. 

Figure 5 (a) Ekman-Birge grab sampler for clay or loamy sediments and (b) Smith-Mcintyre grab... 

The previous section described active samplers where the air is swept of particles using mechanical mechanisms. This section describes passive samplers that do not move, but collect material that deposits by impaction or sedimentation deposition. These types of collector are the most common type for field studies aimed at assessing exposure of aquatic and terrestrial organisms to pesticides. 

Bedload was sampled during competent flows at the same vertical than suspended sediment. Bedload analysis has been based upon 215 samples, 145 during 2002-2003 and 70 during 2003-2004. At SMS we used a 29-kg cable-suspended Helley-Smith sampler with a 76-mm intake and an expansion ratio (i.e. ratio of nozzle exit area to entrance area) of 3.22 (Fig. 2c). Bedload was measured at... 

Emmett WW (1980) A field calibration of the sediment trapping characteristics of the Helley-Smith bedload sampler. US Geological Survey Professional Paper, 1139... 

The devices used for sampling of solid samples (sludge, sediment, and soil) are usually grab samplers or corers. Box corers or multicorers can be employed if more detailed information on the spatial distribution of the analytes is needed. The samples are stored in the dark at 4 °C or more commonly at -20 °C, preferably in glass containers [53]. Very often, solid samples are also dried or lyophilized prior to storage. 

Solid samples collected in the field are usually preserved by freezing immediately, either on board ship, in the field, or at the laboratory [374]. Rapid preservation is vital if the integrity of the sample is to be maintained. Sediment cores should be sectioned and each sub-sample frozen individually. Some core samplers allow the whole core to be frozen in situ prior to sectioning. This technique is preferable, if these facilities are available, since it allows the unconsolidated top sections to be handled more easily [375]. 

Booij et al. (2003b) made an effort to model contaminant uptake by buried passive samplers. The major assumptions underlying this model are that the sampler can be regarded as an infinite sink for target contaminants, that the depletion of the bulk sediment phase is insignificant, and that the contaminant desorption kinetics are not rate-limiting. 

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