Composite sample grab" samples

Before sampling can start however, a sampling strategy needs to be developed. This strategy is based on the conceptual model (hypothesis). Further, the number of samples needed and the required sampling locations are also determined. A further determination is made whether the use of composite or grab samples is appropriate. One needs to know how the data will be used and evaluated. All these are part of the data quality objectives on which a sampling plan is based. 

Figure 14. Comparison ofUV spectra of composite and grab samples of a petrochemical effluent.

Few populations, however, meet the conditions for a true binomial distribution. Real populations normally contain more than two types of particles, with the analyte present at several levels of concentration. Nevertheless, many well-mixed populations, in which the population s composition is homogeneous on the scale at which we sample, approximate binomial sampling statistics. Under these conditions the following relationship between the mass of a randomly collected grab sample, m, and the percent relative standard deviation for sampling, R, is often valid. ... 

Samples from municipal wastewater treatment plants and samples of industrial discharges often are collected as 24-h composites. Samples are obtained using an automatic sampler that periodically removes individual grab samples. The volume of each sample increment and the frequency of sampling may be constant or may vary in response to changes in flow rate. 

Benthic invertebrate communities are taxonomically and trophically complex, and their abundance and species composition in a water body often vary seasonally and among years. Sediment-dwelling invertebrates can be readily sampled but considerable effort is often required to remove benthic organisms from grab samples of sediment, to determine their taxonomic composition, and to obtain sufficient sample mass of a target taxon for analysis. Sampling would not substantially affect target populations. 

If the objective of measuring FMs, or other consumer product chemicals for that matter, in wastewater treatment is to understand FM removal and mechanisms of removal across wastewater treatment processes, then it is important to collect samples at least every 2 h and composite these samples into a single, flow-based 24-h sample. Otherwise, the results may be significantly over- or underestimated depending on the time of the day the sample was collected. However, if the objective is to monitor FMs in only final effluent or sludge, representative grab sampling may be sufficient. 

Sampling. Composite or continuous sampling furnishes water samples that are more representative than grab or batch samples. If possible, such samples should be used and processed immediately to avoid storage problems. However, the collection of small-volume samples may be more easily accomplished by batch sampling. 

An important point that needs to be emphasized is that the isotopic composition of precipitation is variable on all time scales and so the most robust comparisons should be provided by multi-year, preferably multi-decadal or longer isotopic compositions. The fact that what are essentially instantaneous random grab samples that integrate some uncertain aspect of the hydrography of each of these river systems yield any correlation between isotopic composition and hypsometry is pretty remarkable and provides some confidence in estimates based on archival records that integrate isotopic compositions on a very wide range of time scales (Rowley and Garzione 2007). 

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]. 

Effluent sampling and handling Type of sample (i.e., grab versus composite) used during Phase II should be the same as that used during Phase I. One composite or one grab sample should be subjected to both Phases II and in. Once the substance responsible is identified, multiple samples may be analysed for the presence of the toxicant. 

There are two types of samples that can be collected in a sampling event grab samples and composite samples. 

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