Field replicates

J.D. Martin, Variability of Pesticide Detections and Concentrations in Field Replicate Water Samples Collected for the National Water-quality Assessment Program, 1992-97, USGS WRI 01-4178. US Geological Survey, Washington, DC (2001). Also available on the World Wide Web http //water.wr.usgs.gov/pnsp/, accessed August 2002. 

Mevissen, M., M. Haussler, A. Lerchi, and W. Loscher. 1998. Acceleration of mammary tumorigenesis by exposure of 7, 12-dimethylbenz[a] anthracene-treated female rats in a 50-Hz, 100-pT magnetic field replication study. Jour. Toxicol. Environ. Health 53A 401-418. 

Sediment samples used for toxicity testing are subsampled for chemical analyses (following compositing). Samples for benthic community analyses are collected at the same time and from the same areas. Field replicates (typically n = 5 unless a different value is determined by power analyses) are collected for toxicity testing and for benthic community analyses, but generally not for chemical analyses due to cost considerations. 

One major deviation from the typical toxicity test methodologies was that field replicates were used for toxicity testing in lieu of the standard laboratory replication (z. e., one replicate per station rather than 5). This modification was made a priori in order to maximize the number of stations at the expense of statistical power within the available project resources. 

Changing the sample plan to reduce variability between field replicates - for example by stratified sampling or by increasing the number of increment samples that make the composite sample. 

Figure 7.6. (a) Dissolved organic carbon (DOC) and (fc) Cu activity expressed as a percentage of total H2O-soluble Cu in the bulk soil, the outer rhizosphere, and the inner rhizosphere along a soil contamination gradient at Sudbury. Mean values and standard deviations are for three field replicates (Modified from Legrand et al.. 2005.)... 

The sampling was performed at the end of September 1998. At each of the three sampling sites, three Populus tremuloides were uprooted carefully (for a total of nine trees). The three trees are the field replicates used to establish site variability. All soil samples are taken from the upper B horizon (15-20 cm under the organic-mineral interface). This horizon enables the collection of enough roots to provide sufficient rhizosphere mass for chemical analyses, while being deep enough to avoid the lack of contrast between the rhizosphere and the bulk soil that is found in organic horizons. The root diameter was between 0.5 mm and 1 cm. 

Fig. 2. Mean K concentration in the barium chloride extractions in the inner rhizosphere (RHi), the outer rhizosphere (RHo) and the bulk soil (BK) at all sites (0.5, 2 and 8 km from a smelter) and for the three field replicates (A, B and C). Values are means of two or three replicates depending on the soil mass available. Error bars represent the standard deviation when analyses were performed in triplicates.

Note Values in parentheses represent standard deviation of laboratory triplicates. Standard deviations are not given if soil mass was insufficient to replicate analyses. " The sites are coded as follows the first part of the code refers to the distance from the copper smelter in Rouyn-Noranda (0.5 = 0.5 km 2 = 2 km 8 = 8 km) the letter that follows refers to die field replicates. 

The characteristics of the bulk soil, measured on one field replicate at each site, are presented in Tables 1 and 2. The pH values of the bulk soil are more acidic at site 1, close to the smelter, and gradually increase toward site 3. The deposition of atmospheric pollutants, such as sulfur compounds, is most probably responsible for the soil acidification observed close to the smelter. The EC and CEC values of the three sampling locations do not follow any specific trend, although site 1 exhibits both the highest EC value and the lowest CEC. Soil organic C content increases with distance from the smelter, reflecting the increase in canopy density. The amounts of Fe and Al extracted by AAO and DC also increase gradually from site 1 to 3. The proportion of sand in the bulk soil decreases from site 1 to 3 as the silt and the clay contents increase. 

Inner rhizosphere metal concentration (mg kg )/bulk soil metal concentration (mg kg ) Values in parentheses are standard deviations of the field replicates. 

Average inner rhizosphere/bulk soil ratios of Cu " concentrations (mg kg 1) were calculated for each site. The field replicates at site 1 give an average ratio of 1.3. This mean ratio decreases to 0.8 at site 2 and to 0.4 at site 3. Therefore, the relative Cu + content of the inner rhizosphere varies with distance from the smelter, an observation that is probably associated with an increase in OM content in a rhizosphere and a decrease in Cu concentration. 

The symmetry of the structure we are looking for is imposed on the field 0(r) by building up the field inside a unit cubic cell of a smaller polyhedron, replicating it by reflections, translations, and rotations. Such a procedure not only guarantees that the field has the required symmetry but also enables substantial reduction of independent variables 0/ the function F (f)ij k )- For example, structures having the symmetry of the simple cubic phase are built of quadrirectangular tetrahedron replicated by reflection. The faces of the tetrahedron lie in the planes of mirror symmetry. The volume of the tetrahedron is 1 /48 of the unit cell volume. 

Field tests (tests in real environments), in which replicate test samples of metals or alloys —referred to as test coupons or specimens—are exposed to the actual environmental conditions expected in service, e.g. the atmosphere, the ground, the sea, etc. 

Field and Plant Tests Field exposure of test panels offers the benefit of a high degree of control over surface preparation and application. Moreover, through standardised exposure conditions, broader comparisons between both paint systems and locations are possible. More importantly, since replicates may be removed and laboratory tested periodically, changes in properties can be followed in considerable detail. At least four replicates should be examined for each exposure period to minimise the effects of atypical specimens. 

Luciw P (1996) Human immunodeficiency viruses and their replication. In Fields BN, Knipe DN, Howley, PM (eds) Virology. Lippincott-Raven publishers, Philadelphia, PA, pp 1881-1952... 

Kedwards, T.J., Maund, S.J., and Chapman, PE. (1999a and 1999b). Community level analysis of ecotoxicological field studies 1 Biological Monitoring and 11 Replicated design studies. Environmental Toxicology and Chemistry 18, 149-157 and 158-166. 

Recently, however, there have been a number of encouraging developments in the field of antiviral therapy. For example, acycloguanosine (acyclovir see Chapter 5) has been shown to be non-toxic to host cells while specifically inhibiting the replication of herpes vimses. Successfiil clinical trials have led to the introduction of this dmg for the treatment of a variety of herpetic conditions. 

Precision. The choice of 400 mg of soil is arbitrary. It was chosen in order to keep the entire cleanup step within a 2 cc vial. A test of replication was done on one field sample contaminated with about 20,000 ppm of 1254. Three samples of 50, 54, and 54 mg were weighed into separate vials, extracted, and then diluted 1 1000 into hexane in a separate vial. The three chromatograms are shown in Figure 7. Two peaks were quantified to demonstrate how reproducible a measurement can be, even in a field sample. 

Figure 7. Replication of field measurements of contaminated soil.

Repeatability is defined as precision under conditions where independent test results are obtained with the same method on identical test material in the same laboratory by the same operator using the same equipment within short intervals of time. The replicate analytical portion for testing can be prepared from a common field sample containing incurred residues. This approach is used extremely rarely. Normally, repeatability is estimated by the relative standard deviation ofrecoveries, which should be lower than 20% per commodity and fortification levels according to SANCO/825/00. In justified cases, higher variability can be accepted. 

The need for additional samples to compensate for soil heterogeneity must be reconciled with labor, storage, transportation, analytical, and other constraints that add significantly to study costs. Satisfactory results have been obtained from numerous field studies using three or four treated replications with 5-10 soil cores collected from each replication per sampling period. These replication/repetition numbers strike a reasonable balance between the need for samples sufficient in number to characterize agrochemical dissipation versus financial and logistical constraints associated with sample collection and analysis. 

Untreated (control) soil is collected to determine the presence of substances that may interfere with the measurement of target analytes. Control soil is also necessary for analytical recovery determinations made using laboratory-fortified samples. Thus, basic field study design divides the test area into one or more treated plots and an untreated control plot. Unlike the treated plots, the untreated control is typically not replicated but must be sufficiently large to provide soil for characterization, analytical method validation, and quality control. To prevent spray drift on to the control area and other potential forms of contamination, the control area is positioned > 15 m away and upwind of the treated plot, relative to prevailing wind patterns. 

All application verification and soil samples must be individually labeled with unique sample identification (ID) and other identifying information such as study ID, test substance name, sample depth, replicate, subplot and date of collection, as appropriate. Proper study documentation requires that sample lists and labels be created prior to work commencing in the field. Water- and tear-resistant labels should be used since standard paper labels may become water-soaked and easily torn during sample handling. Sample lists should have the same information on them as the labels and are a convenient place to record plot randomization, initials of the individual who collected the sample, and date of collection. As such, the sample list is important in establishing chain of custody from the point of sample collection until its arrival at the laboratory. 

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