Environmental Engineering Reference
In-Depth Information
could be almost anything, such as using the wrong solvent or even using a different
amount of solvent at some stage in the extraction. Variation in the time of extraction, not
using prerinsed glassware, and not using prerinsed drying agent can also lead to sample
contamination or loss. This is one reason for extraction surrogates. They provide a
reference for extraction efficiency. Most extraction methods have set limits for the
extraction efficiency of the surrogates as per the method. When the efficiency is within
limits the data will be readily acceptable. If the solvent recoveries are outside acceptable
limits the extraction is suspect and must be redone.
For clean and relatively low viscosity samples extracts will produce good results. Dirty
or highly viscous samples are problematic and may produce erratic results. For a number
of reasons potential problems from dirty or viscous samples should be considered before,
during, and after analysis and review to determine the usefulness of the data.
11.13. MOVEMENT OF A CONTAMINANT
The movement of a contaminant is usually derived from the site's underlying hydrology.
The water table will move the contaminant plume mainly in the direction of the water
flow. This is especially true for those chemical compounds with very high solubility, for
example, MTBE. For contaminants with low solubility in water the diffusion rate from
the point of contamination will be much lower. Soil, water, and air sampling must always
be carried out downstream from the point of contamination. This does not mean that
sampling to the side, uphill, upstream, and upwind from the contamination is not
necessary. It is. This goes along with mass movement of soil as noted above, which must
also be kept in mind.
11.14. STATISTICAL ERRORS
In looking for sources of sampling errors most literature searches will provide a multitude
of references that are primarily statistical analyses. These are topics and papers that show
how statistics can be used to detect such things as errors. However, the use of the
incorrect statistical tool or method, including the incorrect use of hypothesis or method of
calculation, the use of an incorrect table, or the incorrect interpretation of the statistical
results in the analysis of data, are dangerous traps which must be avoided. To do this it is
highly recommended that an experienced statistician be part of the investigatory team or
be used in doing the statistical analysis on the data [4, 5].
11.15. CONCLUSIONS
Many traps, mistakes, and errors can take place during sampling and analysis of
environmental samples. Personnel and activities in the field are the two biggest sources of
errors in sampling and analysis. Knowing the history of an area, its geology, what is
present, and the rates of inputs, losses, and movement is also important to accurate
 
Search WWH ::




Custom Search