Sampling - Field Sampling: Principles and Practices in Environmental Analysis - page 125

the nitrate ISE was not sensitive or accurate enough for this work.
In analyzing the ten well water samples, the distinction between
contaminated and uncontaminated wells was obvious. The uncontaminated
wells gave no response. There were three wells that gave a response of 1000
ppm nitrate (NO 3 − ), however. This analysis allowed us to identify the
contaminated wells and to predict which other wells should be looked at using
more sensitive methods. It is important to point out that the sensitivity of the
ISE that I was using was 10 ppm NO 3 − , and that the allowable level of nitrate
in drinking water in some places is 10 ppm and in other places higher.
The ISE is an example of a rapid analytical method that is suitable to transect samples.
Although it can be used in the field without elaborate sample preparation, the field office
laboratory is a better place to make the analysis.
The ISE is similar in appearance and operation to a pH electrode. Indeed, the more
sophisticated pH meters are able to accommodate many different ISEs. The use of an ISE
is as quick and easy as determining pH using a pH meter. Typically the time for doing an
ISE analysis is less than 1 min. Portable pH meters, which can do both pH and ISE
analysis, are readily available [14, 15].
Transect sample results can be combined with detailed sampling data to confirm the
detailed sampling data and to make statistical calculations as to the number of samples
needed. These analytical results will also provide information that can be used to
determine if a sample is extraneous or is indeed part of the population being sampled.
More information on this can be found in Chapter 6.
During transect sampling variations in soils and soil types in the sampling area are
noted but ignored. (See Figure 5.2.) However, when the detailed sampling plan is
designed, the soil and soil types present must be taken into consideration. This means that
the whole sampling plan may be designed without regard to soil types. The soil types are
recognized when samples are taken and analyzed, however. In this case the batch number
and the number on each individual sample must be unique, so that they can be identified
later if needed as being from one soil type [11].
When the transect samples have been taken and analyzed, a detailed sampling plan can
be developed. This plan will take into consideration observations and analysis made
during transect sampling and the statistical analysis of the analytical results. (Statistical
analysis will be described in Chapter 6.)
5.10.2. Detailed Sampling
Detailed sampling can be of four types. The first is sampling to determine the levels of
plant nutrients in a field. The second is sampling to determine the extent of contamination
so that the contaminated material can be removed for remediation. In this case, the field
will only be sampled once, and subsequent sampling in this location is not expected to
occur, although monitoring may be called for. In this case, it is important to identify all
the contaminated material and assure that it is safely removed. This is coupled to the third
type, which involves sampling the material once it is in its new location. This will entail