Environmental Sampling Design - Fundamentals of Environmental Sampling and Analysis - page 57

The grid spacing (L) in one-dimensional systematic sampling (such as sampling
in time or along a line in a stream) is easy to calculate. First, calculate the spacing
interval k¼N/n, where N is the total population units and n is the predetermined
number of samples. Round k to the nearest integer. Then randomly pick a starting
position/time point, and select the second point k distance from the first point, the
third k distance from the second, and so on until n samples have been defined. For
example, to design a 5-day sample scheme in a month of 31 days, k ¼ 31 = 5 ¼

6 2 6. If we randomly choose a random number 3 from 1 to 31 using a random

number table, this would give us the sampling days of 3, 9, 15, 21, and 26.
The grid spacing (L) in a two-dimensional area can also be readily calculated
once we know the total number of samples to be collected (n) and the area to be
sampled (A). This results in the following equations:

:

n p for square grid A L ¼ = ð3 : 7Þ

866 p for triangular grid A L ¼ = 0 : ð3 : 8Þ

3.3.5 Other Sampling Designs

So far we have discussed three commonly used probabilistic sampling designs—
simple random sampling, stratified random sampling, and systematic sampling.
There are also a number of other design strategies that may be useful in

environmental data collection. They are briefly described below.
If sampling cost is much less than analytical cost, which is typical for many
trace analysis, composite sampling is a valuable means in cost-saving. If the goal is
to estimate the average concentration rather than the variability or extreme
concentrations for grab samples, this physical mixing will provide the same degree
of precision and accuracy as the mathematically computed average from the analysis
of all samples. Compositing is frequently done in the monitoring of wastewater
discharge and contaminated soils. One exception is that it should not be used for
volatile organic compounds, because mixing is not allowed for these compounds.
If the primary goal is to locate the hot spots (areas with elevated contamination
that exceed applicable clean-up standards), then search sampling is an effective
design approach. Search sampling utilizes either a systematic grid or systematic
random sampling approach to define the minimum grid size in order to locate them.
Generally, the smaller the hot spots are, the smaller the grid spacing must be. In other
cases, an acceptable error of missing hot spots is given; then the smaller the
allowable error is, the smaller the grid spacing must be. Mathematical equations are
available for interested readers.
Transect sampling is another variation of systematic grid sampling. It involves
establishing one or more transect lines across a surface. Samples are collected at
regular intervals along the transect lines at the surface and/or at one or more given
depths. Multiple transect lines may be parallel or nonparallel to one another. The
primary benefit of transect sampling vs. systematic grid sampling is the ease of