Environmental Sampling Techniques - Fundamentals of Environmental Sampling and Analysis - page 89

Unlike surface water sampling, you cannot just get to a well and collect the
groundwater sample. You need to first purge the stagnant water by monitor-
ing conditions of water (pH, temp, level, etc.) until the water is stabilized.
Always keep in mind the potential contamination for the trace analysis of
contaminants. Misrepresentative data can be a result of a combination of
several sources due to well materials, pump, tubing, and containers.
Try to minimize the aeration during sampling. Bailers are often used for both
purging and sampling, but they cause mixing, collection of unwanted
particulates from well bottom or casing, and aerating or degassing VOCs
from samples. Bladder pumps are generally the best under many circum-
stances.
Sampling sequences follow the order of VOCs, SVOCs, metals, PCBs/
pesticides/herbicides, and water quality parameters.
4.2.3 Soil and Sediment Sampling
Soil sampling at a shallow depth is relatively an easy task. Sediment sampling is
analogous to soil sampling at some extent, for example, with regard to sample
compositing and post-sampling pretreatment (homogenizing, splitting, drying, and
sieving). Both soil and sediment can be collected with a grab sampler (scoop and
trowel) for surficial samples or a core sampler if a vertical integrity needs to be
preserved. For sediment sampling, the corer is to further retain fine particles. It is
critical to remove extraneous materials such as rocks, gravels, and debris such as
plant leaves, roots, shells, and insects. But note that not all external materials are
extraneous such as fly ash or waste that are part of the sample characteristics. In such
a case, collect samples of any material thought to be a potential source of
contamination. Composite sampling during field sampling is common for both soil
and sediment because of the suspected heterogeneity and the need for savings of
analytical costs. The only exception is for VOCs analysis since mixing,
homogenizing, and composting of soil or sediment samples containing VOCs are
not permitted. Another common error can be introduced in sample sieving and
mixing procedures is to inappropriately discard nonsoil/sediment or nonsieved
materials (EPA, 1989).
Additional concerns for sediment sampling include factors that affect sample
representativeness, such as waterway width, flow, and bottom characteristics.
Sediments are typically heterogeneous in composition and are subject to variation in
texture, bulk composition, water content, and pollutant content. Therefore, large
numbers of samples may be required to characterize a small area. Generally, samples
are collected at quarter points along the cross-section of the water body and then are
combined into a single composite sample. For small streams, one single sediment
sample collected at the main flow line of the water body may be sufficient. Whenever
possible, a sediment sample is collected at the same location as a surface water sample.
Sediment samples from lakes, ponds and reservoirs should be collected in
approximately the deepest point of the water body. This is especially applicable to