Environmental Engineering Reference
In-Depth Information
stratified are useful prior to sampling. In small streams (
20 ft wide), it is possible to
select a location where a grab sample can represent the entire cross-section. This
location can be at the mid-depth in moving water at the main flow line. This main
flow line is not necessarily the center of the stream, but can be identified by
observing the flow patterns across the channel. For larger streams and rivers,
multiple samples across the channel width are required at a specific sampling
location. As a minimum, one vertical composite (consisting of grab locations from
just below the surface, mid-depth, and just above the bottom) at the main flow line
would be necessary.
For fast flowing rivers and streams, it may be difficult to collect a mid-channel
sample; health and safety concerns must dictate where to collect the sample. When
sampling nearby a point pollution source, two samples from channel mid-depth are
typically drawn: one upstream and one adjacent to, or slightly downstream of the
point of discharge.
In small ponds and impoundments, a single vertical composite at the deepest
point is adequate to characterize the water body. This deepest point of a naturally
formed pond is generally near the center although this may need to be determined. For
lakes and larger impoundments, stratification from temperature difference is often
present in these bodies and is more prevalent than in rivers or streams. Therefore,
several vertical aliquots should be collected. In determining the degree of stratification,
measure DO, pH and temperature in an incremental depth of the water body.
Estuaries are areas where inland fresh water (both surface water and
groundwater) mixes with oceanic saline water of a higher density. Estuaries are
generally categorized as mixed, salt wedge, or oceanic, depending upon inflow and
mixing properties (EPA, 1995). These must be considered in determining site-
specific sampling locations. In addition, estuarine sampling should be conducted in
two phases, that is, during both wet and dry periods. When water depth is less than
10 ft, samples are collected at the mid-depth unless the salinity profile indicates the
presence of salinity stratification. In estuaries where water depth is greater than 10 ft,
water samples may be collected at the one-foot depth, mid-depth, and one foot from
the bottom. In addition, a horizontal sampler such as van Dorn sampler is often used
rather than the vertical Kemmerer sampler (Fig. 4.3).
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Practical tips
When sampling rivers, stream, or creeks, locate the area that exhibits the
greatest degree of cross-sectional homogeneity. Select a site immediately
downstream of a mixing zone. In the absence of mixing zones, select a site
without any immediate point sources such as tributaries, industrial, and
municipal effluents.
In many cases, compositing is needed to reduce the number of sample, but
not to composite samples for VOC, SVOC, TPH, and oil and grease analysis.
For the same reason, no headspace and vacuum filtration should be allowed
for VOCs.
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