Characteristics of the Environment - Field Sampling: Principles and Practices in Environmental Analysis - page 45

what are called alluvial fans, which are similar to deltas except that they are deposited on
dry land. In addition to simply carrying material and depositing it, streams and rivers tend
to meander. Over a number of years a stream might move from one side of its valley to
the other, completely reworking the landscape in the process. Contaminated materials
may be buried in all three of these situations. Figure 2.9 shows a soil profile with buried
material.
In practical terms this means that contaminated material in the stream may be buried
anyplace in the floodplain (Figure 2.10). A field sampling plan must take this potential
burying of contamination into consideration. Also, all floodplains are subject to flooding,
and thus are not appropriate places to store hazardous materials or to set up field office
buildings, particularly those for storage of equipment and samples.
Also as seen in Figure 2.10, a contaminant on a hillside may be closer to the soil
surface at the top of the hill and deeper lower on the hill, because erosion will remove
soil from the top of the hill and deposit it at the bottom. Gravity can also move soil down
a slope and thus tends to bury contaminated areas. Soil moved under the influence of
gravity is said to be colluvial. Figure 2.11 shows a hillside where material has moved
under the influence of gravity.
FIGURE 2.10 Stream terraces and stream meandering across a stream valley.