Traps, Mistakes, and Errors - Field Sampling: Principles and Practices in Environmental Analysis - page 280

11.2. AREA GEOLOGY
Knowledge of the geology of an area can be important in developing and carrying out a
successful sampling plan and avoiding traps and mistakes. Two dramatic examples of this
would be volcanic ash and karst landscapes. Both of these situations are unusual and can
dramatically affect a sampling plan. Volcanic material may contain high levels of gravel-
sized particles that facilitate movement of contaminants under saturated flow conditions.
These soils often have very small amounts of clay, and this also leads to rapid movement
of contaminants.
Karst landscapes develop in limestone rock and result in surface sinkholes and
subsurface cave systems. In this type of landscape water, soil, and contaminants can
move both on the surface and below it. Material can move to the underground cave
system and then long distances until it emerges in a well, stream, river, or lake. Samplers
need to be aware of the geology of the area in which the sampling is taking place and take
into account the movement of material in different geological localities when developing
a sampling plan.
It might be assumed that water and pollutants cannot move through solid rock, but
exceptions exist. One is sandstone, which can be very porous and allow water to pass
through it as rapidly as it passes through soil or sand. The second is the fact that in many
cases underlying rock is not solid but fractured; that is, it contains many cracks through
which water and pollutants can readily pass. Not knowing the type of underlying rock
and its condition can lead to misunderstanding the fate of a component or contaminant in
the environment.
In certain localities it is possible to have both soil and water move long distances. Soil
can erode from sloping fields, in which case the amount of erosion and the final
deposition of the eroded soil will affect the sampling plan. In addition to the erosion, soil
can move downhill under the pull of gravity. This is termed colluvial movement and can
result in the movement of large amounts of soil and regolith or underlying glacial till
down a hillside, further resulting in significant movement of contamination if present.
Both of these potential sources of movement must be accounted for in a sampling plan. If
they are not, it is likely that not all the contamination will be discovered.
Table 11.2 shows soil analytical results that would be typical for a farm in central Ohio.
At first glance, Table 11.2 does not look unusual except that sample WC-15c has a pH of
7.4, while all the other samples have acid pHs. One might then question the validity of
this pH measurement. Statistical analysis would say that this is an outlier and could be
discarded. However, knowing the geology of this area one would know that the regolith
is basic while the overlying soil is acidic. This indicates that the sample is from an area in
the field that has suffered severe erosion. An inspection of the field would show that this
is indeed the case. Not knowing the geology of an area one would not be aware of this,
and thus accurate data would be discarded without regard to its importance to sampling
and analysis.
Another area of concern is water movement. Water can move rapidly over the surface
of soil and carry contaminants into nearby streams, rivers,