Environmental Engineering Reference
In-Depth Information
by a qualified professional. Therefore, some forensics are required. Cross-referencing
the geologic information obtained during an environmental investigation with a water
supply well log that was installed very close to the investigation site can provide infor-
mation on the accuracy and ultimately acceptability of the water supply well log. For
instance, assume a water supply well was installed to a depth of 100 m on the same
property where an environmental subsurface investigation was conducted to a depth of
15 m. If the water well log correctly interpreted the geology in the upper 15 m, a reason-
able conclusion is that the water well driller correctly interpreted the geology at depths
greater than 15 m.
There is another point regarding the installation of water wells worth mentioning here
from a forensic point of view. Historically, it was not uncommon for a qualified geologist
to accompany water supply well drillers during drilling operations, because this was the
only source of affordable geologic information. On many occasions, the geologist's name
would appear on the water supply well log (Sherzer 1913; Mozola 1954, 1969). There is some
benefit then to consult with geologists in the area who were historically employed by the
United States Geological Survey, State Geological Survey, or who teach at local colleges
and universities. This action will help evaluate and cross-reference the information when
reviewing historical water supply well logs.
Geophysical information in various forms is capable of providing valuable geologic
information to assist in geologic mapping both horizontally and vertically. Geophysical
information is collectable in most urban areas without causing any disturbance and can
be extremely useful in interpreting data between boreholes where the geology is known.
Geophysical information is most helpful when the areas are inaccessible to other methods
of collecting geologic or hydrogeologic information and within areas of complex geology.
5.2.3 Building the Conceptual Geologic Model
Following the review of existing information, a conceptual geologic model can generally be
constructed. The conceptual model is a working draft geologic map of the desired region
that identifies areas by their basic geologic processes. For instance, most urban areas are
composed of unconsolidated sediments resulting from more than one type of geologic
process; e.g., fluvial, glacial, alluvial, and marine. The conceptual model simply identifies
geologic processes that formed the deposits along with their general location and horizon-
tal and vertical boundaries.
In many cases, enough information has been gathered to allow the initially identified
geologic processes to be subdivided into individual geologic units or formations. It is dur-
ing this step when data gaps and difficulties in interpreting geologic information usually
become apparent. These data gaps then form the basis for concentrating the field work
activities described in the next section. Listing every data gap, irregularity, or area of inter-
est is recommended until each has been resolved with adequate certainty. After each data
gap, irregularity, or area of interest has been addressed, reinterpretation of the conceptual
geologic model is recommended to evaluate if any changes are necessary.
Another important decision to make while building the conceptual geologic model is
whether anthropogenic deposits such as fill material and natural deposits, such as top
soil or peat, should be included in the mapping process. Fill material is usually noted,
but not included on a geologic map unless the areal extent is significant. Top soil may be
noted at specific locations, but is generally not mapped unless the deposit is either (1) sig-
nificant in areal and vertical extent and/or (2) represents a peat or other similar material
resulting from deposition in a current or former swamp, bog, or wetland as described in
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