Part A: Well Log Interpretation
In this section, you will use information from well logs, records that drill loggers fill out
every time a well is drilled. In Idaho, these well logs are filed with the state and are a very import-
ant part of understanding water rights. The screened section of a well is the portion of the well
casing that is perforated and allows water to flow into the well (Figure 2). Ideally, the only loca-
tion that water will enter a well is in the screened section.
Below is a table with information from a driller's log. Use it to draw a cross section of the
aquifer in the figure on the next page (Figure 3), just as you would a geologic cross section. Be
sure to draw, color, and label all formations and features.
*Note* Colors do not necessarily need to reflect the color of the rock formation.
Well A
Depth (feet)
0-30
30-85
85-95
95-100
Formation
Brown Sand
Gravel
Black Sand
Blue Clay
Well B
Depth (feet)
0-15
15-35
35-55
55-75
Blue Clay
Depth (feet)
Feature
Feature
60
water
water
50-80
screen
screen
1) Using the above data and figure 3, create a geologic cross section of the area where these
wells were drilled. Correlate units over distance if you can, keep in mind that some may "pinch
out". Use "hash marks" to indicate where each well is screened. First fill in the geology in the area
between the two wells, then add the groundwater and screen information.
2) In what formation(s) is the groundwater located?
Depth (feet)
37
35-50
Formation
Soil
or
Brown Sand
Gravel
3) Is this aquifer confined or unconfined (is infiltrating water prevented from moving downward)?
120
4) What formation makes an aquitard that forms the bottom of this aquifer unit?
5) Towards which direction is the water in the aquifer flowing, east or west? Draw an arrow
indicating flow direction on your diagram.
6) What is the hydraulic gradient (change in elevation over change in distance) of the aquifer,
from west to east (1 mile = 5280 feet)?
hydraulic gradinet (unitless) =
distance)
7) Will someone pumping from well A most likely affect water levels in well B? Explain.
(
(
(
(
(
(
(
16
In what formation(s) is the ground water located