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- [Instructor] This video demonstrates
techniques to establish a permanent land surface datum,
nearby reference marks, and a measuring point
from which groundwater levels at a well are measured.
The materials needed include keys,
wrenches, or other tools to access the well,
an engineer's ruler or measuring tape graduated
in feet, tenths, and hundredths of feet,
a field notebook and a writing implement,
a groundwater site inventory,
GWSI System Groundwater Site Schedule form,
metal file and permanent marker,
spray paint, hammer and cold chisel,
survey monuments, such as a nail, spike, or tablet,
camera, protractor, calculator, or other tools
to calculate angles and lengths,
topographic map or rod, leveling instrument,
leveling note sheets, and differential GPS.
Step one, establish land surface datum.
The land surface datum at a well is a fixed reference mark
located at or near the land surface
that can be used to measure the altitude
of the land surface datum,
as well as the distance from the land surface datum
to the measuring point.
Locate a stable, permanent location near the well
to establish the land surface datum.
The land surface datum should be in a location
that can be clearly defined,
clearly marked, and easily located.
Additionally, the land surface datum
should be close enough to the well
to facilitate measuring from it to the measuring point,
and it should be accessible to a survey rod or GPS antenna.
Step two, mark the land surface datum
using a survey monument, brass marker,
or chiseled mark in the concrete pad
at the base of the surface casing.
Step three, take a photograph of the land surface datum
and document the description in the field notebook.
If the land owner does not allow marking of the well,
then describe the land surface datum
as accurately as possible.
Step four.
The altitude of the land surface datum
must be determined for each site.
Locate the well, following the instructions in GWPD 5.
At a minimum, the altitude can be estimated
from a topographic map.
Depending upon the use of the measurements from the well,
it may be preferable to determine the altitude
of the land surface datum using differential leveling
or differential GPS techniques.
Document the measurements in the
GWSI groundwater site schedule.
Step five, establish the measuring point.
The measuring point is a permanent, stable,
clearly-defined and marked reference point
used to measure the water level in a well.
It is often located at the top of the casing
in an observation well,
or at an access point at a production well.
To establish a measuring point for a well,
choose a point on the casing
where a leveling rod could be set directly on it,
and a measuring tape can hang freely into the well
when it is in contact with the measuring point.
If the casing does not have a horizontal rim,
it is preferable to establish the measuring point
on the high or low side of the rim.
Step six.
Using a file, lightly mark the measuring point
on the well casing.
Optionally, mark the measuring point
by a permanent marker or spray paint.
Step seven.
Take a photograph of the measuring point
and document the description in the field notebook.
If the measuring point cannot be marked
directly on the casing,
clearly define it in the field notes.
Step eight, measuring point correction length.
Depending on the use and storage
of measurements from the well,
the altitude of the measuring point of a well
with a vertical stick-up
may be surveyed from a geodetic benchmark
using differential leveling
or differential GPS techniques.
The measuring point correction length
is the distance the measuring tape travels
from the measuring point to the plane
of the land surface datum.
If the measuring point is above the land surface datum,
the measuring point correction is a positive number.
If the measuring point is below the land surface datum,
the measuring point correction is a negative number.
Simple vertical well.
Measure the measuring point correction length
in feet above or below the land surface datum directly
with a steel tape, or by differential leveling
if the objectives of the measurement
require a precise altitude.
More complicated non-vertical well.
If the well stick-up does not allow
the water level tape to hang vertically
from the measuring point through the plane
of the land surface datum,
the measuring point correction cannot be measured directly.
Instead, the length along the measurement path
between the measuring point and the land surface datum
must be computed using geometry.
The resulting measuring point correction length
will be greater than the vertical distance
between the land surface datum and the measuring point.
Step nine.
Document any calculations to determine
the measuring point correction length in the field notebook.
Document the measurements
in the GWSI Groundwater Site Schedule.
Step ten, establish additional reference marks.
A reference mark is a nearby datum
established by a permanent marker
that is used to periodically check the measuring point
and/or land surface datum,
or to reestablish the measuring point or land surface datum
should the original be destroyed or changed.
Establish at least one clearly-marked reference mark
near the well,
such as a benchmark, a lag bolt set in a telephone pole,
a spike in a mature tree, a mark on a permanent structure,
or a poured concrete post.
More than one reference mark is preferable.
Locate a reference mark a distance away from the well
to assure that circumstances that damage the well
do not also damage the reference mark.
Take photographs,
and record a description of the site reference marks
in the field notebook.
Step eleven, establish the vertical relation
between the measuring point and reference marks
by differential leveling.
Document the vertical relation between
the measuring point and reference marks.
At each subsequent site visit,
visually inspect the measuring point, land surface datum,
and reference marks.
Document any damage,
and recheck the vertical relation if damage is noted.
If no damage is apparent,
the vertical relation between the measuring point
and reference mark should be confirmed
at three-to-five year intervals.
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Accuracy.
Keep in mind the following accuracy limitations
of this method.
The accuracy of the measuring point, reference mark,
and land surface datum altitude
depends upon the measurement ethic used.
The accuracy of the measurement point correction length
depends upon the configuration of the measuring point
with respect to the land surface datum,
angle of deviation from vertical,
the ability of the hydrographer to determine the tape path,
and the measurement method used.
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