Environmental Engineering Reference
In-Depth Information
electrode (Fig.
7.5
). Electrodes should be installed to a depth of 10 cm below
the soil surface (which will be 15 cm above the deep electrodes) and should be
labeled as electrodes 10A, 10B, and 10C.
7.
Taking initial Eh measurements (prior to saturation)
. Using either a lab grade
Eh meter or a multimeter in conjunction with a device to create a high
resistance circuit (Rabenhorst et al.
2009
; Rabenhorst
2009
), the voltage should
be measured in the circuit created between the calomel reference electrode and
each Pt electrode. The positive (red) wire should attach to the Pt electrode and
the black wire should connect to the reference electrode. When the electrodes
have been recently installed, there may be some slight drift during the mea-
surement, but this drift should become less apparent on subsequent days.
Typically, these measurements are recorded to the nearest 0.001 V (note
there is too much variability to warrant recording with any greater precision
than this so make sure you are NOT reading to tenths of a mV). Commonly,
students will occasionally reverse the wires on the volt meter when making
measurements. This will result in VERY LARGE ERRORS, because the
voltage will have the opposite sign (
300 mV vs. 300 mV). Be very careful
to ensure that the Pt electrode leads to the red (+) pole on the volt meter and the
reference electrode is connected to the black (
) pole. Your initial readings
will probably be somewhere in the range of 200-400 mV (before correction for
the reference electrode). Over time, the voltages will likely become lower
(especially for the deep electrodes installed below the water table.) Note that
pH measurements must also be obtained at the same 2 depths where the
electrodes will be placed. It is recommended that soil pH at 10 and 25 cm be
collected from a replicate mesocosm so that the instrumented mesocosm does
not need to damaged.
8.
Saturating the mesocosm
. Mesocosms will be saturated to a depth of 15 cm
below the soil surface. In order to saturate each mesocosm, stand the
mesocosm vertically in a container (bucket) where the water can be adjusted
to the proper height, and secure it using duct tape as shown in Fig.
7.6
.
Distilled water should then be added slowly to the bucket which will result
in filling the soil pore space in the mesocosm from below, which should help
minimize the entrapment of air during saturation. The water level should be
raised until it is at the appropriate height (Fig.
7.7
). An alternate arrangement
isalsoshowninFig.
7.7
where the mesocosm is saturated to the soil surface.
This can be used to illustrate differences in properties of contrasting soil
horizons (such as OM content in A vs. B horizons)ifdifferentsoilmaterials
occur at 10 vs. 25 cm.
9.
Eh measurements after saturation
. Approximately 30-60 min after saturation,
collect the second set of Eh measurements from the mesocosm as described
previously. After this, Eh measurements should be made on the mesocosms
daily for the first week, and then every other day through subsequent weeks.
This should be continued for a minimum of 2 weeks and may produce better
results if extended for 3 or 4 weeks. The level of the water in the bucket will
need to be checked and maintained at the proper height.
