CRREL, 300 g of oven-dried ERF sediment ab-
to air shrinks and grows in a cyclic pattern
sorbed 160 g of water, indicating that the satura-
throughout the summer. If the sediments are ex-
posed for a sufficient length of time, their mois-
tion was 53%.
ture content may fall below saturation. While un-
Based on these three estimates, the moisture
consolidated pond bottom sediments in perma-
level at saturation is in the range of 5055% for
nently flooded areas have moisture contents up
the homogenized sediment in the lab. To simulate
to 200% on a dry weight basis, sediments in inter-
moisture levels that may be expected in air-ex-
mittent ponds have much lower moisture con-
posed ERF sediments, the following levels were
tents. For example, in the top 10 cm of exposed
tested: 25% (s = 0.45), 35% (s = 0.64) (measured in
pond sediments in early July 1993, sediment
field in July 1993), 45% (s = 0.82), 55% (s = 1), 65%
moisture levels were between 30 and 35% (on a
(s >1), where s is the degree of saturation.
dry weight basis). Moisture levels between 35
and 40% were measured down to 30 cm deep.
Labeling
Since these moisture levels represent conditions
At each temperature, sediments were moist-
that exist for extended periods between high
ened to one of five moisture contents (Table 3). At
tides, moisture levels in this range were chosen
each moisture content, there were 25 replicates
for this experiment.
prepared. Therefore, each test at a given tempera-
The moisture content at saturation was esti-
ture required 125 sediment samples in individual
mated by a variety of techniques. One way was
plastic containers. Plastic (high-density polyeth-
ylene) containers (8 8 6 cm) were labeled with
by making assumptions based on the properties
of clay-size sediments. This approach is shown
the incubation temperature, the desired moisture
below:
content and a number between 1 and 25 (for ex-
ample: 20C--55%--12).
Assume 100 g of oven-dried clay, with 60%
porosity and a density of 2.65 g/cm3
Pretreatment of sediments
Then:
The equivalent of 300 g of oven-dried sedi-
ment was placed in each of 125 containers for
Volume of solids = 100 g 2.65 g/cm3 = 37.7
each test temperature. The actual amount of sedi-
cm3 = (1 0.60) Total Volume
ment added depended on the moisture content of
Total Volume = 37.7 cm3 (1 0.60) = 94.3
the air-dried sediment. For example, if the mois-
cm3
ture content of the air-dried sediment was deter-
mined to be 1.75% (on a dry weight basis), then
Volume of voids = 94.3 cm3 37.7 cm3 = 56.6
each dish was filled with 305 g of air-dried sedi-
cm3.
ment (300 + 0.0175 300 = 305). Then reagent-
If all voids are filled with water, then:
grade water (MilliQ, Millipore Corporation) was
added to the obtain the desired moisture content.
% moisture = (56.6 cm3 1 g/cm3) 100 g =
For example, a sediment sample containing the
56.6%.
equivalent of 300 g of oven-dried sediment wet-
A tensiometer was also used to estimate the
ted to 55% moisture (on a dry weight basis) re-
quired 165 g water (0.55 300 = 165). Since air-
moisture content at saturation. Pore water ten-
dried sediment had 5.25 g of water associated
sion, expressed as negative millibars, is zero at or
with it, only 160 g of water had to be added to
above saturation and increases (actually becomes
obtain a sample at 55% moisture. Samples were
negative) below saturation. In the laboratory at
CRREL, sediment samples from ERF were wetted
wetted at least three days prior to placement of
to obtain moisture contents over the range of 35
the white phosphorus particles. Samples were
60% moisture (using the procedure given below),
equilibrated at the appropriate temperature and
and a tensiometer from Soil Measurements Sys-
rewetted daily to replace water lost by evapora-
tems was used to measure the pore water tension.
tion. Prewetting of the sediment served to com-
Based on these measurements, the saturation was
pact the sediment to more closely approximate
between 50 and 55% moisture (on a dry weight
field conditions.
basis).
Another experimental approach was to gradu-
Placement of P4 particles
ally add water to a known amount of sediment
At the two lowest moisture contents, a 3-cm-
until no further water was absorbed. In the lab at
long core was removed from the center of each
40
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