The difference in length of persistence at 20C
dized. Factors that slow the change of P4 from a
and 15C in unsaturated sediments may be due to
solid to a vapor are the same as those that limit
oxidation. As discussed earlier, P4O10 is formed
diffusion. Diffusion continues as long as there is
by a branching-chain reaction (Dainton and Bev-
free pore space (Baver 1956). Free pore space,
ington 1946, Dainton and Kimberley 1950). The
however, is reduced by compaction or increased
reaction is slow unless the partial pressures of
wetness.
oxygen and P4 vapor are within certain limits.
Monitoring sediment moisture and
Since the lower limit is inversely proportional to
temperature in Eagle River Flats
the pressure of P4, the vapor pressure at 15C may
be below the critical pressure for a fast reaction. A
Methods
value for the critical pressure is difficult to predict
Eagle River Flats (ERF) is an 865-ha estuarine
since it also depends on the diameter of the reac-
salt marsh at the mouth of the Eagle River on Knik
tion vessel, in this case, the pore diameter. The
Arm in upper Cook Inlet (Fig. 11). As described by
"wall" of the vessel terminates the reaction by
Racine et al. (1993), the main channel of the Eagle
scavenging the oxygen atoms, which are the
River meanders generally through the middle of
chain centers. The vapor pressure at 20C may be
ERF. On either side of the river, levees, mudflats,
marshes, meadows and intermittent and perma-
high enough for a fast reaction. Since the oxida-
tion of intermediate products (P4On, n = 2...9) to
nent ponds occur in zones determined by eleva-
P4O10 is highly exothermic, the temperature sur-
tion, which in turn affects salinity, frequency of
rounding the particle will rise. Any rise in tem-
tidal inundation and sedimentation rate. Perma-
perature near the surface of the P4 particle would
nent ponds along the edge of ERF are also influ-
enced by freshwater influx.
be accompanied by an increase in vapor pressure,
The field experiment to monitor sediment tem-
and therefore greater production of P4O10 and
perature and moisture was conducted in a part of
more heat, until the supply of P4 falls below the
ERF known as "Area C" (Fig. 12). Located on the
critical pressure as the particle diminishes in size.
east side of the Eagle River, Area C includes a sin-
Oxygen is not required for the loss of P4 in un-
gle large pond and several interconnected smaller
saturated conditions. The vapor pressure is high
ponds and inlets (Fig. 13). The main pond (10 ha)
enough that loss will occur as long as there is free
pore space and hence gas exchange between sedi-
is bordered by bulrush and sedge marsh to the
ment air and the atmosphere. For example, when
east and south, mudflats to the west and deeper
field-contaminated sediments were dried at room
small pools with bulrush marsh to the north.
temperature under nitrogen and under air, the
Fresh water enters from the east edge, and three
loss was the same after 60 days (Walsh et al. 1993).
distributary channels drain the area to the west.
The formation of oxidation products in the pore
Distributary channels also allow tidal inflow into
spaces surrounding P4 particles may actually
the pond during flooding high tides. The frequen-
slow loss by forming a diffusion barrier.
cy of flooding during the summer depends on the
In our laboratory experiment, sediment mois-
maximum height of the monthly series of peak
ture was the more important of the two factors
high tides. Monthly peak high tides above about
tested in terms of persistence of P4 in ERF sedi-
31 ft at Anchorage (or about 4.79 m mean sea
ments. As a solid particle in saturated sedi-
ment with free pore space, P4 can change to
the vapor phase and diffuse away or be oxi-
Figure 11. Aerial view looking south across Ea-
gle River Flats.
13
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