Persistence of white phosphorus in a wetland

Extent of contamination of Eagle River Flats

Physical processes of Eagle River Flats

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Table 1. Concentrations of white phosphorus in ERF surface sediments

(CH2M 1997a).

No. of

Maximum

mean

(g/g)

Area

results

Based on tests conducted at an upland impact

to present a lethal dose to a mallard. In one study,

site, we determined that most of the white phos-

the largest particle isolated from collected sedi-

ments was 7.4 3.7 3.0 mm (82 mm3) and

phorus from the detonation of an 81-mm mortar

WP smoke round is deposited in a hot spot with a

weighed approximately 150 mg (Walsh et al.

diameter of approximately one meter at the point

1997). The surfaces of the recovered particles have

of impact. In these tests, substantial amounts of

been lustrous, indicating the lack of an oxide

white phosphorus were driven into the ground

layer, as would be expected in the anaerobic sedi-

within these hot spots. A several-meter-diameter

ments.

halo around the initial hot spot contained much

The size of the particles is important in terms of

lower levels of white phosphorus scattered from

persistence in unsaturated sediments. In mudflat

the initial point of detonation. Beyond this halo,

and intermittent pond sediments that periodically

only a very minor amount of white phosphorus is

dry below water saturation, the solid particles will

deposited. White phosphorus concentrations in

decrease in size due to sublimation from the parti-

the soil can vary three orders of magnitude over

cle surfaces (Walsh et al. 1995, 1996). Because

just a few meters because of this hot spot distribu-

small particles have greater surface to volume

tion (Walsh and Collins 1993).

ratios than large particles, a mass of white phos-

phorus will vaporize much faster if it is finely

divided into small particles than as a single large

Persistence of white phosphorus in a wetland

The cold, water-saturated, anaerobic sediments

lump.

of Eagle River Flats are ideal for storage of white

Temperature is the other major factor that

phosphorus particles. The rapid oxidation for

determines persistence in unsaturated sediments,

which white phosphorus is known takes place

because vapor pressure decreases exponentially

when the white phosphorus is in the vapor phase.

with temperature. During summers at ERF, typi-

The vapor pressure of solid white phosphorus is

cal surface (top 5 cm) sediment temperatures aver-

sufficiently high above 30C to result in spontane-

age around 15C with a diurnal variation of

approximately 10C (Fig. 2). At these temper-

ous ignition if exposed to air. To prevent this

spontaneous combustion in laboratory or com-

atures, sublimation of a 2-mm-diam. particle bur-

mercial settings, white phosphorus is stored and

ied in unsaturated sediment is predicted to take

about 90 days. Oxidation will presumably hasten

manipulated under water. Solid lumps of sub-

the loss. At ERF, the sediments of contaminated

merged white phosphorus can be stored indefin-

permanent ponds could be decontaminated if

itely, although the surface of the solid may devel-

op a thin oxidized coating.

they were allowed to desaturate. Desaturation

requires that either the sediments be removed to

Detonation of WP munitions at ERF has con-

an upland site (dredging) for treatment or water

taminated the sediments with uncombusted par-

from the pond be removed to allow in-situ drying

ticles of white phosphorus. Most of the particles,

determined by microscopic examination of the

(draining). Given the enormous expense involved

sediments, are 2 mm or less in diameter (Roebuck

in removing contaminated sediment for treat-

ment, such as is the case in dredging (Walsh and

et al. 1998). For perspective, a single particle with

Collins 1998), an in-situ approach is preferred.

dimensions greater than 1.5 mm is large enough

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