Contamination from Local Fallout
the activity at carly time intervals. The principal isotopes present at the early
intervals are Sr, Sr#0-Y90, Zr9-Nb*, Rulve-Rhies, Tel2t-129, ]191-182-183-135,
Cs187-Bal87, Balt9-Lal#9, Celt#-Pri4s, Pri47, and Np23*.
Soil
The fallout material appeared initially to be associated with relatively large
particulate matter and to be distributed uniformly over the soil. The residual
activity on the islands was initially contained primarily in the top few inches
of soil.
Radioanalvysis of soil profiles at ] vear indicated that’ little translocation of
the actwity had occurred. Cs!87, Ce!4#4-Pr44, and Ru!*'-Rh}6 constituted the
largest portion of fixed contamination in the soil at this time.
Activity in the lagoon bottomsilt at ] vear appeared to be distributed rather
uniformly to a depth of 6 or 7 inches. Four samples of silt from the northeast
corner of the Rongelap Atoll lagoon had levels of beta activist, ranging from
SANG te 12.0% d msg. The samples were collected at water depths of 40 to
]2uU tt. The activity in lagoon bottom silt is of interest, since it provides a reser-
voir of activity available for future incorporation into marine food specimens.
At 2 vears following the contaminating cvent. most of the activity was still
armk nmxcd te the surface laver (0 to | inchs of soil, Loss of activity from the
surface Javer in the period | to 2 vears was largely the result of radioactive decay
rather than the result of leaching or erosion. At 2 vears, the rare carths, Rul",
Sr") and Cs!* constituted $4, 10, 5 and 1 per cent of the activity respectively
on Rongelap Island.
The Sr’ concentration in the surface soil of the island at 2 years is shown in
table 2. The level of Sr®°/Ca in the soil of Rongelap was 3.4 & 108 yuc Sr8°/g
of exchangeable calcium. The level of a soil sample from Gejen (the northernmost island) was exceptionally high, 6.2 x 10° pyc Sr*’/g exchangeable calcium. These concentrations correspond roughly to the external gamma dose
rates measured on the islands.
The extractability of Sr9° (ie., the ability of plants to extract Sr? from
soil) depends on the soil type, the solubility of the Sr" compounds, water
depth, pH] of soil, moisture and organic matter {Blume and Smith, 1954). In
addition to the Srextractability from the soil, the amount of cxchangcable
calcium in the soil is also a verv important factor. The ability of plants to take
up Sr*" is inversely related to the amount of exchangeable calcium in the soil
'Fuller and Flocker, 1955). For example, plants grown in acid soils low in
calcium take up the largest amounts of Sr, while plants in alkaline calcareous
soil take up the Icast. In the last column of the table (table 2). the ratios of
Sr"/exchangcable calcium ate tabulated. While the calcium content of the
various soi] types analyzed ranges from 23 to 35 per cent, the exchangeable
calcium in the soil represents only a small fraction of the total calcium.
309