This is the correct thought process. The linked article from llnl [1] states that “The final plan called for (1) removing all radioactive and non-radioactive debris (equipment, concrete, scrap metal, etc.), (2) removing all soil that exceeded 14.8 Bq (400 pCi) of plutonium per gram of soil, (3) removing or amending soil between 1.48 and 14.8 Bq (40 and 400 pCi) of plutonium per gram of soil, determined on a case-by-case basis depending on ultimate land-use, and 4) disposing and stabilizing all this accumulated radioactive waste into a crater on Runit Island and capping it with a concrete dome.” It goes on to state that “A estimated total of 73,000 cubic meters of surface soil...was recovered by scapping and deposited in Cactus crater on Runit Island.”

Unfortunately, it does not provide an average specific activity for the material stored, only a lower limit threshold. If we assume it was near this lower limit of 14.8 bq threshold (could be the case depending on how often case (3) was used) and assume mostly plutonium-239. Since isotopic distribution is unknown, this is a conservative assumption because the longer half-life will yield a larger mass in the calculation. The molar density can be calculated from the specific activity listed:

Na/m = a x hl/ln(2) =
14.8[decay/s] x 23110[yrs]/ln(2)
= 1.57e13 atoms/gram
= 2.61e-11 mol/gram

The mass of the dirt collected will be assumed to be wet sand with a density of 1905 kg/m^3 [2] (note that density is actually slightly higher than this due to the plutonium contained). And finally, the volume collected is 73,000 cubic meters, or 1.39e11 grams. So the total amount of plutonium is:

2.61e-11[mol/g] x 1.39e11[g] = 3.63 mol

So our envelope calculation yields about 870 grams of plutonium. So how accurate is this? It is probably on the high side after reading the paper posted by Shivetya [3]. They cite a total inventory of 545 GBq. Using the assumptions above, one would expect an inventory of 2058 GBq.

As for the effects, that’s difficult to tell. Really the two scenarios that come to mind are a constant leaching into the environment or a disaster that results in the total mass being dumped. In the latter case a disaster of that magnitude would likely disperse the material in a biased direction. In this case it would dilute relatively quickly. Impacts from the radiation would probably be apparent close by in the biased direction. A constant leak would depend on the leach rate and leak locations. The NRC defines monthly sewer release limits of material in Table 3 of 10 CFR Part 20 Appendix B - standards for protection against radiation. The number given for Pu is 84 uCi or 3.108e6 Bq. Based on the plutonium inventory calculated and neglecting material decay, the material could leach out within NRC limits over a period of 55 thousand years (2058e9/3.108e6/12). Note that this number is likely highly inflated given the assumption that all radioactive material is Pu-239 and the comparison in [3]. Using the number from [3] and assuming all activity is Pu, the duration would be 14 thousand years. However this is a rough analysis that does not account for isotopic distributions, so please take the results with a grain of salt.

[1]:https://marshallislands.llnl.gov/enewetak.php#cleanup

[2]:https://www.engineeringtoolbox.com/amp/dirt-mud-densities-d_...

[3]:https://marshallislands.llnl.gov/ccc/Hamilton_LLNL-TR-648143...