Now this is a wild guess on my part, not being a nuclear engineer and all, but I reckon that the foot is probably not just as lethal now it is inside this 'New Safe Confinement' thingy, unless they have gone and named it really, really badly.
The New Safe Confinement has a different purpose - to stop radioactive dust and debris from the reactor itself (a bit higher up) from spreading into the nearby area and being spread by the wind in case the hastily constructed original containment structure collapses (they are going to remove that old structure now the NSC is finished so that doesn't happen, I believe).
> Born of human error, continually generating copious heat, the Elephant’s Foot is still melting into the base of the Chernobyl nuclear power plant. If it hits ground water, it could trigger another catastrophic explosion or leach radioactive material into the water nearby residents drink.
I had never heard that it was "still melting into the base of the Chernobyl nuclear power plant". Given that it's only 10% as radioactive as it was initially, that seems unlikely.
While it reaching groundwater was definitely a concern in the 80’s, I don’t think it’s actually moved significantly or is expected to in the future.
"China Syndrome" was an influential film. But until Fukushima, I don't believe that we saw melted core "lava" reach groundwater.
The NSC is so we can dismantle the Sarcophagus (the old containment structure). The fear is a explosion caused by the collapsing Sarcophagus (or perhaps there is another structural concern) might let the Elephants Foot reach the waterbed underneath the empty man-made cooling reservoir.
Here is a cross-section[0] of reactor 4 that shows the Elephants Foot (corium) at the bottom. I wish it'd show how far deep until water. I can't find information as to whether water even exists, or if this was just speculation (but Pripyat is swampy/marshy).
Ex: A has a half life of 100 years and B has a half life of 10 years. If the sample starts at 100% A it will become more radioactive for the first few years until B reaches a steady state.