They have to find ways to transfer water from the basement of the reactor buildings to storage facilities. It's difficult but perhaps not impossible? And even if it's not going to succeed they still have the sub-drain systems.
Regarding the ground water: we have to separate two things, the sub-drain system and the water below the sub-drain system.
The water in the sub-drain system is collected to the sub-drain pits and from there it is pumped to a storage facility and from there it is transferred to the sea (in normal operation mode, because the water is clean, no harm done here). This is a way to control the movements/levels of the ground water underneath the reactors.
So the sub-drain system to some extent takes care of the polluted water underneath the plants. This is what they have already done in units #5 and #6, they pumped the water up from the pits and transferred it to the sea. The water was low-level contaminated so TEPCO thought it was safe to transfer it to the sea.
As for the units #1 - #4, the groundwater is more polluted. So the problem arises: How much water has accumulated in the sub-drain pits? What is TEPCO going to do with that water? As long as the sub-drain system is still working OK, we can assume that the contamination of the ground water is still "under control".
In one of the papers TEPCO gave us a small hint what they are going to do with the sub-drain water underneath #1 - #4:
http://www.tepco.co.jp/en/nu/fukushima-np/f1/images/f12np-gaiyou_e.pdf
(page 22)
But they have not given us any schedule, have they? This is one answer I am waiting from TEPCO, to give more detailed report of the sub-drains #1 - #4, their thoughts about where the polluted water has been coming and when they are going to empty the pits.
But there is also a more severe problem if some polluted water goes underneath the sub-drain systems. Some reasons leading to this might be: 1) The sub-drain systems do not cover the whole area of the plant. 2) The sub-drain systems might not work everywhere as supposed because of the tsunami/quake.
Underneath the sub-drain systems there is only the moving ground water, it's not under control. But even this situation is not necessarily a catastrophe. Because the inland deep well is not contaminated it gives us a hint that the ground water is moving towards the sea. This is also how it is usually supposed to work nea the sea, and the findings from the deep well support this scenario.
What would happen? The pollution in the ground water would dilute little by little into the sea probably not causing any dramatic rise in the radiation levels of the sea water. This is also what the overall trend has been after they were able to stop the mega-leak from the unit #2 pit to the sea: the radiation levels have been decreasing.
So what would actually happen, is this: the nature would take care of some of the cleaning processes of the Fukushima plant - even without TEPCO asking for it.