Tokyo Shinbun has a pictorial showing the locations of the known trenches along the seawall. I think it is more helpful than TEPCO's pictorial, which does not show the embankment which is clearly created artificially. It also nicely summarizes the entire article.
It shows the possibility of seawater and groundwater mixing beneath the sea level. If the highly contaminated water in the trenches from the turbine buildings to the water intake along the embankment bordering the open culvert are leaking somewhere, they will go to the layers of crushed stones underneath the trenches (standard construction method) and leak to the surrounding soil
From Tokyo Shinbun (7/26/2013, English labels are by me):
Tokyo Shinbun's article itself has some misunderstanding:
The biggest problem is that highly contaminated water is flowing into the trenches from the [turbine building] basements which hold 75,000 tonnes of such water.
No it isn't. Tokyo Shinbun doesn't know or forgot to check the density of radioactive materials found this time from the seawater pipe trench for Reactor 2. The density is one order of magnitude GREATER than the density of contaminated water inside the Reactor 2 turbine building, indicating the water in the trench is the same water that leaked into the ocean in April 2011.
I see two big problems, one of which is shared by Tokyo Shinbun.
"Crisscrossing trenches run several meters from the ground surface all the way down to 30 meters deep, and TEPCO doesn't exactly know what their conditions are."
I don't think TEPCO knows what trenches are there, where, or how many. The original blueprint wouldn't help, and all the blueprints of site and facility upgrades are, I believe, still in the condemned building at the plant.
And second, solidifying the artificial soil using the chemicals are possible only up to 20 meters deep.
Some trenches are 30 meters deep? Uh oh.
Since April 2011, TEPCO knows the highly contaminated water from the turbine buildings filled the trenches that goes out to the water intake, but didn't do anything about it until now. Even now, I don't know how they even attempt to empty the trenches, as the water is just too radioactive.
TEPCO's 7/26/2013 handout shows the air dose level as high as 100 millisieverts/hour, and that's when measured from the top of the pit at the turbine building where the trench starts, not near the water surface.