How much ground current vs. neutral wire current?
I've been reading about this phenomenon after reading 
your question and the paper you posted . I can't see how 
that much current could be travelling through ground unless 
a similar percentage of Minnesota either uses SWER or is
 wired incorrectly. If that's not the case then the statement 
must be false.

Claims of current travelling through plumbing 
is a problem with incorrect wiring for sure. If there is a 
solid connection from the neutral at the service, to the 
transformer in the street, to the transformer in substation 
and back to the plant then it must be lower than 59%. 
However there is a parallel path through ground rods connected 
to transformers throughout the grid and at services. I can't 
see it accounting for such a large percentage of current though. 

I measured the resistance between 15 and 25 KOhms through 
1 foot of damp soil with my multimeter. (Curiously it was rising
steadily like I had my meter on a capacitor and then would drop
and restart at different values with no particular pattern). 
Compared to the resistance of utility wires it is obviously very
high.

I have seen (I'm an electrician) a single phase 100a service
lose the neutral in the meter socket. The customer called because 
the screws connecting sections of the baseboard heater (hot water)
were glowing red. All of the power being used in the house was
travelling through any path to ground it could find.