Whatever this substance is, the number is extremely high.
About this "black dust" in Minami Soma, see my past posts, here, here, here.
Minami Soma City's Assemblyman Ooyama reports on his blog (3/8/2012):
Cs-134 1,498,000 ± 1,586 Bq/kg
Cs-137 1,932,900 ± 2,481 Bq/kg
TOTAL 3,430,900± 2,945 Bq/kg
It seems to be part of the report by Professor Tomoya Yamauchi of Kobe University who did the measurement. Mr. Ooyama lists the result of 11 samples tested by Professor Yamauchi.
The NGO in Minami Soma, HCR, seems to have some kind of a fallout with the assemblyman's volunteer group over the "black dust", and they've been making rather wild announcements (telling residents that the black dust was plutonium, when there was no alpha emission was detected, for example). I am still trying to get the overall picture on this matter.