Tepco measures Tritium concentration at 290Bq/L in ground water at a location near Fukushima-Daiichi. Nearby, on July 18, it was 1,700Bq/L.Using numbers from this WHO page. You can calculate that for every Becquerel (Bq) of water contaminated with Tritium, you will get 1.8E-8 mSv of exposure for an adult.

— NucNet Nuclear News (@NucNetupdates) July 25, 2013

1.8E-8 mSv/Bq for tritium

Thus, we can calculate for worst case of 1,700 Bq/L. This will give us 1700Bq * 1.8E-8 = 30,3E-6 mSv per liter consumed by 1 adult.

Let assume now that this adult will drink 2 liters of this water at this concentration every week for 1 year. Which is practically impossible, but just for calculating a worst case scenario.

This give us: 30E-6 mSv/L * 2 L/week * 52 weeks/year = 0,00312 mSv/year or 3,12 uSv/year.

Is this a big radiation exposure?

You can compare this to the exposure I measured while taking is trip from Montreal to Toronto

http://radio-activity-studies.blogspot.ca/2013/01/radiation-durant-un-voyage-en-avion.html

Where I was exposed for a less that 1 hour to 350uR/hr or 3.5uSv/hr.

Conclusion is, you will be exposed to more radiation on a typical plane trip than drinking 2 liters per week for 1 years of Fukushima Tritium contaminated water.

Did I made a mistake on my calculation? Let me know.