This is an attempt to estimate the total radioactivity in the intake canal of the plant. The canal is like a small harbour in front of the turbine buildings, about 450 m long, and open to the sea. Every day TEPCO publishes sampling data for various points in the canal, measured in Becquerels per litre. By averaging those sampled measures, and multiplying by an the number of litres in the canal, the following chart is generated:
A look at Google earth shows the canal is about 450 metres long and 80 metres wide.
The layout of the intake canal is shown below, indicating an average depth of about 7 metres, and :