On 6/19/2015, Tepco announced they measured 1,000,000 Bq/m3 of Strontium-90 at two locations in Fukushima plant port.
This is the highest reading in recorded history. The sample is the port seawater. Sampling date was 5/4/2015.
The location was near the water intake of Reactor 3 and 4, and also the screen of Reactor 4.
The previous highest readings were lower than 700,000 Bq/m3.
Tepco has not made any announcement on this rapid increase.
Source: Fukushima Daiichi