TEPCO, Mar 11, 2015 (emphasis added): Fukushima Daiichi NPS Prompt Report — Tepco Reports Rainwater Containing Strontium90 Found Seeping Into Ground At Fukushima Daiichi Tank Area; No Leakage Into Ocean… approximately 750 tons of rainwater containing Strontium90 may have seeped into the ground… [TEPCO] explained that the water is unlikely to leak out to the ocean through any of its drainage channels but it is monitoring the situation… the total amount of nuclides emitting beta rays such as Strontium90 ranged from 150 to 8,300 Bequerels per liter… it is investigating the cause of the rainwater contamination…
Asahi, Mar 11, 2015: [TEPCO] reported a massive leak of radioactive rainwater… on March 10. It said hundreds of tons of contaminated rainwater breached barriers surrounding storage tanks for highly radioactive water and seeped into the ground… Highly contaminated wateralso leaked from the same area and seeped into the ground in summer 2013… In the latest leakage, workers discovered bubbles at the junctions of side ditches in the area. Officials said this indicated that contaminated rainwater had seeped through the junctions and into the ground.
NHK, Mar 11, 2015: Radioactive rainwater leak at Fukushima plant — The operator of the Fukushima Daiichi nuclear plant says an estimated 750 tons of rainwater with relatively high levels of radioactivity has been leaked… TEPCO officials said the rainwater was 15 centimeters deep at 10:30 PM on Monday, but the figure had fallen to 8 centimeters shortly after 8 AM on Tuesday… They say the water inside the dam contained up to 8,300 becquerels per liter of beta-particle-emitting substances. TEPCO officials say all the water has now been collected. They say the drainage channel to the ocean is buried underground and it is unlikely that the water flowed into the ocean.
Jiji Press, Mar 11, 2015: TEPCO initially put the amount of leaked rainwater at 400 tons. The number was revised to 747 tons.
Arirang, Mar 12, 2015: Another major leakat… Fukushima nuclear power plant earlier this week may have raised radiation levels in nearby waters. [TEPCO] told Seoul-based Yonhap News that the radiation levels in water collected on Wednesday were 30 times higher than the sample collected on Monday. TEPCO said earlier this week that nearly  tons of contaminated rainwater had escaped the plant.
NHK, Mar 11, 2015: Groundwater radiation levels surge after leak – [TEPCO] says levels of radioactive material in groundwater have surged 30-fold, apparently after contaminated rainwater leaked outside a barrier… [It] estimates the leak at about 750 tons of rainwater, and it says levels of beta ray-emitting substances [were] 8,300 becquerels per liter. On Wednesday, TEPCO measured 11,000 becquerels per liter in groundwater… up from 370 becquerels…