Question: What is the name of the system that radioactivity contaminated?
About the article "Chernobyl disaster", section "Rivers, lakes and reservoirs".

Candidate passages:
The level of radiation in Ozyorsk itself is claimed to be safe for humans, but the area of EURT is still heavily contaminated with radioactivity. 
Objects and places.
Topsoil had to be removed from several sites, and several houses were demolished.
All the objects from within those houses were removed and examined.
Those that were found to be free of radioactivity were wrapped in plastic bags, while those that were contaminated were either decontaminated or disposed of as waste.
In industry, the choice between decontaminating or disposing objects is based only on the economic value of the object and the ease of decontamination.
However, in this case, the IAEA recognized that to reduce the psychological impact of the event, greater effort should be taken to clean up items of personal value, such as jewelry and photographs.
It is not clear from the IAEA report to what degree this was practiced. 
Each of the below ground explosions—some as deep as 5,000 feet—vaporized a large chamber, leaving a cavity filled with radioactive rubble.
About a third of the tests were conducted directly in aquifers, and others were hundreds or thousands of feet below the water table.
When testing ended in 1992, the Department of Energy estimated that more than of radioactivity remained in the environment at that time, making the site one of the most radioactively contaminated locations in the United States.
In the most seriously affected zones, the concentration of radioactivity in affected groundwater reaches millions of picocuries per liter.
(The federal standard for drinking water is 20 picocuries per liter (0.74 Bq/l).)
Although radioactivity levels in the water continue to decline over time, the longer-lived isotopes could pose risks to workers or future settlers on the NNSS for tens of thousands of years.
The Energy Department has 48 monitoring wells at the site and recently began drilling nine deep wells.
Because the contaminated water poses no immediate health threat, the Department has ranked Nevada as a low priority for cleaning up major nuclear weapons sites, and it operates far fewer wells than at most other contaminated sites. 
This is a subpage of Environmental radioactivity. 
On 21 April, TEPCO estimated that 520 tons of radioactive water leaked into the sea before leaks in a pit in unit 2 were plugged, totaling 4,700 TBq of water release (calculated by simple sum, which is inconsistent with the IAEA methodology for mixed-nuclide releases) (20,000 times facility's annual limit).
TEPCO's detailed estimates were 2,800 TBq of I-131, 940 TBq of Cs-134, 940 TBq of Cs-137.
Another 300,000 tons of relatively less-radioactive water had already been reported to have leaked or been purposefully pumped into the sea to free room for storage of highly radioactively contaminated water.
TEPCO had attempted to contain contaminated water in the harbor near the plant by installing "curtains" to prevent outflow, but now believes this effort was unsuccessful.
According to a report published in October 2011 by the French Institute for Radiological Protection and Nuclear Safety, between 21 March and mid-July around 2.7 × 1016 Bq of caesium-137 (about 8.4 kg) entered the ocean, about 82 percent having flowed into the sea before 8 April.
This emission of radioactivity into the sea represents the most important individual emission of artificial radioactivity into the sea ever observed.
However, the Fukushima coast has some of the world's strongest currents and these transported the contaminated waters far into the Pacific Ocean, thus causing great dispersion of the radioactive elements.
The results of measurements of both the seawater and the coastal sediments led to the supposition that the consequences of the accident, in terms of radioactivity, would be minor for marine life as of autumn 2011 (weak concentration of radioactivity in the water and limited accumulation in sediments).
On the other hand, significant pollution of sea water along the coast near the nuclear plant might persist, because of the continuing arrival of radioactive material transported towards the sea by surface water running over contaminated soil.
Further, some coastal areas might have less-favorable dilution or sedimentation characteristics than those observed so far.
Finally, the possible presence of other persistent radioactive substances, such as strontium-90 or plutonium, has not been sufficiently studied.
Recent measurements show persistent contamination of some marine species (mostly fish) caught along the coast of Fukushima district.
Organisms that filter water and fish at the top of the food chain are, over time, the most sensitive to caesium pollution.
It is thus justified to maintain surveillance of marine life that is fished in the coastal waters off Fukushima.
Despite caesium isotopic concentration in the waters off of Japan being 10 to 1000 times above concentration prior to the accident, radiation risks are below what is generally considered harmful to marine animals and human consumers.
A year after the disaster, in April 2012, sea fish caught near the Fukushima power plant still contain as much radioactive 134Cs and 137Cs compared to fish caught in the days after the disaster.
At the end of October 2012 TEPCO admitted that it could not exclude radiation emissions into the ocean, although the radiation levels were stabilised.
Undetected leaks into the ocean from the reactors, could not be ruled out, because their basements remain flooded with cooling water, and the 2,400-foot-long steel and concrete wall between the site’s reactors and the ocean, that should reach 100 feet underground, was still under construction, and would not be finished before mid-2014.
Around August 2012 two greenling were caught close to the Fukushima shore, they contained more than 25,000 becquerels a kilogram of cesium, the highest cesium levels found in fish since the disaster and 250 times the government’s safety limit.
In August 2013, a Nuclear Regulatory Authority task force reported that contaminated groundwater had breached an underground barrier, was rising toward the surface and exceeded legal limits of radioactive discharge.
The underground barrier was only effective in solidifying the ground at least 1.8 meters below the surface, and water began seeping through shallow areas of earth into the sea.