I saw this on the news last night and found quite a few articles online after that as well. The Paldang Dam is located on the Han River and is the source of water for the capital area. You can see it here:





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If you zoom out you can see that it's located just to the east of the major urban area where Seoul and all its adjoining satellite cities are. This dam supplies water to 24 million people (so says the second article).



One article can be seen here, and another here (both in Korean). Here's my translation of part of it:





Due to drought continuing from last year, the water quality of the Paldang Dam, the source of water for the capital region, has gotten much worse.



On the 9th the Environmental Ministry announced that the BOD (biological oxygen demand) of the water in the Paldang Dam as investigated in March had reached 2.4 mg per litre. This is an increase of 0.8 mg over the 1.6 mg in February, and 1.4 mg more than the 1.0 mg in January. This is also the highest monthly level reached since 1989 since monthly measurements began to be taken. It is also quite bad compared to the average 1.6 mg average rate recorded from 2004 to 2008.



A large influence for this worsening of water quality is the continuing drought that has continued since last autumn. Precipitation has decreased a great deal along with entry volume into the river, and the flow of the river has slowed down...during the winter the remaining fertilizer and excrement from livestock from farmland has been carried away with rainwater, increasing the pollution.



The Environmental Ministry says that in order to improve the water quality of the Paldang Dam it would strengthen basic facility management such as the sewage system and waste processing facilities.

By the way, over the past three days or so the weather has also gotten extremely warm, reaching highs of 24 or 25 each day. Just a bit over a week ago it still felt a bit like winter.