This summer’s relentless heat wave isn’t just driving up temperatures – it’s also affecting tap water.

On Tuesday, several cities in the Seoul metropolitan area, including Incheon and Anyang, and Gwacheon of Gyeonggi Province, issued public alerts following resident complaints about an unusual grassy odor coming from faucets.

The culprit, officials said, is a widespread algae outbreak in the water reservoirs, triggered by months of scorching heat. The unpleasant smell comes from geosmin, a naturally occurring compound released when algae die off in cooler temperatures.

Water officials assured that the water remains safe to drink, while advising boiling it for at least three minutes as a precaution. They also noted that purification efforts are being ramped up, including the use of activated carbon to improve taste and odor.

The local alerts come after the Ministry of Environment issued a warning Monday about an algal bloom in Paldang Lake, the primary water source for around 25 million residents in Greater Seoul. Prolonged heat and nutrient-rich runoff from heavy monsoon rains have worsened the outbreak, according to officials.

The country’s southern regions are also grappling with algae problems. On Friday, the Daecheong and Boryeong Reservoirs in the Geum River basin, which supply water to the region of North and South Chungcheong provinces, were placed on alert — the first such warning in the country this year. This alert is triggered when algae levels exceed 10,000 per milliliter in two consecutive tests.

Reservoirs in the Nakdong River basin, which serve Daegu and Busan, were already put under a lower-level watch on Aug. 8. The South Gyeongsang Provincial government had earlier requested increased water release from the region’s dams to combat algal growth.