SHREVEPORT, La. (AP) — Unusually high concentrations of the mineral manganese are discoloring the drinking water in the northwest Louisiana city of Shreveport, where officials have been fighting the problem for over a month.

The manganese isn’t considered a health hazard for the general population. But the Shreveport-Bossier City Advocate reports that drinking water from taps in many Shreveport homes began to take on a yellowish or greenish color soon after early June rains were followed by a buildup of manganese in Cross Lake. The lake is the source of the city’s drinking water.

City workers’ latest efforts to fight the problem include scraping the tanks where impurities settle out of the drinking water during the treatment process. The unusually high concentration of manganese coming from the lake created sludge piles on the tank bottoms.

City officials aren’t sure of the reasons for the increased manganese levels. Possible reasons include runoff from construction sites after the heavy June downpours or natural occurrences related to temperature changes,

Water and Sewerage Department Director William Daniel said the manganese in tanks reverts to a gas and goes back into the water. “We’ve never had this much manganese in our water before,” Daniel said.

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Daniel said Monday that dissolved manganese levels in the water were still at about 0.15 milligrams per liter when they should be about 0.05 milligrams per liter. Environmental Protection Agency literature says staining and a bitter metallic taste are problems when manganese concentration in water reaches .05 milligrams per liter.

The agency doesn’t have mandatory water quality standards for manganese but has issued guidance that includes a recommendation that the general population not consume more than 1 milligram of manganese per liter of water for more than 10 days per year. Lower levels are a concern, however, for infants, according to the Louisiana Department of Health website.