DMWW has a certain capacity for removing nitrates from the water.
When demand is too high, the nitrates cannot be removed from water fast enough causing problems.
DMWW has therefore asked to not water lawns and be more conscious of water, to try and lower demand.
The nitrate removal system works and it the water is as safe to drink as any other tap water.
The amount of demand for water is too high right now to keep it that way.
DMWW has facilities plans in place to try and increase flows, but that takes a really long time to design and build. This is also true on the wastewater side with the WRA. The city has grown a TON over the last decade and is projected to continue to do so. More people means they need more flows.
The city tried to sue farm companies upstream for runoff and were laughed out of court. The state legislature makes many decisions with either the goal or effect of negatively impacting the DSM metro. The city is a bit between a rock and a hard place.