With regular rainfall/usage the nitrates don't get far enough down to reach the aquifer. The nitrates get filtered naturally and remain in upper layers of the soil. Amazon concentrated the contamination and drove it far enough down that it reached and contaminated the aquifer, but below that the father down you dig and pull up water the less contaminated it will be. Part of the settlement is a fund for digging new private wells deep enough that the water contamination is under legal limits. I guess if there's no clean water below the contamination they'll have to stop using wells entirely and depend on treated water from the contaminated source.