By March 2, back in Calcutta, we were noticing that the tea we brewed in my inlaws' apartment didn't taste right. No matter what kind of leaf we used, it ended up tasting blunt and characterless. So I popped downstairs to the nearest grocery store and picked up a bottle of water - forgive me, it was from a subsidiary of Coca-Cola - and tried again. The difference was night and day: now it really tasted like tea. Also, after drinking a cup, I saw that tea made with the bottled water didn't leave scummy residue on the china the way the house water did.
Now that we had a phenomenon, the explanation was ours by just asking. In the spiffy high-rise complex where my sister-in-law and brother-in-law live, the water comes from deep wells. This water has a high concentration of iron, my engineer brother-in-law explained. Later on, we tried brewing tea using tap water from another relative's apartment in a different part of Calcutta. It was fine.
The only remaining mystery was how we had failed to notice the iffy tea we'd been drinking during the couple of days in Calcutta at the beginning of our trip, before we took off for the hills. Maybe jet lag. Maybe the distraction of being surrounded by people you haven't seen in a long time. Who knows?
/Lew