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I wasn't aware that the water quality can change. I can see if the water was contaminated, like the dumping of toxic materials, but where would the iron come from if the well tested fine to begin with?"
Actually about the only constant with groundwater is change.
Seasonal changes can be especially dramatic with water levels fluctuations, atmospheric interactions, pH changes, recharge rate increases related to precipitation events/snowmelt/ground freezing, and changes in biologic activity within the water column or vadose zone. On top of the above, the very act of pumping water throws chemical equilibriums out of balance.
Keep in mind that iron is one of the most common elements in the earths crust making up about 5% by volume. The best predictor of the fate of iron in water is the EH-pH equilibrium diagram. See Example(http://www.ozcoasts.org.au/indicators/images/Fe_speciation.gif) With both EH and pH monitored, one can do a decent job of predicting the dissolution/precipitation and speciation of iron between ferrous (Fe2+) or ferric (Fe3+) states and salts and even provide an indication of the potential for iron bacteria growth. Still some complexities exist since there are numerous inter-related inorganic and organic chemical reactions-- happening on the scale of hours while others take weeks.
-Adam |