Noble’s Notes: “A quarterly recollection from 40 years of service to the groundwater community.”
By John Noble
[Ed. Note: The following article originally appeared in our first newsletter edition, published in October 1999.]
Some things that are so obvious turn out to be not true at all. One of these is predicting the presence of iron in ground water – perhaps the most common and pervasive water quality problem with wells in much of the country. Ground water with dissolved iron commonly looks as pure and pristine as a mountain stream. However, it tastes like rusty pipes. The taste is bad enough, but let the water stand in air overnight and the iron will precipitate out into a red floc which is truly ugly. When I used to canvass domestic wells in Western Washington, under negative comments of quality, the commonest was “red water” or “too much iron”. Many drillers I have known have bypassed zones of red sands and gravels that are obviously rich in iron. They wanted a satisfied customer for their finished well. Unhappily, they were often dead wrong. The obvious problem was not really there at all.
The apparent contradiction is that troublesome iron in water must first be in a reduced (unoxidized) state. In that state, all iron is dissolved. It may taste bad, but does not look bad. When the drill goes through reddish formations, and the bailings are all rusty, it is likely that the iron has already precipitated and the water contains no dissolved iron. If a well is completed in these “rusty” zones, and developed or pumped enough to clear the water of turbidity, the tested water quality is generally low in iron and does not have the characteristic metallic taste. The driller’s concern about quality may have been in serious error.
I learned these facts from my first boss in the water well industry. John Robinson had a long suit in pragmatic observations and enough chemistry to see the ironic truth. Between us, we tried to pass on the truth but were not fully successful. Even today, drillers are passing up good red water.
The converse of being incorrectly wary of rusty drill water is not being wary of exceptionally “clean or bright” sands, especially those that have a blue or green color. These formations commonly contain water that is in a “reduced” state, or hungry for oxygen. Drilling samples laid out for future inspection or reference that have the bluish or greenish color commonly are rusty in the morning. Another sign of possible trouble from this reduced water is the appearance of drilling tools pulled from the well on the following morning. The drill stem, which was polished all silvery during the prior day’s work, is not rusty but is instead a bright green. When this is observed, bet that the dissolved iron content will be high enough to fail the water quality tests.
In simple summary, iron that has already precipitated out in the formation will cause you no problems. Iron that is dissolved, but waiting to precipitate out in your glass, will be a problem. Brown is good and blue or green is bad.