Subject: Acid Rain and Tap Water

How does acid rain form? And how much acid is in the tap water in North East United States?

    Acid rain forms when molecules of oxidized sulfur and/or nitrogen in the atmosphere combine with water, forming acidic compounds that dissolve in the water that becomes rain. Typical sulfur compounds (SO2 ans SO3) get into the atmosphere from both natural (i.e. volcanoes, windblown dust containing gypsum-which has SO4 ions in it, etc..) and non-natural (i.e., burning of coal, refining of metal ores) sources. In the north east, the sulfur in the atmosphere is mostly non-natural. In parts of the desert west and in Hawaii, there is a large amount of natural sulfur put into the atmosphere. Nitrogen compounds also get into the atmosphere and form acids, although the natural sources are much more limited than with sulfur. The biggest non-natural source is buring fossil fuels, especially gasoline. Acid rain in L.A. is mostly HNO3 (nitric acid) due to all of the automobile exhaust. Acid rain in the north east is mostly H2SO4 (sulfuric acid) from all of the coal buring that goes on there. In Hawaii, Kilauea volcano puts sulfur into the atmosphere and makes natural acid rain. There is also a small amount of natural hydrochloric acid (HCl) put into the atmosphere this way. Fortunately, our abundant rain and winds dilute the acid in the rain and mostly blow it out to sea, although a large desert area exists in one area of otherwise-lush Kilauea volcano due to acid fallout.
    Your question about tap water is more difficult. Most tap water used in urban areas is not collected directly from rain water, but rather from ground wells. One of the interesting features of groundwater is that its pH (acid content) is controlled mostly by the substrate (rocks) the water flows through, rather than its source. Acids are one of the most effectively "neutralized" contaminants in groundwater (the same can't be said for other things such as heavy metals and pesticides). If the water in your area is drawn from a lake, it has a chance of being fairly acid, however. As far as I know, there is no tap water in any major urban areas significantly more acid than table vinegar (pH= about 5), but I may be mistaken about this since I don't keep up on the details of this subject as much as other subjects. In addition, almost all urban water in the US is treated for bacteria, during which the pH is adjusted to a fairly neutral level (close to pH = 7). Thus, whatever the pH of your tap water is, acid rain will probably not be that important of a factor.
    The danger from acid rain is mostly to forest and lake ecosystems, which are very sensitive to chemical imbalances in their environment. With the losss of these ecosystems that effectively moderate the pH of surface run-off water in many areas, the effect of acid rain on local water supplies could become more pronounced in the future. The technology to reduce acid emissions exists today; the question is: are we willing to pay more for goods and services to help offset the cost of their use?

Dr. Ken Rubin, Asst. Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822

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