Subject: pH of rain and creek waters

In testing local creek water and rainwater, I found a rather substantial difference in pH. The rainwater was about 5.7 pH while the creek waters averaged around 7.9. What could account for these differences? What sort of conclusion might I be able to draw from this?

    First, the rainwater in your area sounds "normal". Most rainwater has a pH of 5.6 to 5.8, simply due to the pressence of carbonic acid (H2CO3). The latter is formed from dissolved CO2 gas and H2O. The source of the CO2 is the atmosphere, which presently contains about 380 ppm CO2. Values of pH below 5.6 or so are considered "acid rain", values above are "basic rain". These names are somewhat in conflict with the common pH scale, for which 7 is "neutral", below 7 is "acid" and above 7 is "base". As far as rain in concerned, abnormal acidity or bacisity is relative to CO2-H2O equlibrium values of 5.6 to 5.8. There are both natural and non-natural sources of materials that cause rain pH to deviate from the CO2-H2O equlibrium values, but this isn't what you asked about so I won't go into it now.
    The creek waters have a pH of 7.9 because they have interacted with a basic substance at some point in their history. Probalby this was in the soils or rocks that formed the solid substrate of the aquifer containing the groundwater that feeds this creek. Without knowing more details of the composition of the creek water, it is impossible to say what the base is, but the most common one is CaCO3. Calcium carbonate may be present as limestone or marble. Another possiblity is the closely related material Ca(Mg)CO3 (dolomite), which is Calcium Carbonate with some Magnesium impurity in it. A second common base found in soils of either agricultural or heavily forested areas in ammonia (NH3). Either of these bases may be the cause of the high pH you measured.

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

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